Staging thermostat to select backup heat source over heat pump depending on outdoor temp

archibald tuttle

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pecmsg

archibald tuttle said:

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X-2 

GGross

pecmsg said:

archibald tuttle said:

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X-2 

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archibald tuttle

@pecmsg I get the X2 staging thing, but this is really a little different. I imagine the X2 is smart function in some units but I kind of figured it responds to rate of change of indoor temp (which is a surrogate for outdoor temp). maybe if the setting for that parameter is adjustable in various heat pump thermostats that would work as a surrogate although i'm looking for an ability to measure outdoor temp (and humidity) and make the switch based on reduced COP rather than on the particularized BTU reduction. Just seems like not an area of focus which should be but people are aimming heat pumps at non-backed up or onboard back up which is the worst of all possible worlds with current electric rates. I can maybe wait til atlanta for perfection (or to discover the current lack thereof- and honestly, it's kind of tough to marry AHR and ASHP, I hate to think I have to switch or add shows, but I suppose I shouldn't complain about another junket) but wanted to get some ideas and discussion going now because I have two customers with fossil backup asking for help on this right now. thanx

pecmsg

With gas or oil furnace its either heat pump or fossil fuel, not both. (The exception is when the evaporator coil is up stream of the furnace)
With HW coils yes you can run both but that's going to satisfy really fast with 120, 140, 180° water! Yes you can get real creative with controls, how fancy do you want to get?

NOTE: With any furnace the evaporator coil "Should" be raised 6 - 8" or more above the furnace.

archibald tuttle

absolutely understand it's not both but one or the other until the next useful technological advance (e.g. second stage of heat pumping inside . . . or self contained propane refrigerant ASHP outside and anti-freeze hydronic transmission to additional selfcontained small charge propane refrigeration stage inside). I'm talking about a really addressable control to make that decision not strictly on the basis of whether the heat pump could theoretically just keep up but when the COP drops minute to minute or in higher defrost necessity circumstances that the controls know enough to call the back up and wait out the poor outdoor conditions.

pecmsg

Honeywell has several Duel Fuel or Hybrid controls. You can lock out the Heat Pump below X. You can lock out fossil fuel above Y. The outdoor board can bring on the back up during defrost.

archibald tuttle

pecmsg said:

Honeywell has several Duel Fuel or Hybrid controls. You can lock out the Heat Pump below X. You can lock out fossil fuel above Y. The outdoor board can bring on the back up during defrost.

that is exactly what i'm talking about. I'll go search honeywell duel fuel controls but if any retentive plumbers out there can quote rank and verse of a honeywell part #s i'll take it. And, by outdoor board, you mean the proprietary board in the heat pump/[sometime] condensor unit? I assume the addressable parameters on whether the backup is called depending on outdoor temp, state of indoor temp or rise compared to setpoint might differ amongst manufacturers although i'm interested to know of particularly well implemented strategies.

thanks,

brian

thanks

archibald tuttle

@pecmsg e.g. honeywell TH8320R1003

operating theory is if the indoor temp drops 2 degrees below setpoint while heat pump is trying to make up then it kicks the backup. haven't read far enough to see if it turns off the heat pump or if that is at a different point. but it has no outdoor temp sensor (nevermind outdoor humidistat).

but as to how fancy do I want to get, given that boiler controls often incoporate indoor and outdoor parameters and rater of change these days i feel like there is relativley little marginal cost to get fancy. and probably these folks will make alogrithms for the uninitiate so differentials or deadbands these days, for instance, are often set by type of heating rather than choosing degrees or rate of rise or whatever with user addressable parameters. Of course I would always prefer user address but just like your car often has a tranmission control that chooses between efficiency and performance, this just seems like a couple afternoons for an engineer . . . (and outdoor sensors for thermstates/control units that don't already employ them.

hot_rod

SpacePak has a nice control that handles all that logic, recently updated. I don't know if it is sold separately. I think the Solstice control comes at it a bit differently with the upgrade where they don't switch the HP off, but supplement with a backup source somehow, as long as COP is above 1, that would be good option.

I took the Solstice installers class a few months back, but have not installed the control for any hands on experience. Jim the trainer for Spacepak would be good to talk with. He has years of installation experience with HPs in that cold climate.

Caleffi has a control to select source based on ODR, but not for the US market at this time.

fentonc

The spacepak solstice one is neat - I think it effectively compensates for the dropping output of the heat pump as the temperature drops, so rather than switching off the heat pump completely at some temp threshold, maybe you get 25K btu/hr from the heat pump and 10K btu/hr from an electric boiler rather than saying “the heat pump can’t provide 35K btu/hr, so I’ll switch to electric backup”.

GregWeiss

I am trying to figure out this same situation. I have an LG heat pump and an oil boiler with hot water fin tube. I would like one thermostat to control both systems and fire the oil boiler based on a certain outdoor temperature. Does anyone have any experience with the LG multisite CRC2 control? LG model number PREMTBVC2

archibald tuttle

hot_rod said:

SpacePak has a nice control that handles all that logic, recently updated. I don't know if it is sold separately. I think the Solstice control comes at it a bit differently with the upgrade where they don't switch the HP off, but supplement with a backup source somehow, as long as COP is above 1, that would be good option.

hmmm, pretty esoteric products but seems like the control is not a stand alone. @GregWeiss but when @Hotrod talks we listen. although i'm going to take issue with his citation above of 1 COP being the cutin for backup unless the backup is reistance electric.

The controls i'm looking for need to deal with the logic of backup that is not electric. Hereabouts with electricity going for .32 kwh and gas at $2.12 a therm, 80% efficiency gas boiler is still cheaper than heat pump at 3 COP!!!!!!!!!!!!!!

can I put enough of those. thanks for all the free energy all your green new deal folks (see also the 7 or 8 pages of comments on the end of gas)

Even with propane at $3 or oil at $5, those both still beat heat pump at 2 COP!!!! and it isn't clear to me that the heat pump logic incorporates its real operating conditions (maybe it does, maybe it can actually compare heat differentials and consumption although it would have to do this over time and be able to know humidty and climate zone to factor in the costs of defrostings. In any event, the control we need is for these decisions that can factor energy costs whereas I anticipate that most of the controls contemplate the same math you did, COP comparing to resistance electric.)

The other problem is, maybe with resistance electric downstream of the indoor coil you could still get marginal input from the heat pump as COP drops, but in terms of indoor comfort, I don't know how you get the differentials in return in standard air to refrigerant minisplit operation to still collect much in the way of btus from the heat pump as outdoor temps drop. maybe its alogrithms allow the TXV function to throttle and raise head pressure to gain better splits with lower volume and less btus so you can still recover a little although I would imagine that might increase electric consumption . . . ? calling all serious HVAC engineers. But where the backup is independent and warming rooms contemporaneously, I would think this would make it even harder to take advantage of partial loads with the minisplit if the return temps remain high because the backup is on.

This seems like a complex equation that could be made simple by a digital gadget. I'm the first one to say, why bother complicating things that don't need it, and I wish they would take all the digital controls out of washing machines where you get very little gain and lots of headaches from them, but this seems like an arena where an algorithm would be very helpful and then you can even wifi in your current costs of fuel as well as outdoor temp and humidity. . .

Just sayin'

calcs:

heat pump at billed rate of $.32/kwh at 300% (i.e. 3 COP) efficiency is $.108/kwh

heat pump at billed rate of $.32/kwh at 200%(i.e. 2 COP) efficiency is $.162/kwh

natural gas at billed rate $2.12/therm and 29.3 kwh/therm at 80% efficiency is $.090/kwh

propane at $3/gallon and 27 kwh/gallon at 80% efficiency is $.139/kwh

heating oil at $5/gallon and 40.6 kwh/gallon at 80% efficency is $.154/kwh