From HVAC to Heat Pumps - kWh usage

[STS-134]

As @STS-134 it doesn't directly add or remove moisture

Heat pumps cannot add moisture to the indoor air but they certainly can remove it. When operating in AC (when it is a desired and likely part of operation when the air is moist) or defrost mode, moisture can condense on the indoor coil (in practice this is very unlikely to happen in defrost mode because the heat pump has been heating the air, decreasing the relative humidity level, while the absolute humidity, i.e. the dew point, remains unchanged), and you'd need to cool the air below its dew point, which is generally the temperature of the outside air or lower, in order for condensate to form. If you are using a gas furnace as your auxiliary heat, this would never happen, because the furnace heat exchanger comes before the indoor coil and the air would first be heated and then cooled; it is possible but highly unlikely when using heat strips, which come after the indoor coil.

Of course, heat pumps can and do remove moisture from the outdoor air when they're in heating mode as they cool the outdoor air below its dew point and condensate forms on the outdoor coil. If the outdoor coil goes below freezing, the condensate will form as ice, which is the entire reason heat pumps need a defrost mode in the first place.

however to the extend that narrower band of temperature helps with air quality then it will help. The main thing with a heat pump is your not going to be swinging the temperature as much. The output temp is lower (which is better in my opinion).

That's more dependent on your thermostat than your heat pump. Most modern thermostats will let you adjust the temperature swing.

 

[Ahh9oolu]

- What should be the expected increase in kWh consumption? Based on this, will I still have "excess credit" for future additions like EV.
- Are heat pump systems effective? Meaning, is it sufficient to heat and cool the house to temperatures such as 70 for heat and 65 for cooling?

Another data point.

I am a little west of Sacramento. We remodeled a 2,300 square foot single story house and insulated it as well as we could (R15 in the walls, R60 in the attic). We went with a 5 zone Mitsubishi heat pump system and have been happy with it so far. It has no problem heating the house to 70 in the winter, though it is more energy intensive than I anticipated. Energy wise, it looks like this so far:

November	250 kWh
December	414 kWh
January (so far)	278 kWh

This winter, we have been leaving the bedrooms mostly off, so temps float around the mid to low 60s. We heat the public rooms to ~70 during the day, so that is the vast majority of our energy usage. If I wanted to heat the entire house to 72, it would probably double my energy usage.

 

[wwhitney]

This winter, we have been leaving the bedrooms mostly off, so temps float around the mid to low 60s. We heat the public rooms to ~70 during the day, so that is the vast majority of our energy usage. If I wanted to heat the entire house to 72, it would probably double my energy usage.

Doubling is not likely.

For a gross simplification, say half your house (the thermal envelope) is at 65 and half your house is at 70, and say the average heat loss rates (per deg F * sq ft) are the same in the two halves. If it's 50F outside, then one half has a 20F delta, and one half has a 15F delta. Your heat loss would 7/8 of what it would be if the whole house was at 70F. I.e. raising the temperature to 70F everywhere would increase your heat loss by 1/7, or 15%. Then if it's 30F outside, the effect is halved, so it would increase your heat loss by only 7%.

Cheers, Wayne

 

[h2ofun]

Another data point.

I am a little west of Sacramento. We remodeled a 2,300 square foot single story house and insulated it as well as we could (R15 in the walls, R60 in the attic). We went with a 5 zone Mitsubishi heat pump system and have been happy with it so far. It has no problem heating the house to 70 in the winter, though it is more energy intensive than I anticipated. Energy wise, it looks like this so far:

November	250 kWh
December	414 kWh
January (so far)	278 kWh

This winter, we have been leaving the bedrooms mostly off, so temps float around the mid to low 60s. We heat the public rooms to ~70 during the day, so that is the vast majority of our energy usage. If I wanted to heat the entire house to 72, it would probably double my energy usage.

I have 10 heads, 4 compressors. I leave 5 of them totally off and rooms closed. I have house downstairs 66 to 68, upstairs 70.
I use like 40 to 60kw per day I LOVE having a zero energy bill. Going to charge a friends Tesla today for free. Never can have enough solar.

 

[HardHitter]

Another data point.

I am a little west of Sacramento. We remodeled a 2,300 square foot single story house and insulated it as well as we could (R15 in the walls, R60 in the attic). We went with a 5 zone Mitsubishi heat pump system and have been happy with it so far. It has no problem heating the house to 70 in the winter, though it is more energy intensive than I anticipated. Energy wise, it looks like this so far:

November	250 kWh
December	414 kWh
January (so far)	278 kWh

This winter, we have been leaving the bedrooms mostly off, so temps float around the mid to low 60s. We heat the public rooms to ~70 during the day, so that is the vast majority of our energy usage. If I wanted to heat the entire house to 72, it would probably double my energy usage.

Are those numbers based upon just heat pump system only or total consumption for your house during those months?

 

[Ahh9oolu]

Are those numbers based upon just heat pump system only or total consumption for your house during those months?

Those numbers are just the heat pump. In general, the heat pump runs 30 to 50% of my energy consumption for those 3 months of heating.

 

[shs1]

There are heat pumps and heat pumps. Most if not all the discussion here seems to be about air source heat pumps where heat is moved from outdoor air into the house in the winter in heating mode, and from the house to the outside air in the summer in AC mode. Air source heat pumps start to struggle as a heater as the air gets to freezing or below and it is not ideal in the summertime when the outside air is really hot, e.g. above 100° F.

The other kind of heat pump is a ground source or geothermal heat pump that take advantage of the fact that about 6’ down into the ground the earth maintains a fairly constant temperature, e.g. 55 °F all year, rather than the outside air that is hot when you need it to be cool and vice versa. So, rather than trying to move heat to and from coils in the outside air, you use coils buried in the ground. There are a couple of ways of doing this depending on how much land you have, and in my case, my coils are located at the bottom of one of my ponds. Geothermal heat pumps are the most energy efficient way to heat your home and better that air source heat pumps in that respect.

We live in the Sierra Nevada foothills of California just at the snow line, and we currently have 6-8” of snow on the ground and nighttime temperatures are in the low 20s. (The ponds freeze over but it does not matter as the bottom of the ponds stays liquid.) My geothermal heat pump (WaterFurnace) transfers heat to and from our pond to hot water tanks in the garage which serve as our domestic hot water source. Our 4000 sq ft house is heated by radiant floors that circulate water that heated via heat exchange coils in one of the hot water tanks. The unit can also heat and cool air and our house also has a central air system that can be used to heat a room quickly in the winter or for AC in the summer. One neat feature of the system is that in the summertime, heat from the various rooms can be transferred into the hot water tanks rather than the pond so we sort of get either AC or hot water for free.

Fortunately, we did the geothermal and radiant heating (using Warmboard) when we built the house, as radiant as a retrofit would be difficult and likely less efficient. Obviously one can replace a central air-based heating and cooling system as well as a hot water boiler with a geothermal system much more easily.

We have 11 kW of solar and 3 Tesla Powerwalls as well as an 18 kW generator to run our all electric house as well as charge our 2 Teslas. Hard to say what the geothermal system takes in terms of kWh, but it takes 4-8 kW when running and may run 10-20% of the time in the winter.

 

[Zinc_Saucier]

Air source heat pumps start to struggle as a heater as the air gets to freezing or below and it is not ideal in the summertime when the outside air is really hot, e.g. above 100° F.

Your info is a bit outdated. Many air source heat pumps are perfectly fine going down well below 0F (see the Mitsubishi Hyperheat units), and how do they not work above 100F?

 

[shs1]

I agree that heat pumps are doing better now, including those now used on Tesla cars, but the heat capacity of air is pretty thin compared to the earth and so it is more of a challenge to for an air source heat pump compared to using the ground for the heat source/heat sink. I was just trying to point out that there is more than one type of heat pump.

 

[Vines]

I agree that heat pumps are doing better now, including those now used on Tesla cars, but the heat capacity of air is pretty thin compared to the earth and so it is more of a challenge to for an air source heat pump compared to using the ground for the heat source/heat sink. I was just trying to point out that there is more than one type of heat pump.

The latest generation of air source heat pumps are really quite efficient even at very cold temperatures. Other sources of heat are also good if you have them available.

For instance, Mr Cool published tables for their 12000 btu unit shows that at 17F outdoor, and 65F indoor it still has 8450 btu of heating available at a cost of 980W. At 5 degrees outside that drops to 6390 btu for 890W.
The 18000 BTU version does even better in the extreme cold, still giving 12130 btu at 5F for just 1.49 kW

 

[HardHitter]

I'm looking at my nest usage and it shows on cold days I'm running heat for about 2-3 hours max. My understanding is that heat pumps are generally something that will consistently run just at variable speeds depending on the temp swings but if I'm only using the heat 2-3 hours a day, would that suggest that the heat pumps are going to be running at the lowest kWh consumption throughout the day? In the example above, he references 1.49kW so if I'm only heating for 3 hours from a gas furnace, would one assume that would be an extra 4-5kWh/day as equivalent?

 

[CWhite]

I would think it has to be more than 5 kWh a day. You use at 3-4 therms a day given your 110 therm monthly usage. For me replacing 1 therm of gas uses about 6 or so kWh. Regardless of the exact figures I think the consensus it that a heat pump will always save you money in the mild climate that you live in. With a dual fuel system like I have you can always adjust the kwh vs therm usage depending on the price changes of each.

 

[DrSmile]

I'm having a hard time justifying the switch for my situation... I have gas heat that costs me ~$450 a month to heat my home in the Winter. It used to get pretty cold here in Northern NJ, but not so much lately... It seems to me that heat pumps have few financial advantages in really cold weather over a traditional gas fired furnace based on the KWh people report (70Kwh average, which here would cost over $14 a day. I have a split level ranch that is hard to heat/cool). So the real reason for getting one would be in the Summer for cooling, but as I produce excess power in the Summer from PV, I don't see the financial benefit there either. My attic space gets cold so I was considering getting a system to heat/cool this space, but an electric space heater and a window A/C unit are a much cheaper option?

 

[Xpert]

How would anything help with dry air unless it adds moisture to the air? A heat pump can extract moisture from the air (in cooling mode) as it condenses on the indoor (evaporator) coil. But when the indoor coil operates as a condenser coil, it doesn't add any moisture to the air. It only adds heat.

the context is comparing with gas furnace, heating up to the same amount of temperature, will it be less dry using a heat pump?

 

[STS-134]

the context is comparing with gas furnace, heating up to the same amount of temperature, will it be less dry using a heat pump?

Nope. Neither changes the dewpoint.

 

[arnolddeleon]

the context is comparing with gas furnace, heating up to the same amount of temperature, will it be less dry using a heat pump?

After some searching, the practial answer is it depends.

Heating the air doesn't dry it. However, if your gas furnace is not the sealed combustion type, and it is drawing its combustion air from your conditioned space, then yes, a gas furnace will have the effect of drying your air because it is taking your nice humid air and sending up the flue as part of the combustion process and that air needs to be replaced by outside air that is drier. With a heat pump (or a sealed gas furnace) you won't have this problem.

Here is one such article that explains things: Does a Gas Furnace Dry Out the Air in Your Home? - Energy Vanguard

 

[STS-134]

After some searching, the practial answer is it depends.

Heating the air doesn't dry it. However, if your gas furnace is not the sealed combustion type, and it is drawing its combustion air from your conditioned space, then yes, a gas furnace will have the effect of drying your air because it is taking your nice humid air and sending up the flue as part of the combustion process and that air needs to be replaced by outside air that is drier. With a heat pump (or a sealed gas furnace) you won't have this problem.

Here is one such article that explains things: Does a Gas Furnace Dry Out the Air in Your Home? - Energy Vanguard

That's a good point. I have furnaces that draw air from the space around the them but I would never put one in the conditioned space. They're all in garages or closets that are open to the attic/crawlspace and thus draw combustion air from outside.

 

[Xpert]

After some searching, the practial answer is it depends.

Heating the air doesn't dry it. However, if your gas furnace is not the sealed combustion type, and it is drawing its combustion air from your conditioned space, then yes, a gas furnace will have the effect of drying your air because it is taking your nice humid air and sending up the flue as part of the combustion process and that air needs to be replaced by outside air that is drier. With a heat pump (or a sealed gas furnace) you won't have this problem.

Here is one such article that explains things: Does a Gas Furnace Dry Out the Air in Your Home? - Energy Vanguard

that explains it, thank you!