Heat exchanger setup on stove pipe ?

[sr73087]

I am looking to downsize my electric water heater due to the rates in CT being mind boggling. I currently have an 80 gallon electric heater that I am renting (don't ask, house came with it a few years back). I want to get rid of that $10/month rental payment and lower my electric bill so I am going to downsize likely to a 50 gallon hybrid water heater, which has a very good rebate as well. I think along with changing out a few higher gpm fixtures I probably can make the 50 gallon work. However, I have been finding some in a similar situation have been using a heat exchanger on their cold water line to up the temp on the water entering the water heater and increase the amount of recovery and reducing the cost of operation. I have seen a few videos on people doing heat exchanger setups on their stove pipe, but not very much many and all in Canada. I don't recall anything in the US. Is anyone here doing anything like this currently? I am interested in the cost, code and payback surrounding it. It just seems like common sense to try and use the heat going up the chimney.

 

[Highbeam]

First just try the heat pump water heater. That device actually heats water by your woodstove indirectly.

Stoves are designed to release heat up the chimney, it's what powers the draft. If you were to cool that exhaust stream you would kill the draft and could lower the flue temps so far that you create a creosote mess. I recommend you not do anything of the sort.

 

[maple1]

First just try the heat pump water heater. That device actually heats water by your woodstove indirectly.

Stoves are designed to release heat up the chimney, it's what powers the draft. If you were to cool that exhaust stream you would kill the draft and could lower the flue temps so far that you create a creosote mess. I recommend you not do anything of the sort.

Agree 100%

 

[spudman99]

Heat exchangers on chimney flues are frowned upon for several reasons. They may have incompatible metals that corrode at different rates. They of course alter the venting aspect of a stove or insert - which was not engineered nor tested by the certifying agencies. Realistically the air moving up the flue is what, 400 degrees. The amount of hot air stripped off by 70 degree room air, moved then to the copper heat exchanger and then try to warm up 45 deg incoming water is simply not efficient.

That is one of the reasons why tankless water heaters have trouble working in CT. The incoming water temperature in the winter is so low that the system cannot make up a 60-70 deg rise quick enough. They are fine in more temperate climates.

Skip the heat exchanger idea....stick with the hybrid (even they are proven to have a long cost recovery period)

 

[weatherguy]

You might be surprised how well the hybrid does on it's own, I've heard good things about them. Kicking myself for not getting one when all the rebates and tax breaks were in place only to have my oil fired shat the bed.

 

[maple1]

Hybrids use less juice, no doubt. But one of the best moves I've made in this house is getting rid of the oil & putting in an ordinary tank heater. It only runs us in the $25/mo range (with 0.18/kwh electric) in the summer for DHW. I'd be really hard pressed to justify the cost of a hybrid, since there are no incentives here for that. Or there weren't last time I checked at least.

 

[sr73087]

First just try the heat pump water heater. That device actually heats water by your woodstove indirectly.

Stoves are designed to release heat up the chimney, it's what powers the draft. If you were to cool that exhaust stream you would kill the draft and could lower the flue temps so far that you create a creosote mess. I recommend you not do anything of the sort.

Makes sense completely. I was probably going to do what you mentioned first anyway and see how it goes with just the heat pump (trying not to have the elements fire if that is possible during the winter). Really interested how well the dehumidification effect works in the basement for the summer too.

There was one heat exchanger setup I found, where they just had it on the back of the stove itself, not the flue. I guess it could affect heat output alittle, but shouldn't directly affect flue temps right?

 

[sr73087]

Hybrids use less juice, no doubt. But one of the best moves I've made in this house is getting rid of the oil & putting in an ordinary tank heater. It only runs us in the $25/mo range (with 0.18/kwh electric) in the summer for DHW. I'd be really hard pressed to justify the cost of a hybrid, since there are no incentives here for that. Or there weren't last time I checked at least.

Right now after the rebate the 50 gallon is only 450ish USD. I feel like I really have no choice at that price.

 

[maple1]

Makes sense completely. I was probably going to do what you mentioned first anyway and see how it goes with just the heat pump (trying not to have the elements fire if that is possible during the winter). Really interested how well the dehumidification effect works in the basement for the summer too.

There was one heat exchanger setup I found, where they just had it on the back of the stove itself, not the flue. I guess it could affect heat output alittle, but shouldn't directly affect flue temps right?

There are some that have jimmied something up, yes, but to me it wouldn't be worth the time effort & trouble vs. potential water heating cost savings. Everyones mileage would vary of course. You also need to be very careful how you do it, water overheating under pressure can be very explosive. Even short of what might appear to otherwise be a boiling point - it could flash to steam in a pipe under the wrong conditions fairly easily.

 

[Highbeam]

Hybrids use less juice, no doubt. But one of the best moves I've made in this house is getting rid of the oil & putting in an ordinary tank heater. It only runs us in the $25/mo range (with 0.18/kwh electric) in the summer for DHW. I'd be really hard pressed to justify the cost of a hybrid, since there are no incentives here for that. Or there weren't last time I checked at least.

I chose not to replace my electric tank heater with a heat pump heater even though there were heavy subsidies that made the price almost equal. Electric tank heaters are very simple, dependable, cheap to buy, silent, and actually cheap to run.

Problem with the heat pump units, they are noisy and need to be in a space that is shared with heated living space. Noisy, maintenance required, tall, heavy, not dependable, etc. If you have a basement or a heated mechanical room then the noise and size should be more tolerable.

To the OP, going from an 80 gallon resistance water heater to a 40 gallon resistance water heater will not save you any money on operating costs. A well installed heat pump water heater can save you a large portion of what you were previously spending on heating domestic water but I think you will find that you are reducing a small number to a smaller number.

 

[Highbeam]

There was one heat exchanger setup I found, where they just had it on the back of the stove itself, not the flue. I guess it could affect heat output alittle, but shouldn't directly affect flue temps right?

They use to call those "wet backs" and often used them on cook stoves to actually heat that tank of water for direct use. Not normally plumbed with pressurized supply water. In your application there would be nearly zero benefit and plenty of risk. Sweating, explosions, etc.

It wouldn't effect flue temps directly but cool the entire firebox so you'd need to increase the burn rate.

 

[maple1]

He could save just by going smaller - depending what he has now. But it would be more from going from old to new. If it's an old one with marginal insulation, sitting on concrete, with no heat traps, there is likely a lot of standby heat being lost.

 

[AlbergSteve]

You could always try this to lower your DHW bills to zero...
I can't seem to find the video where he installed a thermo siphon hot water system on his wood stove without and expansion tank...there was much superheated steam in his kitchen if I recall!

 

[sr73087]

plenty of risk. Sweating, explosions, etc.

That is true, I think you guys have killed this idea going forward. Which is good, too many projects as it is.

I chose not to replace my electric tank heater with a heat pump heater even though there were heavy subsidies that made the price almost equal. Electric tank heaters are very simple, dependable, cheap to buy, silent, and actually cheap to run.

Problem with the heat pump units, they are noisy and need to be in a space that is shared with heated living space. Noisy, maintenance required, tall, heavy, not dependable, etc. If you have a basement or a heated mechanical room then the noise and size should be more tolerable.

To the OP, going from an 80 gallon resistance water heater to a 40 gallon resistance water heater will not save you any money on operating costs. A well installed heat pump water heater can save you a large portion of what you were previously spending on heating domestic water but I think you will find that you are reducing a small number to a smaller number.

From what I have found Connecticut has the highest cost in the lower 48 for electricity (among other things). Our generation rates can be quite good, as we can pick out supplier. However, I get killed on the delivery charges. I compared my bill (UI) with my brother's in Pa (Peco) by dividing the Bill Total/Kwh and found that per Kilowatt we were paying over 74% more overall per Kw. I never realized that there was such a big difference in utility rates. So from simply an operational cost perspective, maybe it depends where you are and what the current rates are in your area.

For what it is worth, if you look at the energy guide numbers ( I think it was calculated at .12/Kw) a standard AO Smith 50 gallon electric water heater should cost in the upper $400 range per year. They are stating the hybrid should use $114 per year, which if it is correct, should be a 74% reduction in yearly electric cost (assuming 450/year for the electric). 9.5/month vs. 37.50/month. I would also be downsizing from the 80 to the 50 which should save as well, not having to maintain an additional 40% of water. Not sure if those figures were calculated under ideal conditions though and what the usage was either. There are so many factors that play into this and I am not married to the idea. Just tired of a $200+ electric bill in the winter with gas heat. Looking for others thoughts to make the best decision. Thanks everyone for your input so far.

 

[electrathon]

Heating water can be dangerous. Woodstoves can be dangerous. If you are going to do this there are a few more things involved than throwing a coil on your chimney (generally not a good idea). All over the world for hundreds of years hot water has been heated by wood fires. If you do it poorly, it can go bad. Use your woodstove poorly and things can also go bad. If you are looking to just run a coil off a water heater to your stove pipe you will likely end up loosing heat. There is some info on the web about this. Look in my history and I have a write-up about it. I have been heating my hot water for years by means of a coil in my stove. Works well (look at the pic in my avatar). I have an insta-hot water heater and a preheat tank. As best I can tell it saves me about $10 a month in natural gas by preheating the water. I am very cautious and do not waste hot (or cold) water either, so you could save more depending.

 

[sr73087]

Good info, I'll take a look for your write-up.

 

[Manly]

I feel your pain and understand your search to cut cost. Some years ago I looked into the possibility of using the wood stove to preheat water. Just isn’t practical. About 6 years ago I installed a Toyotomi oil fired on demand water heater. It gives me use of the oil tank since we never use the oil furnace. We love the Toyotomi water heater. It has been reliable and only draws 85 watts while running. Never run out of hot water. We use a lot of hot water, about 1/2 tank of oil per year. I installed the unit myself and do my own service. The cost of the unit has increased almost $1,000 from the time I purchased mine. If you had to pay for installation and annual service it probably isn’t worth it for most folks. Then again since you live in CT like myself, you’re going to pay through the nose whatever way you go.

 

[electrathon]

Maybe someone else can run some numbers for me and see if I am correct. If you heat one tank of water from 50 degrees to 120 degrees over a one hour period of time and your woodstove puts out 50,000 BTU I am figuring about a 1% heat transfer from the stove to the water. Most stoves put out more than 50,000 BTU and you usually do not heat more than a tank or two of water per day, so I am figuring a performance output drop in your stove of about 1/10th of one percent per day. So low of an amount it likely could not be detected outside of a math environment. As far as slowing the draft, I don't know how to calculate that, but maybe someone smart here can. If you dropped your chimney temperature one degree how much draft would you loose? In reality it would likely be less than a degree of heat drop, but you need a number to work off of.

 

[bholler]

Maybe someone else can run some numbers for me and see if I am correct. If you heat one tank of water from 50 degrees to 120 degrees over a one hour period of time and your woodstove puts out 50,000 BTU I am figuring about a 1% heat transfer from the stove to the water. Most stoves put out more than 50,000 BTU and you usually do not heat more than a tank or two of water per day, so I am figuring a performance output drop in your stove of about 1/10th of one percent per day. So low of an amount it likely could not be detected outside of a math environment. As far as slowing the draft, I don't know how to calculate that, but maybe someone smart here can. If you dropped your chimney temperature one degree how much draft would you loose? In reality it would likely be less than a degree of heat drop, but you need a number to work off of.

Your math is correct if you were able to only extract the amount of BTUs needed when you need them. But the fact is that is next to impossible to do with a Wood stove.

 

[rwhite]

I'm curious how a smaller heater is cheaper to run. I may not understand it completely but if you can get by with a 40 gal then I assume that maybe 50% of that is a normal use. So if your incoming temp is 50 degrees and you maintain 120 degrees at the heater. The water would mix when entering so you would be heating 85 degree water up to 120. 20 gal use of an 80 gal heater is 25%. That would give you 80 gal of 102 degree water. Based on my calcs I only see a very small difference in btus.

 

[maple1]

I'm curious how a smaller heater is cheaper to run. I may not understand it completely but if you can get by with a 40 gal then I assume that maybe 50% of that is a normal use. So if your incoming temp is 50 degrees and you maintain 120 degrees at the heater. The water would mix when entering so you would be heating 85 degree water up to 120. 20 gal use of an 80 gal heater is 25%. That would give you 80 gal of 102 degree water. Based on my calcs I only see a very small difference in btus.

It's not cheaper, all else being equal.

 

[Highbeam]

I'm curious how a smaller heater is cheaper to run. I may not understand it completely but if you can get by with a 40 gal then I assume that maybe 50% of that is a normal use. So if your incoming temp is 50 degrees and you maintain 120 degrees at the heater. The water would mix when entering so you would be heating 85 degree water up to 120. 20 gal use of an 80 gal heater is 25%. That would give you 80 gal of 102 degree water. Based on my calcs I only see a very small difference in btus.

There is no difference. Negligible additional stand by losses because of more surface area of the larger tank of the 80. The only reason you would want a larger water tank is that first hour rating or because you are so wasteful that you burn up 80 gallons of hot water before the tank can heat it.

The 40 is cheaper to buy and smaller. Both come with a pair of 100% efficient 4500 watt elements, though only one fires at a time. There's also not much mixing in a water heater. the cold water is delivered to the bottom and hot water drawn from the top to maximize stratification.

 

[rwhite]

Now if I was renting a heater for $10/mo I'd definitely get rid of the 80gal and install a 40.

 

[sr73087]

There is no difference. Negligible additional stand by losses because of more surface area of the larger tank of the 80. The only reason you would want a larger water tank is that first hour rating or because you are so wasteful that you burn up 80 gallons of hot water before the tank can heat it.

The 40 is cheaper to buy and smaller. Both come with a pair of 100% efficient 4500 watt elements, though only one fires at a time. There's also not much mixing in a water heater. the cold water is delivered to the bottom and hot water drawn from the top to maximize stratification.

The upfront cost difference in this case is substantial. For an 80 gallon hybrid it was roughly $1,900 uninstalled. The 50 is on sale from 1,200 to 800 and then they are offering a $350 rebate on top of that. Originally I figured go the “safe” route with an 80, I can’t end up any worse off. However, it’s around 1/4 of the price, which is what made me consider the 50 in the first place.

I think the difference in operational cost is probably a lot less now than in the past since insulation is much better today and much less heat loss occurs.

I guess one other thing to consider is will it cost more to replenish 20 gallons from a 50 or 20 gallons from the 80. Assuming your use was the same, would having to lower reserve cause the unit to throw on the electric elements sooner, thus defeating the purpose of going hybrid sort-of.

 

[sr73087]

Your math is correct if you were able to only extract the amount of BTUs needed when you need them. But the fact is that is next to impossible to do with a Wood stove.

electrathon, how does your setup handle this issue? I know I’ve seen setups that bleed the pressure off, but wouldn’t the tempature continue to climb?