Thermostat Issue
One of my customers added forced air to an existing radiant system and the contractor installed one thermostat to control both systems, the radiant being the “Auxilliary” system.
The problem they are having is when they just want the radiant heating, the FAU comes on as well.
Is there an installer setting that could correct this?
Two btu per sq ft for degree difference for a slab
Comments
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Looking at the manual, it appears that this thermostat cannot operate the two systems independently. The "auxiliary" system will engage when the setpoint is 3 degrees above the current room air temperature. At that point, the FAU would engage. I do not see any possibility of operating the two systems independently.
Of course, if they don't push the recovery too far, the FAU would not engage.
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It looks like a heat pump. Unfortunately any thermostat set up for heat pump, the auxiliary can't become the primary.
One option would be to program the thermostat for 2 Stage AC, 1 Stage Heat. If the reversing valve is energized in cooling, jump R and O, or Y and O at the condenser. Then of course the radiant is the only heat source, and that's probably not why they purchased a heat pump.
Seperate thermostats might be the best way. There's obviously no floor sensor, so if the radiant thermostat is at 70°, the heat pump thermostat could be set to 69° or lower.
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You could use a 3 pole relay to disengage the G & Ys when the radiant heat comes on.
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Aux is aux, the thermostat is working as designed. What the customer probably wants though is for the floor heat to be primary and the heat pump to be aux.
That thermostat is just sending an on/off signal to the heat pump, so there's no reason why the heat pump couldn't be aux in heating mode. However, there's no setting that allows you to use the aux device for cooling in the summer, so you'd have to rewire between seasons.
What kind of heat pump is it? Would it be happier with a sensing thermostat that allows it to modulate to meet load rather than an on/off? If that's the case then two thermostats set a degree or two apart, with the floor higher, is probably the best solution.
One other note: is the floor using electric resistance heat? I can't count how many times I've had guys say that they'll configure the floor heat so it's only on "to take the edge off" but that the heat pump will do most of the heating. That's not how heat works.
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@Alan (California Radiant) Forbes
What is the net result you want to accopmplish?
- To have the heat pump be primary when above a certain outdoor temperature an the Radiant to be the primary below that temperature
- Have the radiant be the primary all the time and only use the warm air when the radiant cant keep up
- Have the radiant as the primary and not use the warm air in the heating season.
I can draw you a diagram to do 1, 2, or 3.
Need model number of the air handler and the outdoor unit. It gets tricky if the system has a variable speed ECM blower motor. But it can be done.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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@EdTheHeaterMan They want door #3
From what everyone has said, I think the easiest solution is to have two thermostats. There’s a closet behind this one and it would be relatively easy to mount one in there although it won’t get the same airflow.
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour
Two btu per sq ft for degree difference for a slab0 -
2 thermostats is the least invasive, and MacGyver doesn't get involved. You said "Door #3, but the heat pump might help when it gets really cold. Is this your radiant design and install? Wondering why the heat pump if it's only used for cooling. Rebates?
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If they don't want to use the heat pump for heating at all, just set up the thermostat as conventional & jump the reversing valve and the compressor call together, or just tie the reversing valve on to the R to avoid the sounds it makes when it throws. I think someone mentioned the same thing earlier, but this is all my idea.
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Remove Rc Rh jumper
Wire heat circuit to Rh and W
Program thermostat to AC only in the installer menu 05
That should give you the result you want. No special relay or wiring needed.
Even MacGyver can do it the regular way when it is appropriate.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Unless it's a B valve, you're going to want to energize it…
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I didn't think of that. By switching from HP on installer menu #5 to AC, the O will not energize in cooling mode. That can be fixed by connecting both Yel and Org wires to the Y on the thermostat. @HVACNUT might call that a MacGyver adjustment.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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I wasn't referencing MacGyver to you, I was referencing unnecessary relays, rather than a simple solution that even a mook like me could figure out if I showed up on the job. Besides, you're waaaaay better looking than MacGyver. 😘
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@HVACNUT said "I wasn't referencing MacGyver to you,"
I took it as a point of friendly ribbing, and enjoyed the reference. Now I'm disappointed that you didn't mean me. 😫
🤣
I am better looking though!
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Thanks very much, guys. I don’t think a solution would have been found anyplace but here with you.
This is an old radiant installation. PB tubing. I was called in because one zone was not heating. There is a Taco ZVC control in the mechanical area, but it only has thermostat wires running to it. No wiring to the zone valves.
I jumpered the wires at the thermostat and didn’t get continuity at the control, so I presumed the wires were cut somewhere - they had a rat problem - and started talking to them about a wireless thermostat.
We were packing up when we noticed that the zone was now calling for heat and hot water was flowing to the floor and the only thing we did was to reconnect an end switch wire that had been cut.
I realized that the wiring was in the old fashioned way. Low voltage wire was routed through the house, the thermostats were wired in series to the zone valves and the end switches were wired to T-T on the ZVC.
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour
Two btu per sq ft for degree difference for a slab0 -
Operating a ZVC with the ned switch of a zone valve??? That makes about as much sense as braille symbols on a drive thru ATM.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Perhaps, but it was an upgrade from the original. There were no thermostat wires available. At least now you can tell which zones are calling.
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour
Two btu per sq ft for degree difference for a slab
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