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Author Topic: Single thermostat two heat sources  (Read 380 times)

DBeleskey

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Single thermostat two heat sources
« on: September 27, 2019, 08:42:57 AM »

I have looked around to try and see what others have done but everyone seems to have a slightly different configuration than mine, here we go...

I have a Polar G2 OWB, its runs into the house and to the DHW and then to a water to air exchanger in the plenum of the furnace. I am leaving the system circulating all the time with no control vales or anything.
I have a two stage propane furnace that is currently my primary heat source. I have air conditioning but no heat pump. What I want to do seems simple but is proving to be somewhat difficult. I want to make the OWB my primary source of heat and the propane as my backup. I currently have a Honeywell RTH 8500D programmable thermostat, I contacted Honeywell and they said to just change the W wire to W2 and a call for heat would make the fan come on. After some looking, I figured that the fan would come on but if the second stage of the fan kicked on the propane would kick in. I only want the propane on in an emergency type situation.

What I want is for the fan to come on with a call for heat and if the temp drops 2 or 3 degrees below the set point have the propane turn on. So, if the temp is set to 72 degrees, the OWB keeps the temperature there. If for some reason, say pump failure or run out of wood, and the temperature drops to say 69 degrees, the propane will come on.

I can't seem to make this work with this thermostat. maybe I am missing something or perhaps I am in need of a different brand/type of thermostat.
Any suggestions ?
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mlappin

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Re: Single thermostat two heat sources
« Reply #1 on: September 27, 2019, 03:13:06 PM »

How was it wired? You said a two stage LP furnace, was it already using W1 and W2?

Install a jumper wire between W1 and G. Only issue with this is if in the shoulder seasons you want heat but not worth firing the boiler fan is gonna run for awhile before the heat kicks in. You could just install an aqua stat instead
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E Yoder

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Re: Single thermostat two heat sources
« Reply #2 on: September 28, 2019, 08:14:52 AM »

I'd use a ranco control sending incoming water temp to break the signal to the propane (aquastat, like mlappin said), or use a second thermostat to trigger the fan only.
Doing it with only one thermostat and no other control would limit to a system where something won't run correctly I think.
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DBeleskey

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Re: Single thermostat two heat sources
« Reply #3 on: September 28, 2019, 08:34:50 AM »

 The LP furnace is only using  W1.
Not familiar with aqua stat...
Finally heard back from Honeywell, they claim it can be done but want me to call, so will try that Monday am. The old thermostat I had in the last house had an emergency heat setting  that I used when I had a heat pump and electric furnace. If I wanted to run the electric furnace instead of the heat pump I just switched it to emergency mode.  Thought I could do a similar thing with the boiler?
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shepherd boy

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Re: Single thermostat two heat sources
« Reply #4 on: September 29, 2019, 06:31:48 PM »

You can, but you said you want the propane to back it up and if you lock it in em you have no backup.
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shepherd boy

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Re: Single thermostat two heat sources
« Reply #5 on: September 29, 2019, 06:50:12 PM »

Put a second thermostat in and you can chose what you want for primary heat and can always adjust what you want the deferential to be.
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DBeleskey

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Re: Single thermostat two heat sources
« Reply #6 on: September 30, 2019, 04:01:23 AM »

Might be the best option.  I ran two wires to the thermostat location when building, just have to fish it out and try and mount the additional one as close to the other one as possible.  I was trying to avoid the clutter I the area, will have two thermostats and a HRV control too.

I guess I just have to figure out which wires control the fan on the current thermostat and  hook the new ones up to the furnace the same way. All they will do is turn the fan on, that's it.
Any issues if for some reason the both try to turn the fan on at the same time ? I don't want to burn any7out in the furnace...
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Re: Single thermostat two heat sources
« Reply #7 on: September 30, 2019, 05:48:43 PM »

When I was installing furnaces I would always relay it so there would be no chance of two speeds coming on at the same time. anyone knowledgeable about the furnace should be able to do that.
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DBeleskey

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Re: Single thermostat two heat sources
« Reply #8 on: October 02, 2019, 07:54:48 AM »

I was trying to avoid  calling the HVAC guy to do this. Very expensive here would likely cost me $300 plus for him to show up.
How could two speeds come on at the same time ?
If I set the wood thermostat to 72 and 68 at night and the propane furnace to 66 is it possible for the two to come on at the same time ?
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Re: Single thermostat two heat sources
« Reply #9 on: October 02, 2019, 02:45:31 PM »

Some units have only one speed and others are muli speed, and you could have a variable speed. You can excite the green wire and in most cases that will work.however some thermostats will lock the g to the yellow which will make the compressor come on but if you have no air conditioning probably just excite the green wire and the furnace will switch to the appropriate speed if it is different if or when the backup comes on. If you get a back feed in the original stat and the compressor comes on a single pole double throw relay will make it work. I am told newer honeywell stats are open and do not need a relay. But a two stage furnace makes me wonder if there is a possibility it has a variable speed which is a different game to get it to spool up on high. 

 Yes if it drops to 66 both will come on.
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DBeleskey

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Re: Single thermostat two heat sources
« Reply #10 on: October 06, 2019, 07:24:50 AM »

Well...I went with two thermostats. Seemed to be the least complicated way and it will work the way I want.
Thanks everyone for your input !
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E Yoder

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Re: Single thermostat two heat sources
« Reply #11 on: October 08, 2019, 03:16:49 AM »

Sounds great!
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