Makeshift Heat Pump Lockout

[Chevsky]

EDIT2 -- It's apparently confusing. Reworded.

I'd like to setup a HEAT PUMP LOCKOUT. This is for a dual fuel system -- combo Heat Pump and Gas Furnace/2-stage.

Could be creative thermostat wiring, or simple Johnson Controls A421 / other temp sensor and relay.

For two purposes:
• Generator operation -- only run heat stages 2 and up, no heat pump
• Outdoor temp below 30°F

So the question is about how to control (force) heating stage to Aux heat.

I believe there are thermostats that do this natively -- e.g. Honeywell Vision8000pro, Venstar --- not sure, I know they do the temperature lockout (when outside temp gets below set-point, only engage furnace, no heat pump -- typically referred to as "balance point"), but not sure if they do automatic emergency heat.

First generator. How woud that work with a thermostat that does have support automatic emergency heat? When generator is running, send 24vac to E terminal?

What about an external Relay. If first stage of heating is Y, then relay contact set-A breaks Y between air-handler (furnace) and heat pump; Relay contact set-B connects W to W2. Since this furnace is 2-stage, W2 is low-heat, so this tells the furnace to run on low. When thermostat calls for 2nd stage heat, this will already be running. When thermostat calls for 3rd stage heat, it will close W3 and furnace will run on high.

Is it that simple?? Now I can lockout heat pump for whatever purpose. I can energize the relay either/both by connecting to my generator load switch lockouts (SSM module) and via any standalone temperature controller, say Johnson Controls.

What about G -- the heat pump is a MrCool Universal MDUO18 which want's G connected (I think). I may need to leave G disconnected, or break it, requiring a relay with >2 contact sets. Furnace is Goodman 804BN.

I know that in some markets the Utility provides a wire off the meter that goes hot when the temp dips below n°F their electric rate jumps up, and that can be attached to the thermostat or furnace in some way to auto-switchover from heat pump to furnace. It's a disincentive to use electricity. They also put a light somewhere to inform the customer of the rate switch so they refrain from doing laundry and other high-electric-use tasks. So how is that wired up?

Re thermostat -- I wish to use a specific t-stat, RadioThermostat CT80. It does some unrelated things that are important to me, that no other thermostat does, so I'm very egar to make this work, and it supports multistage heat/cool, and multistage Aux.

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[WorthFlorida]

I understand what you want to do. I looked over the T-6 thermostat and there is not enough information. I have a different Honeywell and it allows to use AUX Only but it's a manual override. I have a heat pump with electric heat.

Smart thermostats do control the aux heat in several ways. One is if the room temp and the set temp is 2 or more degrees different, the aux heat is called for. Another is if stage one heat takes too long to raise the room temperature, Aux heat is called for. My Honeywell thermostat will turn off the aux heat when the room temperature is within 2 degrees of the set temperature. I had an older thermostat where the aux heat turned off when the room temperature reached the set point.

Do you have the part number for the remote sensor for outside temperature? I'm very familiar with remote temperature sensor CR7189R. You need to check what features are shown via the WiFi app.

What heat pump model do you have? Most do work below 32 degrees quite efficiently.

 

[Sponsor]

 

[Chevsky]

I'm looking to install a MrCool Universal HP & coil, with Goodman 2-stage furnace, and use standard 24v thermostat wiring -- R, C, G, O, W2, W3, Y, Y2.
So I think...
Y/O will be 1st stage heat (MrCool heat pump)
W2 will be 2nd stage heat (1st stage gas on the Goodman furnace)
W3 will be 3rd stage heat (2nd stage gas on the Goodman

Unfortunately although the CT80 is "Multi-stage Heat Pump with Multi-stage Aux Heat" their notation is a little wonky, referencing connections between thermostat and furnace: W-Aux1, W2-Aux2, W3-Aux3, Y-Y, Y2-Y2, O-O/B, RH-R, G-G, C-C

... and MrCool HP shows terminals Y, B, W1, R, C, G. (So I'm not sure how MrCool interprets the W1 & G... I'm still looking for a complete dual-fuel diagram).

Again, wondering if I can hack a breakpoint feature by providing 24vac to one or more terminals on the furnace via an external temp controller monitoring outside temp. Yeah, and although I may need to go to a proper T6/V8000pro thermostat, those don't have the extra-special-sauce features I want from the CT80 (which are totally unrelated to heat/cool modes -- it's all in the WiFi, baby!)

 

[WorthFlorida]

https://mrcool.com/wp-content/dox_repo/mc-uni-esp-ah-im-um-en-01.pdf

G is an earth ground, not a C lead. W/W1 is second stage heat of the thermostat after the the heat pump.

My first thought was using a micro relay, DPDT. Using a standard thermostat, when warm it would keep the relay off. When the temperature drops It would operate the relay and open the Y and O leads to shut off the compressor. The problem would be the thermostat. W/W1 would not active the furnace until the the room drops below 2 degrees below the set point. Some thermostats this setting is adjustable. There are thermostats for walk in refrigerators and freezers.

There is a bit of miss conception, HP is far more efficient than a fossil fuel furnaces and many relate cost only as more efficient. Since this HP can operate down to 5 degrees, just give it a try this winter.

Another solution is separate the HP and the furnace. Use two thermostats, one for each each plant. With wifi, you can turn off the HP remotely if you think it's to cold to operate.

 

[Chevsky]

I'm not sure what you mean about W/W1. If locking out the heat pump outside of the thermostat (this is all about when it's super cold, say <30'F, or I could set much lower, say 20'F), then the t-stat doesn't know about it--it still thinks it's running a heat pump in heating mode. So it has Y lit up, so it runs the furnace via W. Do you mean that because the t-stat wants to closely track the delta-T (setpoint minus room temp) I will need to be able to set the swing range on the first heating stage to something reasonable like 4+ degrees?, since it's actually running via furnace and should be cycling a bit? Is that what you mean? I'm not sure how much I can adjust the swing.

I've done a bit more research and it seems the accepted way to do this is via breaking Y to HP and sending it to W on the furnace. I haven't seen anything about breaking O/B.

As for the generator, turns out there's a SPDT contact set inside the transfer switch which is intended for such a purpose. So together with a A421 temp controller, I get:

I've sent a note to MrCool asking about W1--my guess is that W1 tells the Furnce the HP is defrosting so please burn gas to temper the cold from the coil).

I will give low temps on the HP a try--the A421 can set the switch-over (balance-point) down to -20F. Honestly, I'm replacing an old system (please no, "Back in my day, those old systems were made in USA and were rock solid, just keep repairin' em!"), and this isn't abput trying to eek every last penny of efficiency; It's more about dipping my toe into the HP pool, and take advantage of the govrnt $$ incentives. Also, since we loose utility power occasionally, if I'm away then I'd like to know that the furnace will pick it up automatically, so the two-thermostats or cloud/wifi thing isn't for me.

 

[Fitter30]

Can't find find a 804 bn furnace on goodmans site or the manual on the Mr cool. Since Mr cool unit is like a single zone mini split from your explanation it can't be used with another furnace/ air handler from anyone else because it has to have communication link between air handler and condenser. Both compressor, air handler both vary in speed with refrigeration and safeties communicating. Since your looking for outdoor temp, generator lockouts for the hp with multi stage furnace operation with emergency heat think its going to take. Someone to program a module with different inputs and outputs to give you everything you want. By making a program will give the flexibility your looking for. Using a off the shelf thermostat adding relays on top of relays will be a nightmare. Were you also thinking of using Mr cool evaporator on the goodmans furnace? Like all the links to the equipment including wiring diagrams, install manuals and mating mr cool to the goodman.

 

[Chevsky]

MrCool Universal series is intended to be "universal" and non-communicating. So no, it doesn't communicatte with the coil. Yes, using a MrCool coil, MDUCC, on the Goodman furnace, which is just a coil (just has a TXV).
The MrCool HyperHeat series is communicating, and I don't think it can be paired with a furnace for dual fuel, (even tho I think the install guide shows the option of a 24vac non-MrCool t-stat).

Yes, Universal series varies the compressor speed, but it monitors the temp of the returning refrigerant rather than comm with AH (furnace). The blower will run at the speed set in the Goodman furnace (adjusted at time of commissioning) based on Y, Y2, W, or W2, and the condenser/compressor will vary based on monitoring the return refrigerant. That's how the Universal series is designed.

The A421 temp controller gives a lot of adjustment/programming like swing and delay to prevent adverse switching on/off the fridge -- these are intended to run refrigeration equipment--that's their bread and butter. The only programming I need is: HP lockout when below a set temp, and whenever the generator transfer switch is engaged.

 

[Fitter30]

Hp doesn't use a expansion valve on the evaporator in heating just cooling. A421 control only has one spdt relay. Honeywell has two sensor inputs and 4 spdt relays each programmed individually. https://www.grainger.com/product/HONEYWELL-Electronic-Temperature-Control-278Y46
Mr cool coil model #? Goodman furnace #

 

[WorthFlorida]

The O lead is for the reversing valve. For most units with system off the reversing valve defaults to HP mode. When cooling is called for voltage is applied to the O lead to activate the reversing valve.

Trane does the opposite, defaults in cool mode and for HP it operates the reversing valve. Digital thermostats asked how to set the O lead when programming.

 

[Chevsky]

Agreed. Two standards -- some brands want to energize the appropriate terminal on the condenser for heating, others for cooling. The CT80 has two terminals, O and B, as do many t-stats, so connect which ever does the right thing for heat vs cool. MrCool wants B to be energized for heating. So connect B from CT80. Was there more to your comment? Does this have anything to do with locking out the heat pump?

 

[WorthFlorida]

Agreed. Two standards -- some brands want to energize the appropriate terminal on the condenser for heating, others for cooling. The CT80 has two terminals, O and B, as do many t-stats, so connect which ever does the right thing for heat vs cool. MrCool wants B to be energized for heating. So connect B from CT80. Was there more to your comment? Does this have anything to do with locking out the heat pump?

No, if you come up with a solution, you do not have to worry about the O lead if it energizes for cooling.