Questions about a add-on storage tank design

[Tseeker]

Hi!
This is my first time posting on here! So hello to you all!

We are now the happy owners of a Englander 28-3500 wood furnace Good bye oil !!

Anyway, being that it's rated for 3000sqr feet and our house is about 1900sqr feet we do have some excess capacity, it's a hot air furnace, so whatever storage that we would add on would be a diy thing most likely. now the furnace is intended as an add-on since it's burn time is 8hrs max, but as we get into our maine winter I don't know if that's going to go down as we run it harder. It's all manually controlled draft, not even a bimetal damper so sometimes we're hot and sometimes we're running downstairs to give it another turn or two. I'm not complaining , just thinking about improving it further.

Now I've only had this idea since this morning, and my head's been fogged up with a cold, so feel free to tell me i'm an idiot

If I were to build an insulated metal frame in the basement (the only way down into the basement is by a narrow set of steps that was a tight fit for our furnace) and put in a bladder tank -500gals a bare minimum?- and then take a clean truck radiator (research showed that they're rated 80k-200k btus) of at least 100k btu rating and construct the ducting so that when the blower on the furnace came on a pump would circulate the water through the radiator. and when the furnace was let to go out during warmer times, we could grab some heat from the tank.

Questions:

1. Am I an idiot?

2. Am I the good kind of idiot or the bad kind? Or should I just take more NyQuil and leave you all alone?

3. Would it work at all?

4. How large a tank would we need for just evening out times so we could extend it to 12hr+ heating times?

5. How long could the the tank keep it's heat if it was insulated with at least 3" of foam?

6. Is there a better way of storing heat from this furnace?

Now unless we do improve this furnace, several years down the road we'l probably get a longer burning more automated furnace. We went with this because the furnace was only $1200 from home depot, and it'll pay for it'self real quick with oil $3.00+ a gallon and tree length wood at $130 a cord

Anyway, let me know what you think!

 

[stee6043]

Neat idea. I'm not sure how practical and/or efficient it would be though. First question-where would you duct the air after you pass it through the radiator? Second-how do you plan to both move heat into storage and out of storage with the same radiator? Your blowers, circs and duct work would somehow need to all be bidirectional. Duct work being the most problematic I suspect.

 

[in hot water]

Get a good quality CO detector for your safety also. Low dose CO symptoms are a lot like cold symptoms. Not trying to scare you but be careful.



hr

 

[Tseeker]

Since the stove is an add-on there is a length of duct work that joins it into the rest of the system, so I was thinking of hooking up the radiator in that ductwork right after the stove before it joins in with the rest of the ducting, that way the 850cmf fan on the furnace (it has a schematic of how to hook in an extra control) would be blowing either hot air from the furnace through the radiator, or cool air from the furnace through the radiator. The circulation pump I think would be alright running in just one direction, probably would be better to pull water from the top during heat recovery mode, not sure how big a difference though.

Thoughts on maybe using 10 plastic 55gal drums with a light frame just for insulating rather than building a strong frame for a bladder? I can get the drums for about $5 ea

 

[stee6043]

Tseeker, I think you would benefit from spending a LOT more time reading about thermal storage. I suspect you're going to answer a lot of your own questions idirectly.

Ultimately, I'd bet on this costing a lot of money and time for very limited success. You should draw yourself a schematic of how you invision both the air and the water flowing in your system. I think you're going to see some roadblocks in the system that may or may not be avoidable.

 

[thecontrolguy]

TSeeker;

I also posted an idea like this some time ago with the idea of (recirculating) the heated air from the discharge of the furnace back to the intake of the furnace during times when the hot water storage needed recharging <and> the house heat call was satisfied. This, I reasoned, would allow the furnace to fire at full temperature for its entire burn cycle thus increasing the overall efficiency and greatly reducing any off-cycle creosote production. Some issues I came up with in the planning were;

1) As the heat storage temperature increases, the return water temperature increases until no more heat is exchanged. (reducing delta-T)
2) You should match, or exceed, the BTU-Hr capability of the wood heat source by the air-to-water heat exchanger.
3) You need to provide ALL the safeties that a normal hot-water boiler would have. (expansion tank, rated relief valve, etc)
4) You should re-circulate the heated air from the supply of the furnace back to the return, via the heat exchanger, so there is an over-all temperature increase in the air as the water temperature increases.
5) Your blower fan is likely NOT rated to operate in a very high-temperature environment so you would have to mount it externally to the ductwork.
6) Be careful to provide minimum flow rates of water through the coil or you might be making a steam boiler.

It might be easier to spend more $$$ and buy an hydronic coil designed specifically for air-to-water heat exchanger duty. That way you would know the BTU-Hr capability at the required flow rates of both air and water and be able to predict the performance that might be had. A little Engineering sometimes helps.

My intention was to duct the supply back to the return via a diverting-damper assembly that prevents recirculation when the furnace is in its normal heat mode. Then, when the house is satisfied, the dampers actuate, and now the air is recirculating. When the water coil (installed in the ductwork immediately above the wood furnace heat exchanger) is hot enough, start the pump and circulate the water from the heat exchanger in the ductwork, to the storage tank. There is a need for several interlocked control devices, but if you're handy enough it could be done. I have a spare A-B SLC500 PLC that is just crying to be installed for this reason.

Now, you would have a storage tank full of hot water that can be used for any heating use, especially DHW preheating, and radiant in-floor heat. Or, if it is hot enough, re-circulate it back to the hot water coil in the furnace ducting, and lock out the gas/oil/electric heat from the furnace you have and heat the air to heat the house. There are LOTS of challenges and limitations doing it this way, but no harm in trying. Also, read about water temperature requirements for different heating emitters (like hot water coils in ductwork) and measure the air temperature in the duct immediately above the wood heater. Now, be prepared for all the purists nay-saying and decide for yourself. Good luck and play safely.