New Wiki Article on Boiler Standards

[Gooserider]

Just to let everyone know, I have just posted a new article in the Hearth Wiki, entitled Boiler Standards. It is a bit of background on the boiler standards we see today, and in particular leads into a paper contracted for by BioHeat USA in their efforts to deal with the current requirement for an ASME "H Stamp" on boilers sold in Mass.

Interesting read in it's own right, and I'd welcome comments and / or edits with corrections or enhancements - feel free to either make the changes yourself, or let me know in this thread, and I can make them for you.

Gooserider

 

[steam man]

I like the idea. Good article. I have a few thoughts on expanding it, particularly in the regards to using storage/pressure vessels. I just need to find time in my life. Good job.

Mike

 

[Northwoodsman]

Gooserider,

Thanks for the info. It's funny you should post ths info. as I met with my local State of Michigan" mechaical inspector yesterday to discuss the European Wood gasifiaction boilers with him.

He then gave me a # of the state boiler organization at our states capital in Lansing as he WILL NOT OK thse boilers for installation in his county and says I will not find an inspector to aprrove them in any county in MI because they do not have the ASME stamp/certification.

Thus, I called the chief engineer in Lansing yesterday and he confirmed my local inspectors opinion. Although he said it is not impossible to get these listed as "special" boilers, it will take much time and research on their part and since the state is already in a tight financial situation it makes matters even harder.

Thus, until these boilers receive ASME certification or a special exemption, I see the following options:

1.) These systems can be installed as "non-pressure" systems without any problems by any state inspection agency. Basically, this requires that a "T" be addded to the pipe system and a length of pipe be connected to the T. Thus, as the system operates it will be unable to build up pressure. The concern of course will be that corrosion will take place within the system because of the oxygen that will be allowed to enter the boiler water.

Once the systems have been installed as an open system, if the owner takes it upon himself to cap this T/vent pipe, than the system will now be operating as a closed, pressurized system.

Has anyone out there oprerated a boiler like this as an open system (with thermal storage)?

2.) Sell the boiler to a customer who installs the system without pulling a permit (many customers will be lost because they do not want/cannot install this system themselves). This also puts the local dealer at risk!

Any feedback is greatly appreicated!

NWM

 

[stee6043]

Gooserider,

2.) Sell the boiler to a customer who installs the system without pulling a permit (many customers will be lost because they do not want/cannot install this system themselves). This also puts the local dealer at risk!

Any feedback is greatly appreicated!

NWM

I'm not sure I'd go so far as to say "many customers" will not or cannot perform a self install. It seems to me "most" of the folks on this site have installed thier own systems. How that relates to the total market for gassers I do not know. But most of us who are willing and able to cut, split and stack wood tend to be moderately handy in general, in my humble opinion...

 

[Gooserider]

NWM, I know you are primarily an EKO dealer, but in addition to calling Lukasz Orlanski from the Eco Olan USA importer, I would give BioHeat USA a call... I know that they are very active in trying to get the MA Boiler authorities to change their rules, I would imagine that they would be just as active in your state.

Another option to avoid running a totally "open" system might be to see if your AHJ's will accept a European style "open" system, where they run a pipe from the boiler up to the top floor attic and put an open tank at the top (which must have AT LEAST as much normally unused capacity as your expansion tank would need for acceptance volume) - this results in a system that is technically "open" but is effectively pressurized by the weight of the water in the tank pushing down on the boiler (roughly one atmosphere (14.7psi) per 30' of height) - while this is an open system, the water in the tank mostly doesn't circulate, so it allows relatively little oxygen to enter the system, especially if you put a layer of oil or other "sealing liquid" on top of the water.

Gooserider

 

[Northwoodsman]

Thanks for the info.

That's exactly what I was thinking about regarding the expansion tank size "open container" elevated above the highest point of the boiler system. I'm also thinking that this tank should preferably be set up in a vertical position so as to have a larger quantity of water pushing down on the system (taller and skinner vs. shorter and wider -all depending on the space available in the specific application).

I didn't think about the oil layer but that's a neat idea also.

NWM

 

[webbie]

note that a simpler open system could use a plate heat exchanger - possibly (if they are rated for exposure to air).....and then the house system could be pressurized.

 

[toddm]

I'll have an open system by this time next year in new construction. If you go here, www.heatweb.com, and click on "wood burning systems" (about half way down the page) you'll see a number of schemes used by the Brits and based on thermosiphon and tank stratification. They were willing to sell a tank to me spec'ed and equipped for my installation, but shipping to the east coast at $900 was a bit much.

The typical approach is to plumb the boiler to a reverse indirect tank, which then supplies heated water under pressure to hydronic and DHW. The water in the tank never changes, except for topping up, so their claim is that open systems can be as kind to boilers as pressurized if you follow their treatment regimens. (Your thoughts on this subject are welcomed.) They have a number of strategies to keep the tank from boiling if the pressurized side stops unexpectedly. In the niftiest, they route tap water through a HX at the top of the tank and dump it to waste. No power? No problem.

I have a pair of crossflow circuits in my boiler stove, so I plan to plumb one directly to the radiant system in my slab, and the second to a storage tank of 120 gallons operating by thermosiphon and heating DHW and staple-up radiant on the second floor . The second circuit will have about 15 feet of head from the expansion tank and a heat dump radiator equal to 10 percent of the stove's output, as per the manufacturer's instructions. I figure I'll need an emergency generator anyway. I won't much heat if I can't keep the hydronic pumps turning. Your thoughts welcome here as well.

 

[Gooserider]

Thanks for the info.

That's exactly what I was thinking about regarding the expansion tank size "open container" elevated above the highest point of the boiler system. I'm also thinking that this tank should preferably be set up in a vertical position so as to have a larger quantity of water pushing down on the system (taller and skinner vs. shorter and wider -all depending on the space available in the specific application).

I didn't think about the oil layer but that's a neat idea also.

NWM

It is rather counter-intuitive, but if you go check your laws of physics as applied to fluid systems, you will find that what produces the pressure is the HEIGHT of the water colum, not it's shape, or how big it is... Doesn't matter if the tank at the top is one inch or 10 feet around, one gallon, or a thousand, if the colum height from the guage to the water line stays the same, the pressure will stay the same...

You could choose the tank to be short and fat, or tall and skinny, and as long as the water line is the same height you would get the same pressure at the boiler. In practice, since the tank level will change slightly as water gets pushed in and out of the system by thermal changes, a short and fat tank should theoretically give you a more stable boiler pressure as a change in the amount of water in the tank would produce less change in the tank level, but I doubt that there would be enough difference either way to matter - I'd pick the tank based on the available space...

Keep in mind that like an expansion tank, the attic acceptance container should be mostly empty under normal operating conditions - the only function the tank has is to give a place for the water pushed out of the system by thermal expansion to go... If you didn't need the expansion space, just the height of a 1/8" pipe full of water would be enough to give you the required pressure.

Gooserider

 

[pybyr]

Keep in mind that like an expansion tank, the attic acceptance container should be mostly empty under normal operating conditions - the only function the tank has is to give a place for the water pushed out of the system by thermal expansion to go... If you didn't need the expansion space, just the height of a 1/8" pipe full of water would be enough to give you the required pressure.

Gooserider

One other factor (or two) --- that I'll mention since I'd given pretty serious thought to one of the euro-style "semi-open" systems where the only pressure is the result of a water column, not a sealed vessel--- for many of us who live in parts of the US/ Canada where a wood gasification boiler is a strongly appealing option, places at the top of the house where an expansion tank might be located, and/or the location of a vent pipe, might very well be prone to freezing-- either of the main tank or potential accumulated freeze up of water vapor in a vent line to the outside.

Also, to the extent that an "attic" might be sealed well enough to not be subject to freezing, you then need to be careful that you don't have any significant amount of water vapor exiting the expansion vessel or vent into the indoor space-- or else you might end up with problems from condensation (mold, delamination of plywood, etc).

Note that I'm NOT trying to cast doubt or ill-repute on the idea of a "semi open" system-- I think they are ingenious, and I nearly went with one, it's just that in my case, I did not need a big expansion tank, and it ended up being more straightforward to flop in an expansion tank instead of designing and installing the details of the other. I'm just trying to note some factors that come into play, and that we in N. America don't generally end up thinking about, as we don't usually have water in un-conditioned spaces at the top of the house.

In my personal opinion, I think the various jurisdictions' resistance to the Euro-boilers on the basis of the standards issue is crummy. As long as it's verified that the Euro standards are at least as stringent as the US standards (which I expect is what Bioheat is documenting- I haven't read the Wiki on this yet, but look forward to), then I think it'd be reat_ if the federal government enacted a standard that gave any boiler meeting the specified Euro standards "reciprocity" under certain US standards- and declared that reciprocity as pre-empting any state or local standard to the contrary. That'd really pave the way for introduction of innovation, which we need (side note, I am a happy Econoburn owner, and the E-burn does meet US standards, and can be had with an ASME stamp- I just think that it'd be good to really get this technology "out there" as far and wide and fast as reasonably possible) Of course, I am not a congressperson, so I have no ability to spur such federal action-- just noting that it's potentially possible from a legal/ technical standpoint if the right constituencies could built an appropriate head of steam behind it....

 

[Gooserider]

I would agree Pybyr - those are important points to consider... Probably worth asking how the Europeans actually handle the issue, but a few thoughts occur to me off hand...

1. I wouldn't expect there to be that much evaporation - My expectation would be that there wouldn't be that much circulation of hot water up into the tank, so the tank would probably stay relatively cool - probably somewhere between house temp and outdoor temp. However it should also be possible to vent the tank through the ceiling, much like a sewer gas vent, so the tank in the attic could be effectively sealed away from inside the attic (I might have a removable access cover for servicing - but that's a detail...)

2. Given a low level of evaporation, I wouldn't expect vapor buildup in the vent pipe to be a big issue, and again I think it could be designed around.

3. It would seem possible to keep the tank above freezing by the simple expedient of re-shaping the thermal envelope a bit - remove the attic insulation under the tank, and don't insulate it's bottom, but heavily insulate it on the top and all sides - hopefully you'd get enough heat coming through from the floor under it to keep the tank warm.

4. If that weren't enough, presumably one could add some supplemental heating under the insulation - shouldn't take much, I'd probably look at some of the electric heat tapes...

As an additional comment on tank setup, I'd also want to be sure there was an easy way to monitor the tank water level, preferably w/o needing to go to the attic, and that there was consideration given to "flood control" in the event of a tank leak / overflow. I don't see a need to have a water line to fill the tank, though it's an option, as it should be just as easy to have a way to add makeup water down in the boiler room - which would fill the tank from below automatically...

Gooserider

 

[toddm]

I'm building a "heat tower" for my installation. (Granted, clean slate in new construction). Looks like an oversized chimney from the outside. Stove in a fake fireplace on the ground floor. Accumulator tank and machinery on the second floor, open to the height of the tower, which is 20 feet. I'll have a heat pump water heater to move ambient heat into the tank, so I wouldn't expect any humidity issues. I'm also using it as a plenum for HRV distribution.

BTW, I'd ask the manufacturer about adding oil to an indirect tank. Mine is very specific: Fernox and nothing but.

 

[Hansson]

The risk of freezing is not so big. As the water in the system expand you get some hot water in the expansion tank.
When we install these system here we got to have a pressure valve close to the boiler.If the the expansion freez the valve will open.you got to insulate the tank but the risk of freeze is not so big.

Remember that you don't have to put the expansion tank in the attic.It only have to be over the top radiator.
I have my expansion tank in a wardrobe in the top floor of the house
(broken link removed to http://picasaweb.google.se/725hansson/Panrumsbilder#5020602465138459858)

From the top of the expansion tank there is a a pipe that goes down to the boiler room. You fill the system from a valve in the boiler room and when the expansion tank is full the water flow over and the water comes down to the boiler room over a sewer well.

I hope you can read what I trying to explain.It`s hard to speak when you don't have the words right

Got to work before the boss hangs me..

 

[Gooserider]

I hope you can read what I trying to explain.It`s hard to speak when you don’t have the words right

No problem Hansson, your english isn't perfect, but it is more than good enough for us to figure out what you are trying to tell us - especially with the links to the pictures (Will admit the captions on the photos aren't as helpful, but most of the photos don't need a lot of explanation) Mostly any problems are a question of word choice, and we can work around that - for instance, most of us think of a "wardrobe" as a large but movable peice of furniture, that serves the same basic use as what we call a "closet" - which is a permanent part of a building - but wardrobe was close enough I could guess what you meant with no problem. We have native english speakers that are harder to understand at times, so you are doing OK.

Thanks for the info on the storage tank setup, your description plus the picture makes it quite clear how your system is setup. I do have a few details that aren't quite clear.

1. I see the pipe that looks like it goes into the top of the tank, and which has a "T" in it where it looks like one branch is going down behind the tank, and one branch is going up, while the third branch goes into the tank - I assume this is the fill line? Is the pipe that goes straight up a vent? Where does it go, just into the attic, or all the way out the roof?

2. I assume the pipe coming out the top side of the tank and going down is the overflow?

3. What I don't see is the connection to the tank from the boiler, which I thought needed to be at the bottom of the tank? Is it just not showing in the picture, or am I missing something?

4. You speak of filling the expansion tank until it overflows - but I thought the tank needed to be mostly empty so that it could accept the expansion of the system water WITHOUT overflowing?

5. How much water does the tank lose through evaporation, and how often do you need to refill it?

Thanks,

Gooserider

 

[Hansson]

1 Yes it`s goes all the way out the roof.
2 Yes it`s the overflow
3 the expansion tank is connected on the pipe between the boiler and tank.
The air,steam in the system can evaporate true the expansion tank easy
4 The expansion tank don't need to be empty. the norm here is that the expansion tank is 5% of the system volume.
A little more than the water in the system expand when heated.

5 Not often.Maybe one time /year.

I done a simple picture of how the system can look.

 

[toddm]

Thermosiphon is a function of delta T and height. You will find a formula at the bottom of heatweb.com's "Wood burning systems." http://www.heatweb.com/Wood/Wood and Heat Banks.pdf Thus you aren't constrained to putting the boiler in the basement and the expansion tank in the attic.

Actually, the Brits much prefer pumps to thermosiphon, although their systems must have a gravity loop somewhere. Look at the circuit schematic on page 6 "Overheat protection using a heat exchanger." The thermosiphon loop here is irrelevant unless the power fails.

For safety reasons, the Brits also insist on a fresh water supply at the expansion tank operated by float valve. The requirement on my stove is 3/4 inch, in fact. In a catastrophic loss of water, the Brits are choosing flooding over grimmer outcomes.

 

[toddm]

You'll have to cut and paste through ".pdf" to get the url to work.

 

[Gooserider]

Thanks for the info Hansson, It sounds like I had the right idea in general, but your sketch and comments definitely helped me understand it...

Toddm, I've looked briefly at that heatweb site, need to go through it more, but I see a couple of possible differences between what they are doing and what most of us seem to be doing...

1. Looking at their descriptions and diagrams, I think their designs are based on the assumption that the wood burner is a naturally drafted / vented unit, essentially a conventional wood stove with a water heating coil in it, that would continue burning uninfluenced by a power failure. OTOH, most of the boilers that we are talking about are forced draft in some way; so a power failure will cut off the draft and signifcantly reduce the boiler's heat output. Even so, most gasification boiler setups still require some sort of provision for heat dumping in cases of power failure.

2. The expansion tank we are talking about does NOT depend on thermosiphon action - it has nothing to do with thermosiphoning (indeed I would want to avoid as much thermosiphoning as possible!) and the only impact delta T has is on how much water expands into the tank... The function of the tank is to pressurize the system, especially the boiler and the pumps, which is PURELY a function of the delta height between the water level in the tank and the point of measurement (which should be the boiler since it is the most pressure sensitive component). Most hydronic system (at least in the US) seem to want a pressure of around 12-15 psi, and it takes a height difference of between 20 to 30 feet to get that... Thus the expansion tank pretty much needs to be up in the attic with the boiler in the basement in order to get the required height difference...

OTOH, a tank intended for hot water storage, can be located anywhere in the system, though most installs seem to put it near the boiler, at about the same height with a pumped circulation... If you do want to heat it with thermosiphon action, it does need to be above the boiler, but how much higher is less of a concern... We are talking different tanks for different purposes...

As to the float valve refill, I can see their point, though I don't agree completely... If I was required to have an automatic fill, I would certainly want it hooked up to a real screamer of an alarm so that if it ever turned on (even a little bit) I would know as soon as possible so that I could deal with the problem...

Gooserider

 

[toddm]

You are misreading the gravity loop in the Heatweb schematic. The loop turns down at the heat dump HX, short of the expansion tank, and does indeed put delta T to work. My manufacturer blesses a scheme that runs radiant right out of the boiler with mixing valves. (Obviously you could still have pressure problems in some pipe runs, but it is easy enough to put them on a heat exchanger.) All my manufacturer says about the gravity loop is that it should be as straight up as possible and that the run shouldn't be longer than 19.5 feet, or 6 meters. That would be measured to the highest radiator rather than the expansion tank. The manufacturer of my stove wants a minimum water temperature of 113 degrees. I want an efficient fire most of all, so I can't say what water temperature I'll need with the hydronic pumps running -- I am using a radiant slab as storage. The manufacturer suggests that much heat will be going up. It suggests a towel warmer as a heat dump radiator, and says it can be useful in the summer to dry towels. (Nope; I don't believe that either.)

Yes, my stove is a blunter instrument than a gasifier. It is also very different in the fact that it's 20 feet away rather than in the backyard. Even so, I'd look hard at the Brits' belt and suspenders approach even if I was putting a wood boiler in the basement. Recall that this thread began with the state of Michigan's willing to bless a system that boils rather than blows up. Why stop there?

 

[toddm]

oops. Make that NOT much heat going up in the thermosiphon loop.

 

[Gooserider]

You are misreading the gravity loop in the Heatweb schematic. The loop turns down at the heat dump HX, short of the expansion tank, and does indeed put delta T to work. My manufacturer blesses a scheme that runs radiant right out of the boiler with mixing valves. (Obviously you could still have pressure problems in some pipe runs, but it is easy enough to put them on a heat exchanger.) All my manufacturer says about the gravity loop is that it should be as straight up as possible and that the run shouldn’t be longer than 19.5 feet, or 6 meters. That would be measured to the highest radiator rather than the expansion tank. The manufacturer of my stove wants a minimum water temperature of 113 degrees. I want an efficient fire most of all, so I can’t say what water temperature I’ll need with the hydronic pumps running—I am using a radiant slab as storage. The manufacturer suggests that much heat will be going up. It suggests a towel warmer as a heat dump radiator, and says it can be useful in the summer to dry towels. (Nope; I don’t believe that either.)

Well I just snagged a schematic off the heatweb site so that we can both be pointing at the same thing... It appears to me that all the schematics use variations on the same basic theme as the one I grabbed.

I see two tanks

1. A large lower tank which they use for heat storage, putting the heat into it by either thermosiphon action, or by pumping.

2. An much smaller upper tank which they describe as a "feed and expansion tank", and in some drawings specify must have a water level 300mm (approx 12") minimum above the highest radiator, and which contains the "toilet valve" fill mechanism - in each case the design incorporates elements that would work to keep heat from thermosiphoning into the expansion tank...

This is just what I said - the function of the expansion tank is to pressurise the system, not store heat or do anything else... In order to pressurise, it MUST be located at the highest point - which presumably would be in the attic or close to it. The boiler CAN be located anywhere in the system as long as it's below the expansion tank, but in order to get the maximum benefit from being pressurised, it needs to be located near where the pressure is highest, namely at the lowest point in the system...

Note that in the schematic I borrowed, they say there is a minimum height above the boiler that must be maintained - and to see the boiler specs for it. Also note that this is NOT a spacially accurate drawing, but rather a functional block diagram that shows the way the various parts interact...

While they do use the lower heat storage tank as part of the thermosiphon based heat dump in SOME of their layouts, it is by no means the only approach they use... The heat dump is a totally different function, and has no real effect on the expansion tank, other than that water expands when it's heated, and thus pushes the water level in the tank up.

I am confused :-S it seems like you are saying that I'm wrong, and that the correct understanding is what I just said that was wrong???

[/quote]Yes, my stove is a blunter instrument than a gasifier. It is also very different in the fact that it’s 20 feet away rather than in the backyard. Even so, I’d look hard at the Brits’ belt and suspenders approach even if I was putting a wood boiler in the basement. Recall that this thread began with the state of Michigan’s willing to bless a system that boils rather than blows up. Why stop there?[/quote]

Actually, if you go back to the beginning of the thread, you will find that I was the one who started it, and talking about the state of Massachusetts (MA, not MI) having rules requiring an ASME approval on a boiler, not a willingness to accept a system that boiled (though that is (barely) more acceptable than blowing up...) However I think Michigan may be one of the other states that is also having a similar problem, so your confusion is understandable.

While many people do put gasifiers in outbuildings, most are actually designed to be put in basements, and if I ever do my system, that's where mine will go - indeed I would be spending most of my time far closer than 20' from the boiler, as it would be ending up in the room directly under my office...

I do agree with you about looking at the British approach, but my feeling is that while they have some excellent ideas (which I plan to incorporate) some of them are not so good in that their "failure mode" is barely any better than the failure it is intended to counter... IMHO a flood caused by that toilet valve trying to refill the system after a line break somewheres else is not an acceptable fix... What I would choose instead is some form of level switch on the tank that would trip a "low water alarm" if the tank level dropped too much, and cause the system to go into "shutdown mode" (shut off air, turn off all circs, maybe close all zone valves, etc) and have a seperately autocontrolled emergency dump zone and / or cold water dump through boiler to drain to deal with any resulting boiler overheat - same net result, without making the flood worse...

Gooserider

 

[toddm]

The expansion tank we are talking about does NOT depend on thermosiphon action - it has nothing to do with thermosiphoning (indeed I would want to avoid as much thermosiphoning as possible!) and the only impact delta T has is on how much water expands into the tank… The function of the tank is to pressurize the system, especially the boiler and the pumps, which is PURELY a function of the delta height between the water level in the tank and the point of measurement

Actually, the Heatweb schematic I pointed out you, reproduced below, uses thermosiphon to generate pressure and thus relies on both height and the temperature difference between the water entering the boiler and the water leaving (delta T). You will see that heat dump HX is the high point of the thermosiphon loop, and that the only person assuming that it reached the expansion tank is you.

I have added a Stratford schematic that uses a similar injector tee to integrate gravity and pumped loops. The only height my Stratford manual mentions is a MAXIMUM height of 6 meters or 19.5 feet. The expansion tank simply has to be mounted above that. Stratford claims that the heat load in the gravity circuit is minimal with the pump running above 113 degrees. In Heatweb's version, the HX would produce even less waste heat.

 

[Gooserider]

Sorry - Thermosiphon can drive a FLOW, but will NEVER generate significant amounts of PRESSURE! Thermosiphon is completely independent of pressure - it will operate in a system at any pressure, you can even get thermosiphon loop action in the same pipe (see "ghost flow") Yes, it does produce a tiny amount, but not enough to be significant - definitely not enough to pressurise a system. Indeed it is because the amounts of pressure generated by thermosiphon action are so low that one has to be so careful designing gravity loops, and use such large piping in them - the flow is easy to stop, and isn't fast enough to transfer a lot of heat unless using a really big pipe...

If you are thinking that the thermosiphon action is producing system pressure, that error may be what is causing the confusion.

Note the following explanatory text accompanying that diagram -

The following diagram shows a suitable circuit which allows both pumped operation to a thermal store, and also has a gravity circuit that comes into play when the pump is not running

The "Termobac" valve has a check in it that prevents the thermosiphon loop from running at any time that the pump is on, as the pump pressure holds the valve closed.

In normal operation, heated water goes up the vertical pipe to the first tee, over and down through the heat exchanger, and then goes to the LEFT at the tee and into the "blender / heat bank (and presumably the unshown heat load) and back to the boiler via the pump and the main branch of the termobac. It only thermosiphons down through the leg of the Termobac if the pump is off, and the boiler is heating significantly.

How does a thermosiphon circuit that isn't even running generate pressure????

Since in normal operation, there should be no flow in the other side of the heat exchanger (unless it becomes a heat source and introduces an undesirable ghost flow in the mains line) there shouldn't be any more heat loss through the exchanger than there would be from a comparable length of plain pipe...

However, the expansion tank is ALWAYS applying system pressure, via that pipe out the bottom of the tank, through the "cold feed" valve and that J-trap (put there to prevent thermosiphoning into the expansion tank), and except as expansion might cause a change in the water level in the tank, that pressure will be constant, regardless of what temperature the boiler or any other part of the system is at, no matter what the temperature is (as long as it's not freezing or boiling) or whether the pump is on or off.... This is from the height of the water colum, and if you were to put gages at every point in the system, you would find the pressure would go up about 1/2 psi for every foot you went down the colum.

The pressure and flow stuff in these systems is a bit tricky to wrap your head around, as there are actually several different pressures and flows involved, and they interact in tricky ways...

Gooserider

 

[toddm]

See the pump in the Heatweb schematic? Follow the pipe left to this thing called a mixing valve, which would keep the thermosiphon circuit moving (better) with the pump operating. Ditto for the injector tee in the Stratford, which is directional but not gated. Therefore, height AND DELTA T.

Yes, the pressure generated is small. Hellooo. This an open system, with limitations on the hydronic side. In a footnote to the Heatweb section "The following data may help in calculating the forces generated through thermo-siphon (gravity) action," DPS concludes, "This force needs to balance out with the pressure losses in the pipework under circulation." Or you can move the hydronic circuit to a heat exchanger. Even in Britain, where most houses have gravity loops similar to the ones above, people are moving to pressurized. I am not claiming that an open system is better. (In my passive solar house, the boiler stove is meant for intermittent duty as a glorified masonry heater.)

That doesn't mean that the Brits don't do it well. They address two other problems with the gravity loop. There is relatively little water involved, so the expansion tank is quite manageable. By using thermostats and mixing valves on pumped systems, they assure a minimum return temperature and eliminate the worst case corrosion of hot boiler and cold water.

Yes, the higher you put the expansion tank the better it works. You say 30 feet. I say BS. I have a million or so homes in Britain to make my point. What do you have?

 

[Gooserider]

See the pump in the Heatweb schematic? Follow the pipe left to this thing called a mixing valve, which would keep the thermosiphon circuit moving (better) with the pump operating. Ditto for the injector tee in the Stratford, which is directional but not gated. Therefore, height AND DELTA T.

Note the Heatweb site that says the Thermosiphon (getting tired of typing that, call it TS) loop is ONLY active when the pump is off...

The Termobac is a non-return valve with a swing gate that provides a route for gravity circulation. It is used to connect to gravity circuits that should only come into play when required.

If the Termobac wasn't there to bypass the pump loop, the TS flow through that loop would be NEGLIGIBLE, if there was any flow at all... Quite aside from the extra elevation changes (enough to block the flow right there) a non-running pump poses a significant restriction by itself. Many modern systems would put the pump on the output side of the boiler (See "Pumping Away" by Dan Holohan of Heatinghelp.com) but a heatweb style setup couldn't use that desired location as a dead pump would block the desired TS loop from functioning as an emergency heat dump...

Yes, the pressure generated is small. Hellooo. This an open system, with limitations on the hydronic side. In a footnote to the Heatweb section "The following data may help in calculating the forces generated through thermo-siphon (gravity) action," DPS concludes, "This force needs to balance out with the pressure losses in the pipework under circulation." Or you can move the hydronic circuit to a heat exchanger. Even in Britain, where most houses have gravity loops similar to the ones above, people are moving to pressurized. I am not claiming that an open system is better. (In my passive solar house, the boiler stove is meant for intermittent duty as a glorified masonry heater.)

An expansion tank can pressurise a system to levels similar to that of a closed system IF the water colum is HIGH ENOUGH!

Your comment on the DPS conclusion "This force needs to balance out with the pressure losses in the pipework under circulation." is a reference to the "flow pressure" (about the same thing as a pump's "head pressure" as a which has NOTHING to do with the SYSTEM pressure!) generated by the TS loop's heat source. If the piping in the TS loop has more head resistance losses than the "flow pressure" you will get NO FLOW in the TS loop. The amount of flow is going to be determined by the difference between the nominal flow pressure produced by the heat source, and the head loss of the pipes. Graphically it would look just like a "pump curve" except the numbers would be MUCH smaller - this is why TS loops usually use large diameter piping, they need to get the head losses down in order to get acceptable flows...

[/quote] That doesn't mean that the Brits don't do it well. They address two other problems with the gravity loop. There is relatively little water involved, so the expansion tank is quite manageable. By using thermostats and mixing valves on pumped systems, they assure a minimum return temperature and eliminate the worst case corrosion of hot boiler and cold water.[/quote]
1. The expansion tank sizing is based on the water volume of the ENTIRE system, not just that of the TS loop. If you are using the heat bank (thermal storage) tank as part of the system water, you MUST include it's volume when figuring the minimum size for the expansion tank (note that if the entire system cooled to near freezing, the properly sized expansion tank would be mostly empty, while it would be mostly full when the system is at maximum temperature. (Unless the toilet valve adds water to it when cold, in which case it will overflow when the system heats up(another reason NOT to use the toilet valve!)) If the storage tank is open, and heated by an exchanger (which it would have to be if it is located lower than other system components) THEN the amount of water involved would be just what it took to fill the boiler and the pipes that make up the system - probably far less... (Expansion tank sizing needs to be the same as the acceptance capacity of the pressure tank in a closed system - from other threads here, when using pressurised storage, this needs to be on the order of close to 100 gallons, while w/ open storage it's usually on the order of under 10 gallons)

2. The use of mixing valves and thermostats is characteristic of any system that is properly designed - pressured or open. It certainly isn't anything exclusive to the Brits.

3. Except that a TS loop pretty much can't have a mixing valve in it - the head resistance of most mixing valves is higher than the force generated by thermosiphon action, especially when using the low temperature differentials that a mixing valve gives. OTOH, note that when the Heatweb TS loop DOES come into play, you are BYPASSING all the mixing valves, etc. and could well (depending on how efficient the heat exchanger is, and an efficient heat exchange would make the TS loop work better by a bigger Delta T) be feeding water back to the boiler that is far below the normally required minimum - Not a big problem since this should be a very rare event, that is an "emergency" when it happens....

[/quote] Yes, the higher you put the expansion tank the better it works. You say 30 feet. I say BS. I have a million or so homes in Britain to make my point. What do you have?[/quote] I have a bunch of guys (many of them long dead) that wrote this bunch of laws called "physics" - which no legislative body, dictator, democratic election, or business sales force has EVER managed to repeal... I am not saying that you "MUST" have 30 feet of height; just that if you want your system to be working at about 15psi in the basement, you need to have a 30' high colum of water - If you are happy with less pressure than that, then you don't need as much height. Bottom line is the pressure at the lowest point on the system will be approximately 1/2psi per foot of height on the expansion tank - no more, no less...

Gooserider