Stretching out storage

[chuck172]

I'm trying to make the most out of my 500 gallons of storage. Seems like the best way is to lower the effective temperature of the heat emitters. In my case, adding more fintube baseboard, and in the future adding some radiator panels.
I have two zones out of 5 that I do have sufficient fintube footage to bring my design temp. down to 150*.
I just added an additional 30% more baseboard to my living/dining/kitchen zone, and the master bedroom has enough.
Now the question, should I look into adding some sort of tempering valves that would lower the load temperature or just run the storage down to the minimum temperature (now 150*) and then re-charge the tank (fire up the boiler)?

 

[in hot water]

150 °F at design conditions? Design condition being the coldest expected outdoor temperature. Is so on mild days 130 °F maybe less could cover the load. The very best method is an outdoor reset controlled mixer. So on design days it gets the full 150 °F or more if needed. Any day less than design the baseboard gets lower temperature.

Properly adjusted you get constant circulation, the control matches the supply temperature to the actual building load. Like a cruise control on a car. This way you maximize the storage and reduce or eliminate the on off cycling of operation. Taco builds a mixing block that works great for that application.

hr

 

[ewdudley]

I just added an additional 30% more baseboard to my living/dining/kitchen zone, and the master bedroom has enough.
Now the question, should I look into adding some sort of tempering valves that would lower the load temperature or just run the storage down to the minimum temperature (now 150*) and then re-charge the tank (fire up the boiler)?

If your radiators are safe at high temperatures I would think you would want to take full advantage of the hot water whenever it is available. The additional radiators allow you to use lower temperature water when it comes to that, but purposely adding entropy at the beginning of the draw-down cycle doesn't buy you anything except more even heating, which might be desirable depending on the circumstances.

--ewd

 

[chuck172]

Entropy- a measure of the uncertainty associated with a random variable

New word to me. Really adds to the confusion!

 

[stee6043]

In my simple mind it seems the tempering valve would certainly help you stretch our storage (which is what your topic is). If you have 190+ in your tanks and you're tempering your zones to 150 you'll be sipping less flow off that tank. As such, it should last longer in my simple mind.

FWIW....methinks entropy was prolly not the best word choice above...in some cases it can be used to estimate lost energy in a system...but I've never heard any HVAC guy use the word! ha....it's definitely a $5 word at least.

 

[chuck172]

The taco mixing block does everything but split wood. At $1300.00 it ought to split wood too.
How about a manual 1 1/4" four way mixing valve.
HONEYWELL V5442N1031 1 1/4 inch 4-way mixing valve with 18.7 Cv
[V5442N1031] $54.88
Would this work?
http://dticorp.com/catalog/honeywel...ing-valve-with-p-4941.html?osCsid=992c783b42b

 

[in hot water]

yes a 4 way would work. Without a motor or electronics it's a dumb control. May as well stack a bunch of ball valves.

Here is a better price on a Taco. Or you could buy the components on e-bay and build your own.

hr

 

[Nofossil]

One major benefit to outdoor reset in this case is that you're returning much cooler water to the storage tank, maintaining stratification much better.

 

[ewdudley]

In my simple mind it seems the tempering valve would certainly help you stretch our storage (which is what your topic is). If you have 190+ in your tanks and you're tempering your zones to 150 you'll be sipping less flow off that tank. As such, it should last longer ...

We have two envelopes, one inside the other. One envelope is the house, the other is the one containing the tank. The tank can offer us a flow of energy into the house because it is at a higher temperature than the house. Likewise energy will flow out of the house because it is warmer than its environment.

If we keep the temperature of the house constant by whatever means, the rate of energy loss will necessarily be constant. In order to maintain the house at a constant temperature we must supply energy to it at the same rate that the house is losing its heat.

Two strategies are suggested.

One is the storage stretching solution where, taken to it logical conclusion, we run the circ 100% of the time and inject water from the tank at a rate exactly sufficient to keep the radiators just warm enough to keep up with the heat loss.

The other is to get the radiators as hot as possible without overshooting too much, then turn off the circ and wait for them cool down, but not too much.

Either way the temperature of the house remains constant, within tolerable limits. Either way all the extractable heat is gone from the tank. Either way the same amount of energy escapes the house into the environment at the same rate. Therefore either way the heat in the tank lasts the same amount of time.

The only extra energy we get in either scenario would be the extra heat generated by the circ pump running 100% of the time. If the cost of the electricity to run the pump 100% of the time is worth it in order to have more comfort due to more even heating, then the tempering strategy would have merit in that aspect.

But in either case the energy in the tank lasts the same amount of time because the same amount of energy has been extracted at the same average rate.

(Although it would be theoretically possible in the case of intermittent extraction to return water to storage at a lower temperature and thereby get better stratification, which means the tank would last longer because it would be colder at the end, but only if entropy or the lack thereof matters.)

Cheers --ewd

 

[chuck172]

So by installing a 4way tempering valve the only benefit would be better stratification by returning cooler water to the storage tank. The only negative would be longer circulator pump running time.
Why then is there an increase in heating efficiency when an outdoor reset controlled mixer is installed to a system?

 

[ewdudley]

So by installing a 4way tempering valve the only benefit would be better stratification by returning cooler water to the storage tank.

The only negative would be longer circulator pump running time.

No, by not having the mixer it would theoretically possible to get some amount of additional stratification if you had a system to take advantage of it. If we're goosing the radiators with hot water they'll overshoot by some amount and so we wait for them to cool down. After they've cooled down they will have undershot by some amount. If you had a system that could take that somewhat colder water and then could keep it segregated from the remaining hot water the average temperature of the tank would be lower at the end, and so the cycle could be somewhat longer, thereby 'stretching out storage', theoretically, but not by much I suspect.

So to the point of the question, will using a mixing valve stretch out storage, the answer is no it will not, and in point of fact it may well shorten somewhat the time period before re-firing is needed.

Nevertheless there may well be advantages to using a mixer and running the circ more, it's just that stretching storage is not an cannot be one of them.

Why then is there an increase in heating efficiency when an outdoor reset controlled mixer is installed to a system?

In systems where we would rather have the radiators as warm as necessary but no warmer for comfort reasons, then an outdoor reset controlled mixer is what we want, but this will not increase heating efficiency.

The way to increase heating efficiency by use of outdoor reset control is to adjust the boiler temperature to be as hot as necessary, but no hotter, in order to minimize flue losses. But wood gasification boilers have fairly high minimum supply temperature, plus the lower the supply temperature, the lower the storage tank starting temperature, and the shorter the time before the next re-fire, so we're kind of in a bind there.

But by all means outdoor reset control of boiler supply temperature in wood-fired systems should be used to whatever extent practicable, even if only by manual adjustment of the supply temperature day by day or week by week as the demands of the season wax and wane, since after all the motivation for civilization in the first place is to provide a venue for drinking beer and fiddling with fire.

The full glory of outdoor reset control might best be realized in systems with multiple natural gas condensing boilers that run constantly at a supply temperature just high enough to satisfy demand.

--ewd

 

[chuck172]

Thanks for your great replies ewdudley.
Besides the obvious of tightening the envelope (insulation, cold infiltration etc.). Is there, in your opinion, any way to "stretch" storage?

 

[ewdudley]

Thanks for your great replies ewdudley.
Besides the obvious of tightening the envelope (insulation, cold infiltration etc.). Is there, in your opinion, any way to "stretch" storage?

The amount of heat is mass times delta-T and how long the heat lasts is dictated by the demand, which can only be affected by insulation, programmed setpoint control, spouse selection, and so forth. Mass is constant as established by design, so delta-T is what we've got to work with.

So max out supply temperature at the end of the burn and have sufficient boiler circ rates to get the whole tank to as high a temperature as possible.

And then we want to minimize the average temperature of the tank at the end of the burn cycle.

What I've seen in some of the more extravagant European systems is where they take great pains to segregate the supplies and the returns in vertical tanks. The hottest water is drawn only for loads that require the hottest water and so forth down the line, or alternatively, return water from a hi-temp load is used to supply a low-temp load.

Likewise the return circuits are injected into the tank at a point no higher than optimum.

So in the end, if the whole affair is perfectly orchestrated, they've consumed the last drop of each temperature at the same time, and tank is perfectly stratified accordingly. Good luck. But nonetheless anything you can accomplish economically towards this goal should be taken advantage of.

If you've got plenty of radiation surface so that you can use low supply temperatures, a heat pump to draw heat out of the tank could double the storage capacity economically in some circumstances.

--ewd

 

[Nofossil]

Let me expand on stratification a bit. The 'two envelope' model is a great way of thinking about it, but it misses one critical point: the game is over when the top of storage is too cool to allow the baseboards (or whatever) to transfer enough heat to keep the house at the desired temperature. This varies with outside temperature. At 30 degrees, I can keep the house warm enough with the top of storage at 120. When it's -10 outside, then my baseboards need 160 to keep up.

Now let's look at two scenarios. In both cases we start with a tank that's a uniform 170 degrees. In both scenarios, we're at an outside temp where game is over when we can no longer provide 140 degree water to the baseboards.

Scenario 1: Running unmixed water from storage through baseboards and back. Water temp drops by 20 degrees going through the baseboards. The return water is initially at 170. This will create a thermocline with 170 degree water above and 150 degree water below. When the thermocline reaches the top, the temp to the baseboard drops to 150 and return to 130. You end up with a tank that's at a uniform 130 degrees. In actuality, there will likely be enough mixing so that the tank will be closer to a uniform 140 degrees.

Scenario 2: Mix the tank water with the baseboard return to provide 140 degree water to the baseboards. The return to the tank in this case is 120 degrees, and the return velocity is relatively low. In this scenario, when the thermocline reaches the top of the tank, we'll have a tank that's a uniform 120 degrees.

This has the effect of increasing thermal storage without changing the size of the tank OR the water temp needed to heat the house.

 

[chuck172]

So does scenario 2 utilize a 4way valve to provide 140* to the baseboards and returns cooler (120*) water to the bottom of storage?

"This has the effect of increasing thermal storage without changing the size of the tank OR the water temp needed to heat the house."

 

[Nofossil]

So does scenario 2 utilize a 4way valve to provide 140* to the baseboards and returns cooler (120*) water to the bottom of storage?

"This has the effect of increasing thermal storage without changing the size of the tank OR the water temp needed to heat the house."

Yes - that's the idea.

 

[DaveBP]

Either way all the extractable heat is gone from the tank.

I think chuck's central question in this thread and one that has been discussed regularly here is how to redefine that term "extractable". All the theoretical thermodynamics can help point to understanding what's going on and man can it kill a whole lot of beer hanging around with grad students but I think we're talking hardware here.

Fin tube baseboard has a very small surface area compared to other emitters used in residential HVAC. To get the heat exchange rate needed for the current weather conditions ('rate' is the important element here) you need to stuff higher temperature water through it than an emitter with a larger surface area (radiant floor or ceiling would be extreme counterexample). Heat exchange is directly related to temperature difference AND surface area. More of one needs less of the other to get the job done.
So, since were talking wood boilers with heat storage tanks, all those BTUs in the tank are not necessarily "extractable" if your emitters can't get the heat out of the water fast enough to keep up with the rate of heat loss of the house. You might just as well not have bothered to burn the wood it took to get them in the tank. Yes, that heat is there and it could be used and it would be 'heating' the house if you kept circulating it (until it was the same temperature as outdoors, at which point we will have achieved maximum entropy) but it would not keep up with the rate of heat loss and it would quickly be felt as something more like cooling than heating in the house.

Lot of people contributing to this forum are in the same boat as you, chuck. Your house was not designed to run off a heat storage tank. Your baseboards were designed for 180F bang-bang hot water supply on, off, on, off.... As long as the wood boiler is ripping it can go forever and keep up the the water temperature the baseboards are clamoring for. But at some point the fire will burn out and so will you and the heat storage tank takes over so you can get some sleep.

Some guys might pick up this line of design thought and work toward getting the lowest tank temp/longest run time between firings. Some guys will race whatever you hand them. And with NASA's budget you could get a WHOLE bunch of heat out of there. But I think the question that is on everyone's mind is how to get not the most but the minimum required sleep (or hours away at work) between firings with simple hardware.

Temperature mixing to match outdoor conditions probably won't help with the baseboards the way they were. I agree with Eliot.
But adding more fin tube to those you have increases their surface area and that will lower the temperature of the water that can usefully keep up with the heat loss of the house. Then you can get more hours/ tank. How far you can go with this will be up to the interior decorator in your family. But it's going to help.

And equally important, I think, is Nofossil's point that the lower the water temperature you send out to those baseboards the lower the temp of the water coming back into the tank. This is where temp mixing might help by using (and therefore returning) the lowest temp water needed. Anything you can do to preserve the stratification and maximize the temp difference in the tank will help you get more "extractable" heat out of it, regardless how many total BTUs there are in there.

And now, some hours after I wrote this, I see Nofossil has given a better example, as usual, of the stratification argument. I'll submit this anyway. Don't want to waste my two cents.

 

[Gooserider]

Another possibly minor point in favor of mixing down as opposed to the periodic overshooting approach...

The baseboard is usually on an outside wall, and inherently is going to dump a certain amount of it's heat straight to the outside world no matter how good the insulation is... Since heat transfer is a function of ΔT, the cooler you keep the baseboard (and the house as a whole for that matter) the lower the heat loss to the outdoors will be...

Gooserider

 

[chuck172]

Seems like we're forming some basic guidelines of improving storage efficiency with woodburning boilers.
Burn good, dry wood.
Install turbulators
tighten up the house envelope (insulate, seal)
Enable the use of low temp emitters (add fintube, panel radiators etc) to lower system supply temperatures which in turn will keep return temperature cool (to improve tank stratification)