Storage

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dpsfireman

Member
Feb 26, 2011
39
Northern Pa
Been lurking here more than a year. The information on this site helped me decide to buy a Biomass 25 last winter when the electric utility decided to eliminate the Residential Thermal Storage rate that I enjoyed for many years. Thanks to all for the help!

When I went with the RTS system many years ago I decided to go with a pressurized water storage system for several reasons. The more common ceramic systems would sometimes overheat a house with clear weather due to the cold nights and daytime solar gain. Also with a closed system there should be fewer problems with corrosion than with an open system. Another reason was my thought that if and when the sweet deal electric rate went away, I would have in place a distribution system and I could just heat the water a different way. That day has come.

I had always supplemented my heating with wood but I wanted to use the wood more efficiently and the Biomass certainly delivers on that one. So, I piped the boiler into the existing storage tanks and have been tweaking ever since to the point now that it works quite well. A few more things are in order but by and large the system works very well.

Which brings me to my point. Over and over again I read about adding more storage by increasing the volume of water. There is another way that I believe should get more attention and that is the temperature differential in the storage system. My RTS system was designed to store enough energy to heat the house for 10 hours during the day with the lowest temperatures we get here, I have the paperwork for the heat loss calculations etc but I believe that temperature was -20 degrees F.

My system was designed to heat the upstairs only I believe (1000 sq ft) but I had also added basement rooms so that the total was 1500 sq ft. Even after adding this, we had a very cold day onetime that was theoretically below what the system was designed for. The electric utility called us on that day to let us know that we could turn on the override function and they would not bill us for that but we never did it because we didn't need to.

The storage system consists of 2 112 gallon well insulated tanks that resemble water heaters. The way this system is able to do so much with so little is by the temperature differential it uses. The baseboard radiators are sized to be able to heat the house with a water temperature of 120 degrees and the max temperature for the tanks is 235 degrees. The temperature of the water flowing into the radiators is moderated by a thermostatic mixing valve.

My question is, why don't the makers of these systems design them to run up to a higher water temperature? I ran the system last winter as it was but the max temp of 195 degrees was OK but not enough. I was not getting anywhere near the BTU capacity my system was designed for. I contacted Zenon and he told be I could increase the max temp by putting a resistor in parallel with the temperature sensor so I did that, carefully selecting a resistor value that raised the temperature 10 degrees when near the maximum temperature. This helped quite a bit but more would be better.

In the winter I can burn the system once a day and get some idling at the end of a burn but by the time it gets to that point most of the wood gasses are gone so I don't think it's a big deal. During the milder weather I can even skip days!

So back to my question, why is there not more emphasis on the temperature differential?
 
Hmmm, a few initial thoughts. Obviously for open storage, boiling the water is a problem. But for closed storage, why not increase the temp to some higher level? Presumably if the tank is rated for this it should be OK. Would there be problems circulating this super hot water? Would their be flash steaming occuring? Seems like there might be around the impeller...no? The whole net positive pressure thing... And how about pumps circulating water at this temp just due to the temp, are they OK at 235? Apparently you have been successful for a long time with it! Obviously any type of pressurization failure would be somewhat catastrophic...with water flashing to steam I'd think.

Another issue might be the inherent increased loss at higher temperatures. Higher temperature differential, higher loss. But clearly, with higher delta T, you have more stored energy. If you have only 250 gallons of water, that's about 2000 BTU/degree, or with 100 degree differential, 200,000 BTUs. Nice that you can use water temps down to 120 with baseboard....that's good design. I'd think that would handle the heating load of 1500 sq ft well insulated around these parts for 12 hours quite easily.

What would happen if you sprung a leak, or if your pressure relief were to open? How else could the water flash to steam inside the system, increase the pressure tremendously, and make you REALLY happy you had a good ASME cert on the tank? That would have to be my guess for the reason temps aren't taken that high. I know the Navy has very hot water around their reactor cores for cooling....I also know they spend millions every year monitoring everything to do with those lines and that system....
 
I few things on the max temp. I checked the max temp on circulator in the loop between the boiler and the tanks and it is 230 degrees. The circulator for the base boards doesn't see water any hotter than 140 or 150 because of the regulator. There is another consideration and that is the expansion tank has to be appropriately sized. One thing to keep in mind, the electric system would only reach that temperature if it was very cold outside. There is an outdoor temperature sensor that is used to determine how much to charge the system.

I do not intend to run the system any hotter than it is now, around 205 to 210 depending on which gauge I look at. The point I am trying to make is that there is another way to get more storage than by having a bigger tank. Depending on individual circumstance, to increase storage capacity it may be easier and cheaper to add more baseboard radiators and/or heating the floor so that the water temp can be lower and still heat the house.

Think of this example, if a system is operated with a max temp of 180 and a min of 140 that gives a delta T of only 40 degrees. If that range can be extended a bit on both ends, say to a max of 195 and a min of 130, the storage capacity is increased to 1.6 times its previous capacity which is very substantial. IMHO that is worth looking at.
 
I think instead of going dangerously hot, add more heat emitters so you can use lower temp water. That would be a safer alternative. I can use down to 90 f water in my floor.
 
I've been thinking of retrofitting my bathroom with in floor heating, maybe I should take a serious look at doing the whole house while leaving the baseboards in. Woodmaster, did you ever do a calculation to see how cold it could be outside and still be able to heat the house with 90 degree water?
 
dps, you are running your boiler at temperatures (and probably pressures) that are dangerous. Wood boilers have a tendency to creep up in temperature when in idle and if your OPERATING limit is set at 205 or more, it is VERY likely that you will trip your pressure relief valve. This is not the end of the world, of course, merely dramatic, BUT, if you were to have some kind of unintentional, uncontrolled breach in your boiler or tank system, the results could be very serious. Boiler systems are designed around 180 degree operating temps and 30psi relief valves for good reasons. Please consider adding additional storage volume and/or replacing or adding panel rads to your system so that you can take advantage of even lower tank temperatures instead of focussing on higher operating temps.
 
I am not without redundant safety systems in place. The controller for the system includes the provision to dump excess heat into all zones and the trip point for that is set just a bit higher than the temp where the boiler idles. I have only seen this occur maybe once or twice in almost 2 seasons.

As far as pressure goes, my electric storage system (manufactured by Axeman Anderson at http://www.axeman-anderson.com/) was designed with an expansion tank large enough to handle expansion caused 235 degree water temperature. Although the volume of water was increased a bit by the addition of the boiler, I do not run it to 235 degrees any more so the pressure typically maxes out in the low 20's, about the same as it did before the addition of the boiler due to the lower max temperature. I have NEVER had a pressure relief valve open in more than 15 years of operation of the system with and without the boiler and there are are now 3 of them including the one on the boiler.

Furthermore, all valves in the system are Automag valves that open in the event of a power failure. The loop between the boiler and the tanks will convect quite well due to the piping layout which is just about optimal for this to occur. The zones to the radiators do not have flow valves but instead I used heat traps to stop unwanted convection. If the zone valves are open and the circulator is off it will still convect taking a few minutes to get started.
 
Baseboard like hot water so being able to heat your house with 120F water suggests a lot of spare capacity. You probably have 3x the amount of baseboard that you would need if you ran it at the 'normal' temperature. The cost of that extra baseboard could have been spent on storage.

The best combination is low temperature (radiant) which means that you do not have to run your storage so high with the issues that come.
 
dpsfireman said:
I am not without redundant safety systems in place. The controller for the system includes the provision to dump excess heat into all zones and the trip point for that is set just a bit higher than the temp where the boiler idles. I have only seen this occur maybe once or twice in almost 2 seasons.

As far as pressure goes, my electric storage system (manufactured by Axeman Anderson at http://www.axeman-anderson.com/) was designed with an expansion tank large enough to handle expansion caused 235 degree water temperature. Although the volume of water was increased a bit by the addition of the boiler, I do not run it to 235 degrees any more so the pressure typically maxes out in the low 20's, about the same as it did before the addition of the boiler due to the lower max temperature. I have NEVER had a pressure relief valve open in more than 15 years of operation of the system with and without the boiler and there are are now 3 of them including the one on the boiler.

Furthermore, all valves in the system are Automag valves that open in the event of a power failure. The loop between the boiler and the tanks will convect quite well due to the piping layout which is just about optimal for this to occur. The zones to the radiators do not have flow valves but instead I used heat traps to stop unwanted convection. If the zone valves are open and the circulator is off it will still convect taking a few minutes to get started.

It certainly sounds as if you have a well designed system and are being very safe with it. And certainly higher temps allow you to store more BTU in a given water volume. Although your system does make me a bit nervous (less now), I am also concerned that someone without as much knowledge as you have might try the same thing after reading this thread - they might have just enough knowledge to be dangerous, as the man said. I think we are much better off promoting the use of lower temperature emitters than higher temperature boilers and storage.
 
Keep in mind that this is a commercially built storage system that was designed to run at these temperatures, not something put together out of parts made for something else, I would not want anyone to think otherwise. When my boiler idles at 205 degrees the storage system is a full 30 degrees from its maximum designed OPERATING temperature. As delivered the system included a safety cutoff aquastat in each tank set to 240 degrees! With the wood boiler the max temp never gets anywhere near this.

Question: Has anyone ever supplemented baseboard radiators with radiant heating in the floor? It looks like a good way to further increase my storage capacity by allowing me to run down to even lower water temperatures.
 
dpsfireman said:
I've been thinking of retrofitting my bathroom with in floor heating, maybe I should take a serious look at doing the whole house while leaving the baseboards in. Woodmaster, did you ever do a calculation to see how cold it could be outside and still be able to heat the house with 90 degree water?

I dont Think I could run that low for very long if it was very cord. I usually don't let it get that low but have a few times and it was still comfy. Never did any calculation for that.
 
Large delta T systems have been around for years. Maybe 15 years ago a company on Long Island built some setpoint controls that allowed you to adjust to a 100 degree delta T. Articles were written about tanks charged to 200F and pulled down to below 100F, heating radiant slabs, for a 100 degree delta T. I have one or two of those controls still in my shop.

District heating systems in Europe leverage large delta T as do some of the district steam systems in this country. There is a handful of cities in the US that still offer district steam. In NYC they do some large refrigerated warehouses with ammonia chillers powered by steam.

There is a movement on to build new emitters that can use low water temperature. Solar thermal, and GEO are another use for low water temperature emitters, and it's renewed popularity is driving some more low temperature emitters on the market. Two common methods to use low water temperature are large, tight corrugated fins, and small ECM reset controlled fans to force the convection. Here are a couple, examples from recent seminars.

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