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DIY fan coil unit with water heater (through heat exchanger)? #5317340 01/09/20 02:45 AM
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PandaBear Offline OP
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As mentioned previously my home has an oversized furnace that is quick cycling, loud, and uncomfortable. I had considered a few options to make it better including downsizing a new furnace, dual stage, condensing, venting to the side of the house (it is the flue in the wall between 2 bedroom that's loud), going mini split HVAC, etc.

Today I was reading a few post on the internet and came across a home water heater (not a dedicated boiler) based hydronic system, using a double wall heat exchanger to keep the hydronic side water away from the potable water in the water heater. I also ran into the fan coil unit in commercial systems and was wondering: is it possible to use the same concept and put a radiator inside the duct, right after it come out of the furnace, heated by the water heater through the heat exchanger, to warm the air going through the duct?

The disadvantage of this system would obviously be lower efficiency (assume water heater is only 60% efficient), but it would be quiet, and tune-able to adjust the vent temperature via water flow rate, to make the system always on. It would be very comfortable if I can tune the coolant temperature in the duct (radiator) to be slightly above my desired thermostat temperature, and let the circulation fan run continuously. It would put some extra load on the water heater but it should be fine for my moderate climate.

Let's say I can gather all the parts (i.e. potable grade pump, valves, hydronic grade heat exchanger, copper flexible pipe, small radiator, long life coolant), and I can do the electrical signal wiring myself (splice the thermostat output into the pumps), what would be the reason not to do it? Also why aren't there residential size fan coil unit?

My goal is to get a quiet (as in no forced air combustion sound) adjustable temperature system with minimal conversion risk and minimal hired labor. I can live with a slightly reduced water heater life, and the drop of heating efficiency from 80% to 60%. My other options of side venting + new furnace or mini split HVAC would likely cost me 5-10k, so it is really not worth the energy efficiency difference. Most importantly, I can remove this rig if I ever want to go back, and I can avoid ever replacing my furnace again.

Last edited by PandaBear; 01/09/20 02:51 AM.

"You keep asking questions PandaBear and you'll end up a vegetarian like my wife" - Camu Mahubah
Re: DIY fan coil unit with water heater (through heat exchanger)? [Re: PandaBear] #5317523 01/09/20 08:56 AM
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LazyDog Offline
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There are a few ways to deal w/ an oversize furnace. I would first start by timing how long does the unite run in an hour period. For instance, outside temperature
is 40F, the run time is 4 minutes and 5 times an hour.

1. Get a good Thermostat and read advance settings, you can set the number of runs allow per hour. You can change it to 3 runs and this will increase the run time. For me an ideal run time is 8-10 minutes depending on the outside temperature.

2. In some furnaces you can change the orifice or jet to smallest size. In other cases, you actually eliminate a burner form let say 5 to 4.

3. After you do all these, the blower speed will have to be adjusted and the fan delay dip switches. This can be found in the owners manual.

Last edited by LazyDog; 01/09/20 08:57 AM.
Re: DIY fan coil unit with water heater (through heat exchanger)? [Re: PandaBear] #5317652 01/09/20 12:17 PM
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AZjeff Offline
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Good stuff to know LazyDog but I remember the OP having other concerns as well. Very loud furnace operation or exhaust vent noise because of placement?



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Re: DIY fan coil unit with water heater (through heat exchanger)? [Re: PandaBear] #5317659 01/09/20 12:21 PM
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PandaBear Offline OP
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Thanks and yes to both of you.

It is a loud furnace that run about 1 min out of 15 mins and off 14 mins out of 15 mins. From my understand in the prev discussion it is probably 3x to 4x oversize for my climate. It is probably sized like this to get the CFM for 4 ton AC ready, back in 1994. It is turned on about 10 days a year only, and about 3 hours per day it is on.

Nobody I talked to here wants to take on a 2 decade old furnace, nor modifying a furnace. The liability is too big and the money is not there, plus no warranty to cover their butts on any screw ups.


"You keep asking questions PandaBear and you'll end up a vegetarian like my wife" - Camu Mahubah
Re: DIY fan coil unit with water heater (through heat exchanger)? [Re: PandaBear] #5317758 01/09/20 02:14 PM
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Fitter30 Offline
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Gas water heaters are about 40,000 btu's. Hot water coil for heating needs a minimum 120* water to 180*. For the price of one pump( for the wh side) and a plate heat exchanger a heating only water heater might be possible . So ether way you go a pump, expansion tank, matching coil and air elimination device. My house has two mini split systems both two ton and three wall unit each. Advantages - zoned, quiet, no duct loss and energy efficient. Disadvantages - availability of parts most are a couple of days away, no heat during defrost mode, air filtering is at a minimum and when the wall units need cleaning its a big job water , cleaner and tarps are needed. Bosch has come out with a varible refrigerant flow heat pump. Check with your utility companies to what rebates they have and if they offer a energy audit that includes a blower door test that will tell you how tight the house is.

Last edited by Fitter30; 01/09/20 02:16 PM.
Re: DIY fan coil unit with water heater (through heat exchanger)? [Re: PandaBear] #5317833 01/09/20 03:19 PM
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AZjeff Offline
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Originally Posted by PandaBear
It is turned on about 10 days a year only, and about 3 hours per day it is on. .


See your point, how much would you want to spend for 10 days of loud operation? I'd consider buying 2 or 3 electric space heaters and run them as needed then put away til next year.



14 RAV4 2.5 5W-20 PP
18 Nissan Titan 5.6 DI, 0W-20 QSUD

The most important thing to do in your life is to not interfere with someone else's life. - Frank Zappa

Re: DIY fan coil unit with water heater (through heat exchanger)? [Re: Fitter30] #5318059 01/09/20 06:47 PM
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Originally Posted by Fitter30
Gas water heaters are about 40,000 btu's. Hot water coil for heating needs a minimum 120* water to 180*. For the price of one pump( for the wh side) and a plate heat exchanger a heating only water heater might be possible . So ether way you go a pump, expansion tank, matching coil and air elimination device. My house has two mini split systems both two ton and three wall unit each. Advantages - zoned, quiet, no duct loss and energy efficient. Disadvantages - availability of parts most are a couple of days away, no heat during defrost mode, air filtering is at a minimum and when the wall units need cleaning its a big job water , cleaner and tarps are needed. Bosch has come out with a varible refrigerant flow heat pump. Check with your utility companies to what rebates they have and if they offer a energy audit that includes a blower door test that will tell you how tight the house is.


How much effort was it to power the outside unit of the mini split? I know there's DIY units like Mr Cool but I don't think you can run your own electrical for 240V, right? Also forgot to mention my house is 2/F, and already has a 3 ton carrier 12 or 13 SEER outside.

Last edited by PandaBear; 01/09/20 06:48 PM.

"You keep asking questions PandaBear and you'll end up a vegetarian like my wife" - Camu Mahubah
Re: DIY fan coil unit with water heater (through heat exchanger)? [Re: PandaBear] #5318549 01/10/20 10:18 AM
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Fitter30 Offline
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Where I'm at there are no inspections or permits. Power and control wiring isn't a big deal for me because im in the trade and been doing service work and industrial installations for years.

Re: DIY fan coil unit with water heater (through heat exchanger)? [Re: Fitter30] #5318616 01/10/20 11:52 AM
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AZjeff Offline
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Originally Posted by Fitter30
Where I'm at there are no inspections or permits. Power and control wiring isn't a big deal for me because im in the trade and been doing service work and industrial installations for years.


Good, so you wouldn't mind running over to PB's place and wiring something up?



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The most important thing to do in your life is to not interfere with someone else's life. - Frank Zappa

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