Heat Pump Aux Heat Tstat Setup

bradleyj · December 1

I've currently got a Honeywell TH6320ZW2003 T6 Pro thermostat controlling a dead-simple gas-fired hydronic system; red and white wire from boiler to "R" and "W" terminals on the tstat. This is a switch closure input to the boiler. There is no 24V power. The tstat runs on batteries. Everything runs fine like this.

I'm ultimately planning on installing a heat pump system for primary heating and cooling, keeping the gas-fired system for auxiliary heat. The T6 Pro is very capable of doing this, so I'm trying to set it up to test the aux heating feature to confirm that this will work. I've reprogrammed the tstat to tell it that it's controlling a 2-stage heat pump system with a single auxiliary heating stage and programmed the auxiliary heat to kick in if the indoor temperature is two degrees or more below the setpoint. I've moved the wire to the boiler that's normally connected to the "W" terminal on the tstat to the "Aux" terminal. Remember that the heat pump system is not installed so the two wires to the boiler are still the only wires connected to the tstat. All I've done is reprogram the tstat and move the white wire from "W" to "Aux". When I turn the tstat set point way above room temp (way more than 2° above), the tstat indicates that it is calling for heat, but the boiler is not kicking in. I do not hear the click of a relay closing in the tstat as I do when it's set up to use the boiler has primary heat. I would expect that the tstat would control the auxiliary heating system the same way as it does for primary heat by simply closing a relay between the "R" and "Aux" terminals. If it does, then I must not have it programmed correctly because I can't seem to get the boiler to start. Maybe the tstat can detect that there is no heat pump connected so it won't call for aux heat? Maybe it needs 24V power for this to work?

Can anyone offer any advice? I'd really like to test this before I get a heat pump installed.

HydronicMike · December 1

You could just have the heat pump installer do it.
If you have 2 stage heat, you have it programmed wrong. 2nd stage heat would kick in on the 2° droop. Aux should be programmed to lock out the heat pump and use the gas boiler. And that should be at a certain temperature.

EdTheHeaterMan · December 1

Some companies would call your system a Hybrid design. The Hybrid design is usually done with a gas (or oil) furnace. That presents a problem for the heat pump A coil on top of the furnace. Hear me out on this to understand the wiring logic. Since the standard AUX or EMER heat is usually electric resistance that is installed downstream of the indoor coil, the heat has no effect on the A Coil in the air handler. With a furnace, however, the additional heat of the furnace passes thru the A Coil atop the furnace. The heat pump can not operate under those conditions, so the compressor must be shut off when the fossil fuel furnace starts.

Depending on the Hydronic Heat Coil location, you may not have that problem. If the hydronic heat coil is first in the air flow, then you must shut off the compressor and program the thermostat for fossil fuel hybrid operation. If your heat coil is after the refrigerant coil in the air handler then you can use the heat pump when the temperature drops. That may preheat the air entering the Hydronic coil and offer you more efficient operation for a short range below the break even point of the Heat Pump alone. If that temperature is say 30°F then you may be able to get some benefit from heat pump operation down to maybe 20°F. Below that temperature, you should shut off the compressor with an outdoor temperature thermostat.

I can help based on your system setup. Send me the equipment you are thinking of adding and how the heat pump air handler is configured to determine which way you need to go.

You may need to add a "HEAT RELAY" to the air handler in order to make the boiler start and pump heated water to the hydro coil at the AHU.

EdTheHeaterMan · December 1

here is the Installer Setup (ISU) chart for the TH6320ZW2003 T6 Pro.

Depending on your heat coil location tou will need to select the correct opetio for ISU 255.

bradleyj · December 1

This would definitely be a hybrid system. The T6 is very capable of doing what I would want it to do, but it needs to be set up correctly. It has a setting to tell it whether your aux heat is electric (where you need to be concerned about lockout of the heat pump for the reasons you describe above) or hydronic. My existing system is a gas-fired boiler with baseboard radiators, there will be no aux heat coil in the AHU. The existing hydronic system will be completely independent from the heat pump, other than being controlled by the same tstat. I'm hoping that someone here has specific experience with this particular tstat in this application who can help me set it up correctly and make sure that I've got the control wires connected to the correct terminals.

Here's the installation guide for the tstat

Honeywell T6.pdf

I've been thinking that a relay may be necessary. If the "Aux" output in the tstat isn't an internal relay like the "W" terminal is when the tstat is programmed to control the boiler as primary heat then I would need to take whatever signal the "Aux" output is giving to convert it to a switch closure, which is what the boiler wants to see. I can't find any specific information on the tstat in the manual or online describing the nature of the "Aux" output. If it's a 24V output then I would need a relay. I'll check the output with a multimeter to see if there's 24V there when it should be.

I haven't selected a heat pump yet. It'll probably be a typical 2-stage split system with a reversing valve capable of both heating and cooling.

bradleyj · December 1

https://forum.heatinghelp.com/discussion/comment/1874726#Comment_1874726

I want to test this myself to make sure that this tstat will work in this application. It's SmartThings compatible and I like it, so I'm hoping I won't have to have it replaced if I have a heat pump installed.

If you set the equipment type in the T6 to "Heat Pump", it allows you to select the number of compressor stages, not heating or cooling stages. I set the number of stages to 1 for this test. With this setting, I would expect the tstat to call for aux heat if the setpoint is >2° above room temperature.

bradleyj · December 1

https://forum.heatinghelp.com/discussion/comment/1874737#Comment_1874737

Thanks, I have the full installation manual describing these settings.

If you set the equipment type in the T6 to "Heat Pump", the selections for 220 changes to "Compressor Stages", which to simplify this for now I've set to 1, and the selections for 221 changes to "Aux/E Stages", which I've also set to 1. I have all others set the way you describe; 255 is set to "Gas/Oil".

I'm thinking more and more that I'm going to need a relay for this.

EdTheHeaterMan · December 2

Since your heat is not from a heat coil on the air handler, then you can run the thermostat like the AUX/EMER heat is electric. The W (or the E/AUX) from the thermostat will then operate an additional heat relay in the AHU and keep using the heat pump while the next stage if the thermostat pulls in the hat relay to ramp up the baseboards. Otherwise you may get a short time of cold air blowing when the heat pump compressor drops out and the fan still operates to move the air in the AHU with no electric backup.

This may be a trial and error design to get it right. The heat relay may need to disconnect the fan and the compressor so you don't get cold air blowing. Setting 255 to Fossil may be the best idea. The compressor will stop, then the “Add On” Heat relay will turn on the boiler and shut off the fan. This will stop the AHU from blowing cold air.

Now you will get the problem of seesawing from gas boiler to heat pump as the thermostat gets partially satisfied and goes back to stage 1 then the temperature drops to stage 2 then back to stage 1. This will continue until the outdoor temperature is high enough to operate by heat pump only.

Perhaps a better idea would be to have an outdoor sensor that will select a specific outdoor temperature where the heat pump just does not operate at all when it is colder than that temperature and only the gas boiler will operate. That control can have a 4° or 5° differential so the seesaw will not happen when the outdoor temperature is close to the outdoor thermostat set point. Outdoor temperatures don't usually see 4° difference causing short cycles the way the thermostat will.

EdTheHeaterMan · December 2

@bradleyj said: "Remember that the heat pump system is not installed so the two wires to the boiler are still the only wires connected to the tstat."

The thermostat will want to see that you have power from a 24v. transformer from the air handler unit (AHU) and then send that power to a heat relay in the AHU by way of the E/Aux thermostat terminal. With no air handler yet, there is no 24 VAC from the AHU to power the relay.

Do you have any idea what the model number of the AHU is going to be? I can look up that wiring diagram and get you an idea of what additional after marker relay you might add the the AHU when it arrives.

Kaos · December 2

https://forum.heatinghelp.com/discussion/comment/1874726#Comment_1874726

Lot depends on fuel cost and heat pump. New units will run down to very cold temperature at a decent COP, so cold climate units don't need to be locked out, they can (and in cases of expensive fuel) should run when aux heat is called.

For example, this is a 2.5ton unit that almost produces 3 tons of heat at 5F with a COP of 2.

https://ashp.neep.org/#!/product/259983/7/25000/95/7500/0///0

Around me, that would be significantly cheaper to run than propane or oil.

If you are installing a heat pump, make sure to pick one appropriate for your climate.

HVACNUT · December 2

Change it to 1 stage heating (you said you set it up as 2 stage) and 1 stage aux.

Heat Pump Aux Heat Tstat Setup

Comments

Categories