Dual fuel (heat pump with auxiliary gas furnace) - what settings for maximum efficiency and/or cost savings?

[Anonymous]

We got a heat pump (a Lennox XP20) with an auxiliary gas furnace this year and I'm a little unclear on the following:

1. In the winter, should we keep the temperature exactly the same throughout the day and night? Or can we lower it a couple degrees overnight/during the workday?

2. The default settings are for the furnace to start "helping" the heat pump when the temp drops below 50 degrees and for the furnace to work on its own below 25 degrees. Do people just use trial and error to figure out the best settings?

3. In the summer, if a heat pump is in cooling mode can we use it like a regular AC (e.g. turn it off or turn up the thermostat when we are not home)?

[Anonymous]

We have a new heat pump with auxiliary gas furnace. My understanding is that with ours, the Daikin, it's better to just leave it at the setting we really want it at. But there are some good youtubers who might have better answers for you.

[Anonymous]

I'm struggling to see the point of a heat pump that needs "help" at air temps of 50 degrees.

Might as well just have a gas furnace.

[Anonymous]

Anonymous wrote:We got a heat pump (a Lennox XP20) with an auxiliary gas furnace this year and I'm a little unclear on the following:

1. In the winter, should we keep the temperature exactly the same throughout the day and night? Or can we lower it a couple degrees overnight/during the workday?

2. The default settings are for the furnace to start "helping" the heat pump when the temp drops below 50 degrees and for the furnace to work on its own below 25 degrees. Do people just use trial and error to figure out the best settings?

3. In the summer, if a heat pump is in cooling mode can we use it like a regular AC (e.g. turn it off or turn up the thermostat when we are not home)?

I'm the PP and did some YouTube research based on this question. You should check it out. They said it's fine to turn off the A/C in the summer when you're not home.

Our gas furnace only kicks on at 15 degrees or so, so I'm not sure what to tell you about best settings. But maybe a video is out there about your particular furnace?

[Anonymous]

ughh heat pumps are the biggest scams, just get a furnace if you have gas

[Anonymous]

Mech engineer. Set it to whatever temps you want to be comfortable. Let it make its optimization calculations.

[Anonymous]

OP here. Thanks! Just to clarify, the settings for when the furnace kicks in can be changed. It's not that the heat pump "needs" the furnace. I think it's more of a preference if you want to use gas or electricity or a combo. We can set it to kick in only below 15 degrees if we want to. I don't know what is best. Will look for YouTube videos but maybe this is a trial/error thing.

We are very happy with the heat pump so far, though of course it is early days. We used to have a lot of cold spots in our townhouse and big differences in temperature between floors in the winter. Now it feels very even and comfortable. And maybe less dry as well. Costwise I do get that heat pumps don't make as much sense if you have natural gas, but the federal tax credit helps.

[Anonymous]

This site:
https://ashp.neep.org/#!/product/34047/7/25000///0

Has detailed performance specs for that unit.

There are two questions: first is whether the unit has sufficient heating capacity, and the second is which is more economical to run, gas or heat pump.

The answer to the first depends upon what the heating load for your house. On that page you can put in your zip code and the heating load for your house (which your contractor should have calculated) and it will tell you how many hours a year of supplemental heat you'll need and at what temperature you will start needing supplemental heat.

If your contractor didn't calculate a heating load, or it's not available to you, this article tells how to estimate your heating load based on past energy usage and weather:
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler



The answer to the second depends upon your local cost of electricity and gas, and the coefficient of performance (COP) of your heat pump. The COP declines as the temperature drops. The page I linked to above shows COP by temperature.

In our climate heating loads are generally higher than cooling loads, if a heat pump was sized for the cooling load it's going to need to be supplemented on the coldest days. The unit you have is rated for 33,800 BTU/hr of cooling at 95F, but only 20,600 BTU/hr of heating at 17F.

[Anonymous]

Anonymous wrote:This site:
https://ashp.neep.org/#!/product/34047/7/25000///0

Has detailed performance specs for that unit.

There are two questions: first is whether the unit has sufficient heating capacity, and the second is which is more economical to run, gas or heat pump.

The answer to the first depends upon what the heating load for your house. On that page you can put in your zip code and the heating load for your house (which your contractor should have calculated) and it will tell you how many hours a year of supplemental heat you'll need and at what temperature you will start needing supplemental heat.

If your contractor didn't calculate a heating load, or it's not available to you, this article tells how to estimate your heating load based on past energy usage and weather:
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler



The answer to the second depends upon your local cost of electricity and gas, and the coefficient of performance (COP) of your heat pump. The COP declines as the temperature drops. The page I linked to above shows COP by temperature.

In our climate heating loads are generally higher than cooling loads, if a heat pump was sized for the cooling load it's going to need to be supplemented on the coldest days. The unit you have is rated for 33,800 BTU/hr of cooling at 95F, but only 20,600 BTU/hr of heating at 17F.

Thank you! This is so helpful. Yes our contractor did not want it to be oversized for cooling, this was one of the reasons they recommended dual fuel. I had forgotten about that.

[Anonymous]

Anonymous wrote:

Anonymous wrote:This site:
https://ashp.neep.org/#!/product/34047/7/25000///0

Has detailed performance specs for that unit.

There are two questions: first is whether the unit has sufficient heating capacity, and the second is which is more economical to run, gas or heat pump.

The answer to the first depends upon what the heating load for your house. On that page you can put in your zip code and the heating load for your house (which your contractor should have calculated) and it will tell you how many hours a year of supplemental heat you'll need and at what temperature you will start needing supplemental heat.

If your contractor didn't calculate a heating load, or it's not available to you, this article tells how to estimate your heating load based on past energy usage and weather:
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler



The answer to the second depends upon your local cost of electricity and gas, and the coefficient of performance (COP) of your heat pump. The COP declines as the temperature drops. The page I linked to above shows COP by temperature.

In our climate heating loads are generally higher than cooling loads, if a heat pump was sized for the cooling load it's going to need to be supplemented on the coldest days. The unit you have is rated for 33,800 BTU/hr of cooling at 95F, but only 20,600 BTU/hr of heating at 17F.

Thank you! This is so helpful. Yes our contractor did not want it to be oversized for cooling, this was one of the reasons they recommended dual fuel. I had forgotten about that.

From my eyes as a contractor your contractor had a clue about what they are doing. The bulk would have installed an oversized heat pump. The 25 degree switchover point to gas is a good starting point. You can probably go lower but unless we have an unusually long cold spell or natural gas prices multiply your talking savings measured in the tens of dollars over a winter.

[Anonymous]

Heat pump switchover can be much lower than you’d think. In Canada they use heat pumps at very low temps. That said, if you set it too low you may actually be using more energy as the heat pump struggles to recover indoor temp. 20-25 degrees is certainly an easy default for most systems.

[Anonymous]

For efficiency, you have to balance your cost of gas and cost of elec against your specific appliances and house. Generally, figure out where the heat pump begins to struggle then supplement with gas. For my parents house, it was around 35F, but that was 30 years ago in a different climate zone.

For higher and lower temps across a day, that depends so much on how consistent your schedule is and how your house is set up. The real benefit when turning the heat down is how long it's at equilibrium at the lower temp before it changes back - that's where the savings are. The cost to heat back up was your savings from letting it cool down - those are about a wash. (Some say it "stresses the system" too much to do this, but I don't buy that argument outside rare cases of worn out systems. Programmable thermostats aren't some conspiracy for the early demise of HVAC units)

We have two zones at our house: basement/first in one and second floor in the other - and have kids off at college. All bedrooms are on the second, home office is in the basement. Our schedules have one or both of us working from from home for all but three mornings a week.

In the winter, we have our first floor at 72 from 6am until 9pm, then it's 62 overnight. It doesn't really cool off much until after 10:30pm or so unless it's really cold out. The second floor is at 62 from 8am until 8pm - we really don't spend time there during the day and with heat rising, it generally doesn't run the heat unless (again) it's really cold out. The second is set for 68 overnight and 72 from 6am to 8am weekdays and 7am to 9am weekends.

In summer we don't do quite as much temp differential due to A/C dehumidifying. The first is 72/76 (day/night) and second is 74/70 (again, day/night).

These change during holidays, especially when the kids are home and both zones are used 20+ hours a day.

[Anonymous]

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:This site:
https://ashp.neep.org/#!/product/34047/7/25000///0

Has detailed performance specs for that unit.

There are two questions: first is whether the unit has sufficient heating capacity, and the second is which is more economical to run, gas or heat pump.

The answer to the first depends upon what the heating load for your house. On that page you can put in your zip code and the heating load for your house (which your contractor should have calculated) and it will tell you how many hours a year of supplemental heat you'll need and at what temperature you will start needing supplemental heat.

If your contractor didn't calculate a heating load, or it's not available to you, this article tells how to estimate your heating load based on past energy usage and weather:
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler



The answer to the second depends upon your local cost of electricity and gas, and the coefficient of performance (COP) of your heat pump. The COP declines as the temperature drops. The page I linked to above shows COP by temperature.

In our climate heating loads are generally higher than cooling loads, if a heat pump was sized for the cooling load it's going to need to be supplemented on the coldest days. The unit you have is rated for 33,800 BTU/hr of cooling at 95F, but only 20,600 BTU/hr of heating at 17F.

Thank you! This is so helpful. Yes our contractor did not want it to be oversized for cooling, this was one of the reasons they recommended dual fuel. I had forgotten about that.

From my eyes as a contractor your contractor had a clue about what they are doing. The bulk would have installed an oversized heat pump. The 25 degree switchover point to gas is a good starting point. You can probably go lower but unless we have an unusually long cold spell or natural gas prices multiply your talking savings measured in the tens of dollars over a winter.

Second this opinion. Oversizing of cooling is rampant, it causes a lot of comfort problems.

[Anonymous]

Anonymous wrote:
The real benefit when turning the heat down is how long it's at equilibrium at the lower temp before it changes back - that's where the savings are. The cost to heat back up was your savings from letting it cool down - those are about a wash.

Right, if you don't run the heat for a few hours, you lose somewhat less heat by having the difference between your interior and outside be lower. But to the extent the house itself cools off, you have to put that heat back when you warm the house up again so that's a wash.

With heat pumps though it gets a little trickier, because the efficiency of a heat pump varies depending upon the outside temperature. The classic setback thermostat program is to turn the heat down when you go to bed, and have it come back on an hour or two before you wake up. But that's typically the time of day when it's coldest outside, and the heat pump is least efficient. So you may not even end up saving money because you run for fewer hours, but at lower efficiency.

Note that for cooling it can be the opposite. If you turn off the AC when you leave in the morning, and have it turn on again a few hours before you come home, you avoid having it run during the hottest part of the day, when it's least efficient. Also, AC gives better dehumidification if it runs continuously, having it run straight for a couple of hours will give better comfort than the same amount of cooling coming from cycling on and off during the day.