Pressurized system

[wardk]

I have posted that we have a garn on the way, decided to call a hydronics consultant to help with the distribution design. I gave him a description of system needs and he said the first thing to do was install a large wtwx and pressurize the output system . Is this an efficient way to operate a garn? He will be here next week for a close look,any good questions I should be asking would be helpful .

 

[jebatty]

I gave him a description of system needs and he said the first thing to do was install a large wtwx and pressurize the output system . Is this an efficient way to operate a garn?

Without knowing what your told the consultant, it is not possible to answer your question with reference to your system. Short answer is that the Garn operates as an open system and if your system needs require a pressurized system, then the wtwx likely is needed. A wtwx can be very efficient and also can be very inefficient. Careful design is required. Much will depend on the temperature of the hot water required in your system, the flow rate of that water, the temperature drop between system supply and return, btuh heat load, friction loss (pump head) in the system and in the wtwx, size of piping required, proper sizing of the circulator(s) and proper sizing of the wtwx.

 

[Eric Johnson]

It's designed to be an open system, so I don't know why you'd pressurize it. Talk to a Garn dealer or rep, and they recommend de-pressurizing existing pressurized systems.

As Jim says, heat exchangers can be hit or miss. They can be very sluggish and inefficient if not sized and installed correctly, IME. And, they're not cheap.

Personally, I go pressurized all the way, mostly because that's the only kind I've ever had and it's what I'm comfortable with. But there are certainly compelling reasons to go non-pressurized, with the Garn being the prime example. It works, why mess with it? My guess is that it would be easier and cheaper to depressurize your existing delivery system than installing an hx.

 

[jebatty]

What I think I know about open systems of the Garn-type is that the system being served by the Garn cannot be at an elevation higher than the Garn. If the Garn is located at the high point in the system, then normally using the Garn-system as an open system would be OK. But if the Garn is not the highest point in the system, then water will rise in the system only to the elevation of the Garn, and higher points will not be served. That often is why systems are pressuriized. Each 1 psi of system pressure raises water about 2.3 feet, i.e., a 12 psi system will raise water to about 28 feet, usually sufficient for a 2 story house.

There are pressurized systems with open expansion, in the attic for example, and I suppose these are pressurized-open systems rather than pressurized-closed systems.

There are other issues with both open and pressurized systems related to flow rate, NPSH and cavitation issues. This is a whole additional area that usually does not pose a problem in most home installations when using common components, but does become an issue that must be addressed in high flow applications.

 

[wardk]

What I think I know about open systems of the Garn-type is that the system being served by the Garn cannot be at an elevation higher than the Garn. If the Garn is located at the high point in the system, then normally using the Garn-system as an open system would be OK. But if the Garn is not the highest point in the system, then water will rise in the system only to the elevation of the Garn, and higher points will not be served. That often is why systems are pressuriized. Each 1 psi of system pressure raises water about 2.3 feet, i.e., a 12 psi system will raise water to about 28 feet, usually sufficient for a 2 story house.

There are pressurized systems with open expansion, in the attic for example, and I suppose these are pressurized-open systems rather than pressurized-closed systems.

There are other issues with both open and pressurized systems related to flow rate, NPSH and cavitation issues. This is a whole additional area that usually does not pose a problem in most home installations when using common components, but does become an issue that must be addressed in high flow applications.

I think you have the reason for pressurizing, the house heating in the basement below the boiler but the shop will be above. I did a heat calc on the house about 89,000 not including dhw, not sure on the shop 1800sq well built but the two garage doors are a little suspect. Doesn't need to be very warm,

 

[Eric Johnson]

I think there's a way to do it even if the boiler is lower than the highest point in the system, but your system has to have no leaks. Once you fill the radiators or baseboards (or whatever) and close the air vent, the water level won't drop and it will circulate. That's what I've been told (by a Garn dealer) and it worked the one time I've tried it--putting a cast iron radiator on the second floor of a house being heated with an OWB. I'm not exactly sure why it worked, but it did.

 

[jebatty]

I think there's a way to do it even if the boiler is lower than the highest point in the system ...

It works based on creation of a vacuum, and in the same way that if you fill a tube with water by sinking it in a bucket, holding the tube vertical; then plug the top end, lift the tube, and the water stays in the tube. Not only can there be no leaks, but also any air in the system will seek high points and collect. Careful venting may be needed from time to time. It seems that such a system would have to be filled from all top points down, or filled with a pump from the bottom, with the "drain" end(s) plugged. As water is added, air will rise to the top and needs to be expelled with a vent valve. As each high point segment is filled, the vent valve would be closed.

I'm not sure I would want to do this in my heating system, but maybe it works just fine. A pressurized system is so easy and avoids the air problem and makes it easy to add make-up water, work on the system, and auto-vent. I haven't heard of anyone using such a system in a two story house with the boiler at a low level. If anyone has done this, it would be good to share the experience, issues, solving problems, etc.

To the best of my knowledge, Garn never brought up this possibility in the one Garn application with which I am experienced, and I believe Garn spec'd the plate wtwx in this system. Maybe there are certain applications where this is feasible and others where pressurization is the best practice.

 

[Eric Johnson]

It works based on creation of a vacuum, and in the same way that if you fill a tube with water by sinking it in a bucket, holding the tube vertical; then plug the top end, lift the tube, and the water stays in the tube.

Or like putting your finger over the end of a straw when it's full of liquid.

I did it at my dad's house. He said it would work and I really didn't believe it, but we installed the radiator in an upstairs bathroom and it worked just fine. I think we filled it by connecting a garden hose to somewhere on the system and forced the water up into the radiator that way.

I'm not saying I recommend it, because it requires constant monitoring and it doesn't take much for the water to drain out of the high points on the system, but it can be done and if done right, would probably work well.

Along those lines, I installed an old steam-only cast iron radiator in my hydronic system. It's a "steam-only" radiator because there's no way to vent the columns when you fill it, i.e., no top pipe. Being too cheap to scrap the radiator after I bought it (not realizing the potential filling problem), I figured out how to valve it so that I could turn the radiator over, fill it, then connect it to the system full of water. That was about 8 years ago, and it still works like a champ. No heating professional would do this, of course, but since it's my house and my heating system, I figured what the heck. I should add that it doesn't accumulate air because it's downstream from a vent-able cast iron rad, which catches all the air.

Here it is. This is a very old radiator. Later, all rads had top pipes so that they could be used in both hydronic and steam systems.

 

[BoilerMan]

The thing about non pressureized system and elivated radiators is the potential for boiling in the radiator due to the vaccuum. As we with pressureized systems know water boils at 212 at ATMOSPHERIC pressure (sea level) and around 250 in or pressureized 30psi systems which is referred to as superheated water (hotter than boiling due to higher than atmospheric pressure). The opposite is true in a system with less than atmospheric pressure, the boiling point drops. So say we have an unpressureized system working at 200 with a radiator 20' above it on the second floor. The 200 degree water is just fine in the boiler, but as it travels up to the second floor it starts to boil due to the vaccuum, and when it boils it makes steam which collects in ..... you guessed it in the radiator making flow stop due to the circulator not having enough head to overcome the gas in the top of the system. Making a banging and water hammer racket up there. Enough said.

Not to mention all the extra cossosion inhibitors one would need to treat not only the vessle (GARN) but also the entire system water. If a wtwhx is used it's only the GARN that has to be treated and we avoid all the other issues.

my .03 on open systems
TS

Eric, I like your use of the steam radiator. I've thought of doing the same thing myself, filling it upside down, as no one ever wants the sheet metal steam ones, just the cast HW ones which were also used for steam in the later years.

 

[wardk]

Okay we pressurize. Do I use two wtwx one for the house the other for the shop each sized for heat load or one big one for both?

 

[hobbyheater]

The thing about non pressureized system and elivated radiators is the potential for boiling in the radiator due to the vaccuum. As we with pressureized systems know water boils at 212 at ATMOSPHERIC pressure (sea level) and around 250 in or pressureized 30psi systems which is referred to as superheated water .

One downside that likely is not considered with a large pressured system. Should the scenario of a total consuming house fire occur and where a 1000 gallon storage tank is involved with 250 F temperature at 30 psi, the expansion factor is approaching 300,000 to 1. The smallest pinhole leak and the explosion will be large!
The Garn has the best of both worlds, large unpressurized storage and a heat exchanger for pressured loops!

 

[Eric Johnson]

Eric, I like your use of the steam radiator. I've thought of doing the same thing myself, filling it upside down, as no one ever wants the sheet metal steam ones, just the cast HW ones which were also used for steam in the later years.

Actually, this one is cast iron. They're not all that uncommon around here--especially since most people don't have steam systems anymore.

 

[StihlHead]

What I think I know about open systems of the Garn-type is that the system being served by the Garn cannot be at an elevation higher than the Garn. If the Garn is located at the high point in the system, then normally using the Garn-system as an open system would be OK. But if the Garn is not the highest point in the system, then water will rise in the system only to the elevation of the Garn, and higher points will not be served. That often is why systems are pressuriized. Each 1 psi of system pressure raises water about 2.3 feet, i.e., a 12 psi system will raise water to about 28 feet, usually sufficient for a 2 story house.

There are pressurized systems with open expansion, in the attic for example, and I suppose these are pressurized-open systems rather than pressurized-closed systems.

There are other issues with both open and pressurized systems related to flow rate, NPSH and cavitation issues. This is a whole additional area that usually does not pose a problem in most home installations when using common components, but does become an issue that must be addressed in high flow applications.

Uh.... in my experience this is simply not correct. Open systems can and do service heat exchangers elevated above or located below the systems. I have designed and installed several in both configurations. Actually some head in the open system will help pressurise the Taco pump feeding the system and cut down on cavitation, which is the only real advantage to a pressure system that I can think of (avoiding cavitation). I am an open system fan and I would only have an open system, as they are far safer. Yes, I have had to argue as to why I bleive this many many times. It is circular, like cat vs non-cat stoves, Chevy vs Fords, etc.

At any rate, a Garn, Cantral Boiler, or any other open system that is above or below the Hx's will work just fine. In my designs, I actually run the supply line (hot) fairly high to get good head to back-pressure the Taco (to help avoid pump cavitation). Usually a second story is about the right elevation for this. Otherwise in all the designs, head height has not even been a factor and they are all still in operation (no Taco pump failures yet). Of course these systems are driving flat plate Hx, to heat DHW and radiant floor loop systems, and not old style steam radiators... if you are driving a system that was previously pressurised, you may have issues with that.

 

[Bob Rohr]

The temperature at which water boils is related to the pressure on the water. At sea level 0 psi gauge pressure (14.7 absolute pressure) water boils at 212F

Either the pump needs to keep running to add the pressure to the system, or the temperature of the fluid reduce to stay below the boiling temperature at sub atmospheric conditions.

So limit the piping height or lower the water temperature to prevent flashing. With either condition met, you will not have steam flash in an open system

Also pump like to have some positive pressure to prevent boiling (cavitation) at the impeller. Same principle here, as the water temperature increases, more pressure is required to prevent the boiling. Pump specs often show that pressure for 140F and 205F as an example.
Water will boil at 15 feet above the water level at 190F. I suspect OWF do not often supply 190F to the upper part of the piping considering them piping loss in the ground etc, even if you run them up to 200F at the stove.

 

[jebatty]

StihlHead -- I understand your point. When the circulator shuts down, in the Garn open system won't the system drain back to the Garn, and doesn't there need to be sufficient capacity in the Garn to accept this water, else overflow? I suppose that may not be a problem with the Garn, but I don't know how much top tank space the Garn has to accommodate drainback before overflow.

This may be less of an issue or no issue with an open system that does not have overflow at a low point in the system, which I think is the pressurized-open system I mentioned in my first post in this thread.