Designing a heat exchanger for domestic hot water capture / solar HW tank

[cbrodsky]

We have a local company working up a solar domestic hot water system quote for us that may provide me a convenient opportunity for woodstove heat collection as well.

Currently we have a tankless coil that we use through our cast iron oil boiler.

We are looking at installing 120-160 gallons of water storage tank capacity as part of this system. I have discussed the wood stove recovery idea with the company designing our system. We have an option to install an extra heat exchanger coil into the storage tank that would give us an opportunity to link up to our woodstove to help supplement the solar hot water in winter.

We have a Woodstock Fireview in an installation that does not require a rear heat shield. Therefore, I am considering buying the rear shield for the purpose of sandwiching a simple finned tube exchanger on the back of my stove to trap and capture some heat thrown off the stove. I would use a differential switch to allow circulation when the wood stove is hotter than the storage tank, along with a pressure relief valve for safety in this heat loop.

Based on a quick back of the envelope calculation, I think this may be a fairly useful method even after making what I think are fairly conservative assumptions to not be overly optimistic. I would be interested in input from some of the other folks on the board - especially those who may be using similar setups.

I start out with max rating of stove at 55,000 BTUs hr - we know that is a wild guess at best. Assume we only get 1/3 this output in normal steady use with the surface running 500-600 degrees. I think this is reasonable in that I frequently hear a modern average home requires 10-20,000 BTUs/hr in steady state to stay warm in our area, and we heat nearly exclusively with the stove, so I assume that is about the level of heat we are getting out of it. Then assume the stove heat exchanger sandwiched between the stove rear and heat shield is exposed to about 20% of the firebox area radiating heat. Then assume only half the heat generated in that area actually transfers into the pipe. This puts me at about 1800 BTUs/hr into the heat exchanger:

55,000 * 0.33 * 0.2 * 0.1 = 1800

If I assume I have a solid fire going 12 hours/day and discount the rest of the day when it is coaling and dropping into the 300 degree range, I can get 21,600 BTUs/day. Assuming my hot water has to be warmed 100 F compared to groundwater:

21,600 / (8.33 lbs/gal)*(100 F) = 26 gallons of hot water produced per day.

This would be more than adequate for our household and could quite possibly let me shut down my oil boiler in the winter with the exception of severe cold snaps or travel away from home. Another interesting way to look at this is that capturing 21,600 BTUs/day would require about it would save about 6.2 gallons per month of oil burning at 75% efficiency. At the likely $2.50/gallon we're up to this year, and 6 months of burning, that is not a half bad savings rate of $93/year. That would give me payback in fairly short order - on top of that, I would also avoid a lot of standby losses in my boiler when I have to keep it hot just to make domestic hot water in milder winter periods.

Now obviously there are a lot of assumptions here - interested in other people's thoughts!

-Colin

 

[elkimmeg]

Use a car radiator for your exchanger between the stove and heat reflection shield

 

[cbrodsky]

Thinking about it some more -

One other option that might work would be to simply have two different loops sharing the same heat exchanger coil inside the solar storage tank. It would basically look just like our multi-zone baseboard system. A pair of the Honeywell electrically actuated valves that would enable flow to solar only, woodstove only, or both, when their independent differential controllers call for circulation. This would let me make a decision later and is probably a lot cheaper than putting another heat exchange coil inside the solar storage tank.

-Colin

 

[cbrodsky]

Use a car radiator for your exchanger between the stove and heat reflection shield

Was thinking about that - however, I'd need to find a pretty small car - one of the requirements to this project is that it is fairly "stealthy" and doesn't look like a ridiculous contraption. Spent way too much effort putting in a slate hearth and surround to trash it up

Actually, this does give me a good idea though... perhaps a transmission cooler would do the trick - might be just the right size!

-Colin

 

[elkimmeg]

Good idea Some small cars like Geo metro have small radiators.. I am thinking you need to have some vollume and half pints of capacity would be useless A little exageration but you know what I mean

 

[stovepipe?]

hey colin,

interesting idea here. I've been thinking about putting in a solar domestic HW system too-- will be interested to hear how things go with this woodstove idea.

also would be interested to know what you've found out about a solar system-- what kind of solar collector are you looking into and what kind of storage tank? I would really like to get a set-up going myself, but they are on the expensive end for sure, and I'm also unclear on what kind of effiencies I can expect. what have you been told? and where/what kind of climate are you in?

 

[cbrodsky]

hey colin,

interesting idea here. I've been thinking about putting in a solar domestic HW system too-- will be interested to hear how things go with this woodstove idea.

also would be interested to know what you've found out about a solar system-- what kind of solar collector are you looking into and what kind of storage tank? I would really like to get a set-up going myself, but they are on the expensive end for sure, and I'm also unclear on what kind of effiencies I can expect. what have you been told? and where/what kind of climate are you in?

We are going through a local company but I did some research myself prior. They came out to do a solar path map and I should have more details on the exact specs shortly. I'm expecting it will be three flat plate collectors with either single 120 gallon or two 80 gallon Rheem storage tank(s). Primary tank will have electric backup (cheap option to add and if we sell the house someday, probably important for most people), but I suspect we will rarely use it - in the winter, when my oil furnace is running, I'd use the tankless coil as it would be cheaper. I emphasized that I want to size it a little on the large end given that the incremental cost drops off quite a bit as you add panels/storage once you've done the install.

In NY State, we get both 30% back for federal tax rebate, and 25% back for state tax rebate. This eases the pain on the cost considerably as it ends up being about half price. The company is also willing to let me do any parts of it myself that I desire like have them install the roof panels and I handle the internal work. But if I get half the cost back in rebates, may just pay them to do it and spend my time on other projects. Should have more price details in the next week or so.

-Colin

 

[webbie]

Now obviously there are a lot of assumptions here - interested in other people's thoughts!

-Colin

I did this with success with my Efel coal stove - which did not get too hot on the rear. I had finned coil made into serpentines - maybe 4 or 5 back and forths, and they were against the rear of the stove. As per your design, another piece of metal covered the rear.

It used a aquatstat and small brass pump to control the heat. I think I made the pump turn on at 140 and off at 120,

 

[Sandor]

Been thinking about this in my own application.

Wanted to use a cast iron radiator behind the stove to capture heat. Problem is the water pressure. My well typical turns on at 40lbs and off at 60lbs. Could not find a radiator that would operate above 30psi. Car radiators typically operate at 20psi, or less, before the cap vents the pressure.

I have room in back of the stove, that is installed in the corner, for a 30 gallon stainless stell cylinder. Can paint it flat black to blend in.

The thinking is:

Run the hot water inlet through the cylinder (behind the stove) first, to capture the heat from the stove. I measured the temp behind the stove last year at 100-140 deg. In the summer, it would capture heat in the room and give me a slight prewarm for the water heater and limited room cooling. Water inlet temp is 58 degrees.

Install the undecided solar water heater with the 80 gallon Rheem tank.

I get anxious every time I use the garden hose this summer and the first couple of gallons coming out is scoulding hot!

Gotta do something.

 

[cbrodsky]

Been thinking about this in my own application.

Wanted to use a cast iron radiator behind the stove to capture heat. Problem is the water pressure. My well typical turns on at 40lbs and off at 60lbs. Could not find a radiator that would operate above 30psi. Car radiators typically operate at 20psi, or less, before the cap vents the pressure.

Good point - however, looks like a transmission cooler may be just the answer. Site below lists coolers tested to 300 psi for use at 100 psi+. Additionally, they look like they have compression fittings rather than rubber hose clamps - would not want to trust a rubber hose clamp for any part of this system.

http://www.tciauto.com/cooling.htm

The other thing I like about these is they are < 1" thick which would tuck them in very nicely between a stove and rear heat shield, and they come in many different sizes which would give you some flexibility to swap out later if you find your heat production is too high or too low.

They also all have mounting flanges so it could be a no-weld bolt-on to your rear heat shield making for a very easy installation. Finally, they are generally quite cheap - $50 or less brand new.

-Colin

 

[Sandor]

The tranny cooler is going to lack volume.

You will get a pint of very hot water then what? Water rushing bye once the hot water is running (shower, D/W) is not going to pick up alot of heat. Maybe you will have a circulating pump? Hey, that may work!

 

[elkimmeg]

What about a pressure reduction valve?

 

[cbrodsky]

The tranny cooler is going to lack volume.

You will get a pint of very hot water then what? Water rushing bye once the hot water is running (shower, D/W) is not going to pick up alot of heat. Maybe you will have a circulating pump? Hey, that may work!

The plan is to take advantage of the fact that as part of the solar install, we will have somewhere around 120-160 gallons of highly insulated hot water storage in the basement - conveniently right below the stove on the floor above! So I will need to install a small circulator and form a loop to the same heat exchanger the solar system will be using to transfer heat into that tank.

The one other thing I may need is a battery backup to protect against a scenario where we lose power but the stove is still burning and overheating water in the tranny cooler. Ideally, I could find a DC operated pump for small volumes and link it to a battery - will have to investigate that a bit. That is the downside of the stove being over the tank - can't use thermosiphon effect.

-Colin

 

[Eric Johnson]

Been thinking about this in my own application.

Wanted to use a cast iron radiator behind the stove to capture heat. Problem is the water pressure. My well typical turns on at 40lbs and off at 60lbs. Could not find a radiator that would operate above 30psi. Car radiators typically operate at 20psi, or less, before the cap vents the pressure.

If I understand what you're saying, Sandor, you wanted to run DHW through a cast iron rad. Pressure might be an issue, but a CI rad would start to rust out pretty quick if you tried to run regular, oxygenated water through it. Better under that scenario (which, for aesthetic and other reasons I like a lot), to run closed-loop water through the radiator and then into a heat exchanger, IMO.

One other question: Don't some auto radiators have plastic parts in them?

 

[Sandor]

Been thinking about this in my own application.

Wanted to use a cast iron radiator behind the stove to capture heat. Problem is the water pressure. My well typical turns on at 40lbs and off at 60lbs. Could not find a radiator that would operate above 30psi. Car radiators typically operate at 20psi, or less, before the cap vents the pressure.

If I understand what you're saying, Sandor, you wanted to run DHW through a cast iron rad. Pressure might be an issue, but a CI rad would start to rust out pretty quick if you tried to run regular, oxygenated water through it. Better under that scenario (which, for aesthetic and other reasons I like a lot), to run closed-loop water through the radiator and then into a heat exchanger, IMO.

One other question: Don't some auto radiators have plastic parts in them?

Eric, I am looking for a system that could be utilized year round. So yea, I wanted to run the domestic water through the radiator before it hit the water heater. Let the stove heat it in the winter, and capture some room heat during the summer. Better trying to heat 78 degree water in the summer, rather than 58. I know the cast radiator will not work, but the 26 gallon stainless will do the job I am looking for. This is just to supplement a solar water heater setup, especially during the winter months.