Dual fuel changeover settings

[Tithis]

Might not be the right forum for this as it does involve a wood stove, but its not the main focus.

So earlier this week I got a new 2stage furnace and heat pump installed. This install included a HZ432 Honeywell zone controller for the newly seperate zones for the top and bottom floor. Each floor has a ecobee3 lite that can only call for heat, cool or fan, and the zone controller decides how many stages of heating or cooling to use depending on if one or both floors are calling. Additionally it also handles the automatic switch from using the heatpump vs using the furnace.

Right now its set to do that swap at 32f, and the installer instructed me to change that if it seemed like the heatpump could or couldn't keep up at that temp.

The heatpump beats out propane in cost/btu across its entire operating range if the spec sheet is to be believed, so if it can keep up with the homes heating demands I would obviously always want to use the heatpump.

Issue that occurs to me though is I still have a wood stove that I intend to heat primarily with. If I got the wood stove + heatpump going they should have absolutely no problems covering my homes heating requirements with the stove serving as the bulk heat source and the heatpump adding a bit on top while helping to circulate the heat. However if we are away during a particularly cold time I doubt the heatpump could keep up on its own. I thought these two operating situations were simply at odds and I'd have to settle with just setting that crossover temp at whatever I felt the heatpump could handle alone, even if its more costly.

However while looking over the manual for the HZ432 I did notice two settings that look like they might be of value.


Dual Fuel Changeover; which has the following two options

• Dual Fuel Changeover by OT Temp (outdoor temp)
• Dual Fuel Changeover by OT Temp + multistage call

Notes:
Allow panel to changeover from heat pump to
fossil fuel based on outdoor temperature, or
allow second stage call to changeover panel
to fossil fuel for a minimum of one hour when
outdoor temperature is above OT balance tem-
perature setting. Multistage changeover is only
available on dual fuel applications configured
for 2 heat pump stages.


and then this setting

Dual Fuel Mstage Changeover Time; which can be set to anything between 15 and 180 minutes

Description: Sets fossil fuel changeover delay time.
Notes: Sets number of minutes to delay fossil fuel when calling for second stage heat.



Now at first glance it looks like one of these may solve my problem. So far it seems the 1st floor is the primary one calling for heat, with the 2nd floor only calling for it occasionally for a few minutes at a time. If the heatpump was falling behind I'd assume both floors would end up constantly calling for heat, and it almost seems like one of these two settings would let the system detect that situation and changeover to the furnace automatically after either an hour, or some time between 15-180 minutes. Not sure if we got anyone who might have experience here with something like this, cause I might just be misunderstanding it and these mean something else entirely than what I'm thinking.

 

[stoveliker]

@Brian26 has a lot of knowledge on heat pump performance (and I presume systems in which they are incorporated).

 

[Brian26]

Do you have a model number on the heat pump outdoor compressor and indoor coil? I could look up the performance of it at various outdoor temperatures.

The savings from a heat pump over propane is astronomical right now. Even with Massachusetts high electric rates your heat pump is way cheaper than propane probably even in some extreme cold. With the current savings I would run the heat pump as much as possible. In Maine right now its estimated burning propane cost $3200 more this winter over a heat pump.

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Here are some rough numbers. I pulled the average statewide costs online for Mass for electricity and propane from the EIA.

https://www.eia.gov/dnav/pet/PET_PRI_WFR_DCUS_SMA_W.htm

Here are the costs per million btu.

Heat pump at a cop of 3 with .24 electricity would cost you around $22 per million btu..

Propane Furnace at 85% efficiency at $3.86 gallon is around $49 per million btu.

Your propane essentially costs double or more per btu.

I would set the outdoor switchover temp much lower than 32 at those rates. Depending on your insulation, air sealing, windows etc your heat pump might have no issue heating your home down to 20 degrees. If you live in an old farmhouse with no insulation it might not be able to keep up. Unless you had a manual J and know your homes heat loss the only way to find out is to experiment with the outdoor switchover temp.

It looks like Western Mass is going to see some cold this weekend and Monday night is in the low teens. Would be a great time to only run the heat pump for a few days and see at what temperature it can't keep up. Then use that as your rough switchover temperature.

 

[woodgeek]

Easy. You don't want a 'dual fuel' configuration. You want a 'staged' configuration, where the HP is the first stage, and the furnace is the second stage. That is easy with one ecobee. Not sure how to do it with two and your fancy furnace controller.

But you just need to reconfigure the control scheme, rather than trying to BS a dual fuel setup.

 

[Tithis]

@Brian26 : Sure, here are the model numbers.

Outdoor Unit: Bosch BOVB-60HDN1-M18M
Indoor Coil: Bosch BMAC4248CNTF

Together they are listed as 46,000 BTU in AC and 32,000 BTU at 17 degrees F, 16.0 SEER, 11.0 EER and 9.5 HSPF, but with the Bosch 80,000 96% 2 stage furnace the efficiencies are listed slightly higher. We got a carrier 80,000 96% 2 stage , so should basically be equivalent.

I know the cost per BTU is pretty dramatically in the heatpumps favor, when I did estimates based on the spec sheet I was getting equivalent propane costs lower than anything recorded here in the last decade, hence my desire to minimize how much the furnace runs. I also looked at our energy mix from ISO New England to estimate co2 per kwh to compare against the furnace on that front, and it too is pretty drastic.

Edit:

Additionally a prior to all this being done since we were downsizing the furnace from 100,000 BTU to 80,000 BTU I followed the instructions on this site (https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new) to estimate my heating load based on some heating degree days data from a nearby airport, as well as two propane bills a month apart to gauge my propane use during that time (this was before we got the stove)

Bills were on 12/18/20 and 1/19/21
224 gallons were delivered on the Jan bill.

224 gallons * 91,500 BTU/Gallon = 20,496,000 BTUs 20,496,000 BTUs * .96 (old furnace efficiency) = 19,676,160 BTUs sent into the ducts.

Nearby airport recorded 1071 degree days @ 65f from 12/18/20 to 1/18/21

19,676,160 BTU / 1071 Degree Days = 18,371 BTU/DD
18,371 BTU/DD / 24 hour/day = 765.5 BTU/Degree Hour

Design Temp here is 0f, so thats a 65f degree difference
65f * 765.5 BTU/DH = 49,744.5 BTU/HR load

But they say if you set your thermostat low often, which we did, to bump that up
70f * 765.5 BTU/DH = 53,585 BTU/HR

And if you were sizing a furnace and going with a 1.4 sizing factor to cover record extreme cold snaps
49,744 * 1.4 sizing factor = 69,642 BTU/HR
53,585 * 1.4 sizing factor = 75,019 BTU/HR

Now that estimate is assuming the old furnace was actually running at 96% efficiency, however it was red tagged for a cracked heat exchanger the next year and often triggering a flameout as 3/5 flames were organge and blowing around to an extent. So I'm doubting it was actually running at full efficiency, it smelled awful even back then and had soot deposits.

The house is insulted with pink fiberglass, but air sealing under the siding is minimal, when we had one gable ends siding redone it was just scraps of rosin paper under the clapboards. The new siding has tyvek house wrap under it, which we'll continue as we reside the front and other gable end. That should hopefully reduce the heating requirements as bit in the coming years.

For now based on what I've seen the my complete system should have a similar HSPF to the BOVA 2.0 48, with only slightly lower CFM for high, so its probably the closest equivilent that has full performance data. Based on that I'm guessing the heatpump would start to struggle around 22f if it was the sole heat source. Although it could probably be enough to keep the pipes from freezing while we're gone lower than that.

 

[EbS-P]

It really seems that you are looking for a vacation mode. In that case I would set the the crossover temp to say 20 degrees. Alternatively you could set the propane furnace up as Thrid heating stage without any temp crossover. This is how my two stage Heatpump with electric resistive is set up. I can’t find any outdoor crossover temp setting. I do know that I’d the the call for heat happens and the room is more than 2 degrees below the the set point the strips kick on to give a boost. If the set point isn’t sastified in say 8 minutes it’s kicks up to stage two and if that still won’t satisfy the set point the resistive strips will come on.

So the heatpump is a single stage? If that’s correct then you have 3 stages of heat. Seems simple enough. Use stage one (heatpump until crossover temp then it will call stage two gas low stage then stage 3 gas high stage if when call is not satisfied. Zone control on single stage is difficult. Is the stage zone control just on the furnace. I would not use the low furnace stage and set a low crossover temp. I favor simple controls and once you get zoned controls in the picture it’s more that i know.

I’m a complete HVAC noob and all I know if from dealing with my System and watching “anti diy hvac” on YouTube

 

[Tithis]

It's actually an inverter heatpump, so the compressor has a range of 30%-100%. Its unusual in that its non-communicating and supposedly how it works is the outdoor unit monitors the pressure on the line and how often its being called to decide how hard to run the compressor and is trying to aim for long, low intensity heating/cooling cycles.

The early version of my model came with a single speed air handler, and the current version can come with a 2-stage air handler. Their current line basically has 2 series of outdoor units and 2 series on indoor units

V1 Outdoor Unit + Single Stage Air Handler = 15 Seer range
V1 Outdoor Unit + 2 Stage Air Handler = 18 Seer range
V2 Outdoor Unit + 2 Stage Air handler = 20 Seer range

I've effectively got the middle one with furnace have two fan stages for cooling/heatpump.

The zone controller is configured to only call for the 1st stage when one floor is calling for heat, and calls for 2ndstage when both floors are calling for heat.

So far it seems to work well with the 2nd floor calling for heat a fraction of the time that the 1st floor does.

 

[EbS-P]

Ok that makes more sense. Variable compressor with two stage blower.

So you have a compressor control that’s not integrated to the thermostat. That probably works well. I would experiment next winter and see where the crossover should be set. I’m guessing it could be be quite low given the high cost of fuel for the foreseeable future.

Thinking a little more about it setting the crossover really low and utilizing the gas run time above the crossover seems like the most efficient from a cost perspective. Even at a COP of 2 with high gas prices it may be cheaper to run heatpump at really cold temps.

So say Heatpump can’t keep up and has not satisfied the the call temp instead of kicking gas right on in the second stage call (not sure how this works with two gas stages) you would let the heatpump run say an hour then run the gas. Temp satisfies all systems shut off. It cooled down heatpump comes on. Second stage calls again and it waits an hour before running gas again. If I am interpreting this correctly I wouldn’t ever want my heatpump to shut off as long as it was cheaper than the gas. (That’s gonna take some math but doable. Been done. On an earlier thread I think one minisplit user found that temp for his unit was -10F or -20F. That was before prices spiked.). You should account for defrost cycles.

If I am understanding the crossover temp correctly it will completely shut off the heatpump. 32 is way too high. The crossover temp is to make sure you are using the cheapest heat. With a multi stage call for gas your comfort will be taken care of.

My gut says if the system works I you have described and I interpret med (you gotta trust your installer here) you probably don’t even need the cut out temp for the heatpump. If it’s not keeping up you need gas. Sure there is a temp where it wont save you money but I’m guess a new system it’s probably (hope if really) its 0F or colder

 

[ebrodnax]

This is a similar question to the one previously discussed here. I have a Bosch 18 seer heat pump (Bosch BOVB-60HDN1-M18M 18 seer) paired with a Fujitsu 80% efficiency gas furnace (couldn't install a higher efficiency gas furnace due to venting constraints in a 100 year old row house). I'm trying to figure out the temperature at which I should force the heat pump to switch to the furnace. Cost of electric is $0.1648 per KWH, cost of gas is $1.605 per CCF. Any advice would be much appreciated.

 

[EbS-P]

This is a similar question to the one previously discussed here. I have a Bosch 18 seer heat pump (Bosch BOVB-60HDN1-M18M 18 seer) paired with a Fujitsu 80% efficiency gas furnace (couldn't install a higher efficiency gas furnace due to venting constraints in a 100 year old row house). I'm trying to figure out the temperature at which I should force the heat pump to switch to the furnace. Cost of electric is $0.1648 per KWH, cost of gas is $1.605 per CCF. Any advice would be much appreciated.

the correct answer I don’t have. You can ask over at hvactalk.com. The pros have spreadsheet tools for this. Why they don’t share is beyond me. My rule down here isonce heatpump needs to go into defrost I need to shut it off and fire the wood stove.

I’m going to guess is somewhere around 35 degrees
Estimate your cop here and plug into the cost calculator.

ASHP

ashp.neep.org

Residential Heating System Cost Calculator | Efficiency Maine

Our heating cost comparison calculator can help you estimate your annual home heating costs for different heating systems.

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5 ton unit is large. Do you have any resistance coils? What thermostat are you using? Unless you have a huge or drafty space you probably want to set the crossover temp based on comfort rather than cost. 5 degrees either way won’t change the cost much but running. Running gas at temps above 40 doesn’t use much gas.

 

[stoveliker]

Mine indeed starts defrost cycles at 35.
It killed me to hear that. So that is the absolute minimum T where I use the mini split.

 

[woodgeek]

Huh. The defrost thing is not that cut and dried. When its 'dry' outside, as in low dew point, defrost does not use that much energy. When it is wet (humid or precipitating) it does.

For my (old, low tech) HP I think the COP without defrost is still 3 in the 30s, and I would spitball that it is 2.2-2.4 under dry conditions, and closer to 2 under humid conditions (and maybe 1.5 under rare whiteout or freezing rain conditions).

Ofc, since stats switch on outdoor temp, and not outdoor dewpoint (which would make a LOT more sense), you could be leaving some savings on the table assuming COP = 1 or <1 as soon as defrost cycles start.

It also means that there is no one perfect crossover temp to be calculated, but a best guess.

 

[stoveliker]

That may all be true, practice in my climate and with my equipment has shown that defrosting does not happen above 35 here. Hence my decision to not run below that T.
Data trumps theory.

 

[woodgeek]

That may all be true, practice in my climate and with my equipment has shown that defrosting does not happen above 35 here. Hence my decision to not run below that T.
Data trumps theory.

My statements were based on cycle timing and usage measurements about 10 years ago. Not theory. The dry condition COPs I got matched that stated in the Goodman manual and were around 2 down to 25°F including defrost. With an old tech, non-inverter Goodman split system.

If your economic breakeven is at COP=2 or 2.5, then sure, 35°F is reasonable. If your breakeven is at a lower COP, you are leaving money on the table. Actual money, not theoretical money.

Ofc with woodburning, the breakeven is more preference, rather than financial.

 

[stoveliker]

Yep. My point was that my situation is cut and dry, contrasting with what you said, as these were my observations.

None of my heating costs me any variable costs (solar+mini split, and wood) and I was not talking about break even, not COP, I was talking about not wanting it to go through defrost cycles as one way or another it decreases efficiency (even if it's still good because "it doesn't use that much energy").

No offense meant or taken, I was only reporting that mine does not (ever) defrost above 35 F, whether it is foggy or crisp blue sky, and it does so below 35 - and I made the choice to not want that.

 

[EbS-P]

My statements were based on cycle timing and usage measurements about 10 years ago. Not theory. The dry condition COPs I got matched that stated in the Goodman manual and were around 2 down to 25°F including defrost. With an old tech, non-inverter Goodman split system.

If your economic breakeven is at COP=2 or 2.5, then sure, 35°F is reasonable. If your breakeven is at a lower COP, you are leaving money on the table. Actual money, not theoretical money.

Ofc with woodburning, the breakeven is more preference, rather than financial.

I think we’re getting down to the point where it probably only matters to very few people

A new uses asked a question I looked cop for equipment and plugged numbers into a cost calculator. My investigation said 35 was reasonable. No need to spend more time unit the OP comes back and says here is data and saving 20-30$ a year matter.

Yep. My point was that my situation is cut and dry, contrasting with what you said, as these were my observations.

None of my heating costs me any variable costs (solar+mini split, and wood) and I was not talking about break even, not COP, I was talking about not wanting it to go through defrost cycles as one way or another it decreases efficiency (even if it's still good because "it doesn't use that much energy").

No offense meant or taken, I was only reporting that mine does not (ever) defrost above 35 F, whether it is foggy or crisp blue sky, and it does so below 35 - and I made the choice to not want that.

That means your coil is only 3 degrees below ambient whenever the dew point is below 35. If and that really good engineering.