Looking for info on water to air heat exchangers/hydronic forced air

[Streblerm]

A little background. I have a geothermal heat pump that I have never been satisfied with. Neither performance nor economy have ever been what I thought it would be, at least on the heating side. I refer to it as my $16k air conditioner. Well, last year it went from not working well to not working at all. I have had four techs look at it and only one has offered a solution @$1200 with no guarantee that it will work when he is done. I can buy a new unit for about 2k with the tax credit so putting 1200 in an 8yo unit with no guarantee it will be fixed seems like a bad idea.

This is what led me to burning wood. The wood stove has never let me down, but the wife has this crazy idea that the whole house could be the same temperature and if you were cold you would just push a button and hot air would come out of the duct work. crazy right?

I am kind of hooked on the geo. I like my air conditioning and I have the wells so the cost of a new geo unit is similar to the cost of installed ac. The problem is how to take advantage of cheap gas (30 cents per therm in my area) for heating. I was all set to install a new gas furnace and tie it into my plenum but two furnaces seems like a new can of worms.

My latest hair brained scheme is to design a hydronic system with a heat exchanger in the plenum of the geo. I would use a tankless water heater as a heat source and my current tank as a preheat tank since the geo will heat the water to some degree. I realize this will still be a slightly complicated system but at least it will be of my own design.

So the info I am looking for is around size of heat exchanger, loop design, sizing, pumps, water temp, etc.

I need about 80k btu output to keep my house warm down to -10f which is 10 degrees below the design temp for this area. I was trying to explain this to the last "professional" who came out and wanted to put in a 140k btu furnace.

 

[Guswhit]

 

[CoreyB]

 

[Magic_Man]

I've seen it done before, ( I'm in the hvac trade ) and it will work although I'm not sure how efficiently. Another thing to take into consideration is that you shouldn't mix your drinking water with your radiant water. You will need a water to water exchanger to isolate your heating loop.

 

[fordf150]

Why tankless? 40 gallon water heater with forced air vent. Hot water storage and boiler in one unit. It would give you a short period of heating without the need for the burner to be lit. My cousin switched his shop over from a LP boiler system with radiant in floor heat to a 40 gallon water heater and couldnt be happier. LP bill went down dramatically compared to the 5 yr old "high effiency" $5000 boiler that he replaced. he is heating a 2 stall garage plus 2 medium size offices with that 40 gallon water heater.

 

[Streblerm]

Yes, absolutely I will separate the potable water from the heating water.

I was mostly curious if any of the OWB guys are running a water to air hx, what size they are, and what kind of flow they are using.

I realize I will lose some efficiency with what I want to do vs a he gas furnace, but gas is so cheap right now... A he gas furnace doesn't suit my house with its footprint, the forced air zone setup I am running, and the woodstove. Basically as long as it is cheaper than my electric resistance backup heat at roughly 80 cents per hour for 30k btus I will be happy.

 

[Streblerm]

Why tankless? 40 gallon water heater with forced air vent. Hot water storage and boiler in one unit. It would give you a short period of heating without the need for the burner to be lit. My cousin switched his shop over from a LP boiler system with radiant in floor heat to a 40 gallon water heater and couldnt be happier. LP bill went down dramatically compared to the 5 yr old "high effiency" $5000 boiler that he replaced. he is heating a 2 stall garage plus 2 medium size offices with that 40 gallon water heater.

I have a gas water heater but it is only 40k btus which is about half what I think I need. I also don't want to run into a scenario where I need to shut off the heat to take a warm shower.

My plan is to start cheap with just the a plate HX and a water to air hx hooked to the taco pump on the desuperheater of my non functional geothermal as an air handler. I will use the current water heater for proof of concept and to calculate if it is financially feasible. If it is, I will upgrade to the tankless. It isn't much more than a second water heater. I would probably need an 80 gallon to do what I want to do. That is a lot of water to keep hot all year.

 

[fordf150]

I have a gas water heater but it is only 40k btus which is about half what I think I need. I also don't want to run into a scenario where I need to shut off the heat to take a warm shower.

water heater dedicated to just heating needs in addition to you potable water heater. I am not an hvac guy by any means but the 40 gallon of stored hot water should be good for 15-20 minutes of furnace runtime before the water temp would start dropping dramatically and have a 15 min recovery time to get back to full temp. Just guessing on that. I know several guys other than my cousin heating garages with water heaters and in floor pex. might be worth a shot....if it doesnt work out you will already have the plumbing figured out for the tankless and it wont be a total waste of money since water heaters only seem to last a few years anymore

the tankless water heaters are continuous hot water but at the cost of a fairly massive amount of energy used to generate it.

 

[Streblerm]

I hear what you are saying about the recovery time. It would provide some heat but not enough. 40k btu input means it will never produce more than 40 k btus and that will only keep me warm down to about 20 degrees. That is assuming 100% efficiency which I won't get.

There is no need to run two water heaters. I will separate the potable water from the heating water with a plate HX.

I have always wanted to play with hydronic heating. I'm a big fan of radiant. I could always expand the system to include some baseboards or radiators in other parts of he house.

 

[benp]

Neighbor has an air to water heat exchanger plumbed into the duct work. Has a big fan on the end of it.

The house stays 75 all winter

 

[CoreyB]

Neighbor has an air to water heat exchanger plumbed into the duct work. Has a big fan on the end of it.

The house stays 75 all winter

Would you be able to share some photos of that set up?

 

[Streblerm]

Here is some info I have come up with. Each point is up for discussion as I don't know for sure that any of it is accurate.

Water to air HX also called hydronic fan coils are in the range of 90-95 efficient. This is directly related to size so basically the biggest one you can fit in your ductwork is best. With a high efficiency boiler this should still give me efficiency in the high 80s. The heat loss in a short run of pipe should be negligible.

Flow rates should be in the range of 3-10 gpm

Here's a link to a spec sheet for a Rheem residential hydronic air handler. Lots of good info here as well as a piping schematic

http://cdn.globalimageserver.com/fetchdocument-rh.aspx?name=rw1t-specification-sheets

 

[benp]

Would you be able to share some photos of that set up?

I'll see what I can do.

Here's the gist of it.

I think it's a Dragon Breath heater.

http://www.dragonbreathheaters.com/products/unit-heaters/

It is plumbed into the end of the ductwork in the basement. The heater is on it's own thermostat that it in the living room. It is completely independent from the furnace.

 

[Streblerm]

That's pretty cool. My plan is to just buy a coil and use my geothermal as the air handler.

http://www.ebay.com/itm/20x20-Water...970234?hash=item25a4efd1fa:g:bAsAAOSwA4dWGSHV

 

[H-Ranch]

Water to air HX also called hydronic fan coils are in the range of 90-95 efficient. This is directly related to size so basically the biggest one you can fit in your ductwork is best. With a high efficiency boiler this should still give me efficiency in the high 80s. The heat loss in a short run of pipe should be negligible.

I'm no HVAC expert, but I think you'll find most OWB's are set up with the largest heat exchanger that will fit in the plenum. I don't know about the efficiency numbers as that seems kind of high for the whole system.

One thing I was wondering about the tankless water heaters: are they rated for continuous duty cycle? When it's 0° outside is it a problem if it has to run for 2 hours straight?

 

[Streblerm]

One thing I was wondering about the tankless water heaters: are they rated for continuous duty cycle? When it's 0° outside is it a problem if it has to run for 2 hours straight?

I don't think it will be an issue. The operation of the water heater is similar to a furnace. I don't think it is a question of the duty cycle, rather the limit would be the ability to heat a large amount of water when demand is high. That's why the tankless water heaters have output ranges from 20k-200k btus. They are also sized by gpm. Duty cycle is definitely a question I will research before I buy a tankless unit

The issue would be more pronounced with a conventional water heater where flow rates and ability to heat water are measured in gph rather than gpm.

 

[Magic_Man]

But you will need far less btu's if you set it up in a closed loop. If you set your water heater outlet temp at let's say 150 degrees the water coming back out of the water to air exchanger is still going to be at 100 degrees minimum and is not going to take nearly as many btu's to reheat.

 

[Streblerm]

But you will need far less btu's if you set it up in a closed loop. If you set your water heater outlet temp at let's say 150 degrees the water coming back out of the water to air exchanger is still going to be at 100 degrees minimum and is not going to take nearly as many btu's to reheat.

This illustrates the beauty, efficiency, and flexibility of hydronic heating. Heat not extracted from the air coil isnt lost, rather it is returned to the system. With a hot water tank the closer you run to the limit of the tank, the more efficient it will be.

While my 40k btu hot water heater would do most of the heating season, there is no magic in hydronics. It takes a minimum of 70k btu output to heat my house at negative temperatures. I have lived it and my heat loss calculations support it. I don't want to build a system that is incapable of supplying 100% of my heating needs. If I assume 80% effeciency total and add a fudge factor I'm probably looking at 90k input minimum. A tankless heater or more efficient condensing boiler would allow even more flexibility and economy by not trying to heat 50 gallons of water when you don't need it.

The geothermal is about 70k output for both the ground loop and the resistance heat. It will heat my house but the comfort level and economy is lower due to the low register temp common to heat pumps and The resistance backup heat is just ridiculous expensive.

The more I think about this the more exited I am. There are areas of the house that need more heat but ductwork changes are difficult. It will be easier to pipe water or glycol than air to heat those spaces. There are lots of options like radiators, floor heat, even small self contained fan coil units to get the heat into the air.

I have also considered that I could use my ground loop independently of the compressor by circulating the ground loop through a heat exchanger. I recently found out that this is something that is done and it is called a "water side economizer". As I get into the calculations for the heat exchanger I should be able to figure out how much cooling I would get by circulating ground temp water through the system.

This is really getting outside of the firewood scope. I guess I should mention that I still plan on using the wood stove and its use is a major reason I want to build a flexible, zoned heating setup. I like burning wood but I think I would be just as happy burning two cords a year and paying 100$ for heating as I am burning 6+ and no bill. I know the wife would be happier. She likes the stove but she gets tired of feeding it.

 

[NSMaple1]

With a heat load that high, I don't think there will be many times that you won't need that 50 gallons of water you have already heated. Also not sure an on-demand heater would be designed to stand a continuous output that high - the load to heat a house is a LOT more than the load to heat DHW. Something to check out. It would either lead to the tankless heater running most all the time, or a lot of short cycling. I also don't think you will get much out of tying the heat pump water heater (desuperheater?) to this setup - coils need much hotter water than a desuperheater can produce, and you would lose some of the heat pump liquid heat to efficiency losses during the heat exchanger part of that. I think most people I know that heat their DHW with their geo system can heat their DHW with it, but only adequately.

I would likely do a condensing boiler with the biggest coil you can easily fit.

And replace the heat pump.

 

[Streblerm]

A gratuitous firewood stack pic to keep it forum appropriate. about 15 cords with what I already moved into the garage for the year. I'm trying to bring in another three before the end of the year.