Heat Pumps-Need some splaining

[scrinch]

We've recently contracted to replace our 17-yo solar panels and inverter with a new system to start a new 20-year NEM2 contract with PG&E in northern California. We are doubling the size of our previous solar installation (which served our needs under NEM1) in order to accommodate an EV and a heat pump in addition to our current loads.

We have a 16-year-old natural gas furnace that is currently working okay, but it has had several problems in the past few years that cost several hundred dollars to fix, so we think it is reaching the end of its life. We have never had AC in the house, but the heat last summer convinced us that it may be time to invest in it now. We had 15 days over the last summer when the indoor temperatures approached 90F despite our cooling the house down overnight and buttoning up tight and using shades during the day.

So prior to replacing the HVAC system I've been trying to learn about heat pumps. I know that there are whole house systems and mini-splits. We have all the central heating ducts in place, so a whole house system seems the way to go for us...possibly even a dual fuel system. And the sizing and efficiency numbers seem to be straightforward enough. What I don't quite get is the single stage/double stage/variable speed stuff. Do these terms refer to the air blower or the compressor? And what goodness do they provide...energy efficiency, cost efficiency, mechanical efficiency, soft start capability, ability to fine-tune the heating/cooling output? I have googled these things and mostly come up with websites that are marketing a product. The references to those terms are more sales and jargon than actual explanation. Can you shed some light on what multi-stage and variable speed mean wrt heat pumps? What I ultimately want is a system that doesn't have a huge start-up amperage and that can effectively heat our 2000 sq ft house when it is down into the mid-20's outside, which is where our overnight winter lows usually fall.

 

[Wheel]

You'll definitely want a dual fuel system. Heat pumps aren't that efficient at heating when the ambient temperature is in the 30 degree range.

 

[Flyfish1]

You'll definitely want a dual fuel system. Heat pumps aren't that efficient at heating when the ambient temperature is in the 30 degree range.

I second that.
Our home in Coastal NE gets some cold nights and the radiant floor propane heating system struggles to adapt. Usually the AC/heat pump system installed in 2019 will kick in to cover the deficit , but on extremely cold ( <10 F) it struggles as well. So , don't follow my lead on systems, but agree with dual sources.

 

[steveark]

Engineersplaining

The variable speed, multistage jargon refers to the compressor and the more you spend the more energy efficient the heat pump/AC will be. Here in the south heat pumps have back up resistance electric heating coils usually rather than a back up natural gas furnace. My home has a split unit and a whole house unit as well as a third natural gas furnace/AC unit.

 

[38Chevy454]

The compressor and the blower fan can both be variable or various speeds. Basically more speed ability is more cost and more potential repair. But also little higher efficiency.

Agree on having nat gas backup for vfc when it gets real cold. My heat pump switches over to propane at 25F.

 

[folivier]

There has been lots of progress in the last couple years on heat pumps. You can get heat pumps that work efficiently at a much lower ambient temperature now. Worth looking into these low temp units.

 

[frank]

there is a lot of talk out there about different stages of heating and cooling efficiency with the new hvac units. I have a single stage furnace and a/c unit that is 20 years old and it has heated and cooled my house without fail. An hvac guy that is a friend told me the more stages and electronic devices on a furnace and a/c unit the more chance there is that it will break down. If I were you I would get the simplest gas furnace and a/c unit on the market with the longest warranty and go with that. the temps where I live run from upper 90's in the summer to -30 in the winter. I don't know anything about heat pumps, but it seems to be redundant to require two or three systems to heat and cool the same area.

 

[retire48in2018]

We’ve had heat pumps in Texas border, 2 places in Tennessee, 2 places in NC and 2 places in Illinois.

If outside air is much below 20, they don’t work well. Below 32 - maybe.

Depending on size (measured in tons) - they deliver heat (or cooling) to home. Factor in home size (square footage) and how well it is insulated, direct sunlight through windows (sun heating home), etc - adds complexity to how big a system.

I would advocate for a larger system than math (or contractor) specifies. Why? During cold days or hotter days - is when system is pushed to limits. But - that’s when you want it, most. So, if your system is sized perfectly (and perhaps home is not perfectly insulated) and outside temp is below 32, it might take more than 30 min to increase whole house by 1 degree - and below 20 - it won’t keep up. Extremely well insulated homes, and little direct sunlight (when cooling)give a little more margin and can size units a little smaller.

Stages -
There is the heat pump for heat (1), for cooling (2) and usually some kind of auxiliary system for additional heating (3). Commonly that auxiliary system is electric heat strips. They are usually sized in KW. This auxiliary system is VERY inefficient and as a result expensive. In colder climates, that’s when people make the auxiliary be a gas furnace or something similar.

Heat pumps have 2 parts - the outside “box” (compressor is there) and inside box - (air handler). In more recent years, more work has been on that air handler part - variable speed, and other mechanisms assisting air flow. For example, slowly ramping up air flow, helps heat a little better. That’s the “variable speed”, etc

When heating pump compressor activates - they can be loud. In more recent years, efforts have been made to try to reduce this noise. However, regardless of what marketing says - you don’t want that outside box (compressor) near a bedroom, especially on the other side of a wall.

Bottom line -
If you can locate heat pump compressor away from bedrooms, and you appropriately size (larger than calculator IMO) it will work below 32, and marginally or not at all below 20. You will need another system to heat below 20. Cooling - if you have lots of windows allowing direct sunlight and/or not perfectly insulated home - again upsize unit.

 

[retire48in2018]

That was all related to air source heat pumps. I have no experience with geothermal heat pumps ( used ground instead of air as heat/cool source).

 

[mpeirce]

The compressor and the blower fan can both be variable or various speeds. Basically more speed ability is more cost and more potential repair. But also little higher efficiency.

If you occasionally are running on backup power, variable speed is nice in that it doesn’t surge nearly as much when turned on. It comes on slow and ramps up as needed.

 

[mpeirce]

If outside air is much below 20, they don’t work well. Below 32 - maybe.

If by “they” your mean your old units, that may well have been true, but many modern heat pumps work well into the sub zero °F range today.

 

[Piper20]

Agree. Todays VS HP are NOT like the old ones you keep talking about. I’ve had one since 2018. 95-100 degree air at -5 and resistance heat is not on. Heat strips are set for -10. Has not been that cold since it was installed.
All this “it’s only good to 30 degrees” is wrong. Yesterdays HP aren’t todays HP.

 

[Chuckanut]

I've been looking at mini-split heat pumps lately to augment my electric heating which has performance issues when the temp drops below 40 degrees. And to add some AC for the two or three weeks of 90+ weather we have during the year. According to my research the normal heat pump works well at 40 degrees where the dropoff starts. It still keeps 70% of its heating capacity down to 20 degrees. I can get one that has 70% at 0 degrees, but it's bigger, more expensive, and uses more electricity whenever it runs. Below 20 degrees is rare in my area and I am keeping my current electric heaters. So, I feel OK with the 'normal' unit. If I ripped out my electric heaters, I would get the bigger 0 degree unit.

 

[retire48in2018]

I recently had 3 of the best brands in this area - none work much below 20 without auxiliary heating. (Heat strips in air handler).

If you are including this auxiliary stage - they can go colder.

They all said going below 20 without auxiliary heat is very suspect.

 

[retire48in2018]

If there is something out there - air based heat pump working down to 0 - please send me a link (excluding heat strips, backup furnace, etc).

I could not get any one to quote that.

Many state they can get to -4 - but that’s with the auxiliary heat strips

 

[Iusta]

I have two electric variable speed blowers and 2 electric air source heat pumps for 4,000 sf. I have 34 solar panels and two EV’s. I live in Southern California and we still pay 4,500 per year in electricity costs. The installers told me not to use electric and go with gas units because of the cost of electricity vs. gas. I didn’t listen and tried to be Mr. Tree Hugger and went with electric. Bad decision. Use gas units.

 

[Piper20]

If there is something out there - air based heat pump working down to 0 - please send me a link (excluding heat strips, backup furnace, etc).

I could not get any one to quote that.

Many state they can get to -4 - but that’s with the auxiliary heat strips

Lots of info. Last page summary design temp in heating mode -15.

https://www.shareddocs.com/hvac/docs/1009/Public/06/25VNA4-02PD.pdf

 

[Texas Proud]

Be careful with the suggestion that you buy a bigger AC unit than what is calculated for you... if you buy a single stage you will be very disappointed...


The problem is they cool your house down too fast... without taking out the humidity... so it feels muggy and you lower the temp... which does the same again... my BIL had a bigger unit and moisture would form on the walls etc. as the humidity stayed in the house all the time...


I would think a variable unit would work if oversized... At times mine is only cooling at a 25% range.. they tend to run a lot longer as they try and keep the temp in a more narrow range...

 

[retire48in2018]

Here is what carrier gave me on their quote

https://www.shareddocs.com/hvac/docs/1009/Public/0B/OM25VNA8-03.pdf

Last paragraph on bottom of page 2 - goes to
Auxiliary heat at 10 degrees

This looks like a slightly different model than earlier link - I’ll have to ask them more

Thanks

 

[Piper20]

The 18VS is a 5 speed. The link I included was a variable speed

 

[Time2]

You'll definitely want a dual fuel system. Heat pumps aren't that efficient at heating when the ambient temperature is in the 30 degree range.

The OP is in the San Fransisco area, the lowest temps there are above 40*F. I don't think they need a dual fuel heat pump. I'm in NW Florida, it gets colder here and we have not had the supplemental electric heater strips turned on for over 10 years and the heat has been fine.

 

[scrinch]

That may be the temps in San Francisco right by the water. I live an hour north, and my weather station shows about half of the nights in Jan-Mar have been below freezing, with the lowest at 23F.

 

[Time2]

That may be the temps in San Francisco right by the water. I live an hour north, and my weather station shows about half of the nights in Jan-Mar have been below freezing, with the lowest at 23F.

OK, I searched North Bay and it kept telling my it was outside San Fransisco, after it said it was in Canada!
So, dual heat may be needed. The installers would have the best data.
But even so, the electric heating elements will do the job, just not as cheap as natural gas.

 

[scrinch]

Okay, thanks all for the great input. What I'm seeing is that both the indoor air handler fan and the outdoor compressor can be variable speed, but usually when used in conjunction with "heat pump", the term "variable speed" refers to the compressor. It seems there are single speed, two speed, multi-speed, and variable speed compressors. The additional speeds in the compressor allow operation at lower heating/cooling rates which can be more energy efficient and allow for better air filtering and dehumidification. But more speeds also mean greater complexity which lead to higher cost and potentially lower reliability. Oversizing a heat pump for greater heating/cooling capacity is probably not a good idea unless the system is multi-speed and can operate at partial load under conditions when all that heating/cooling capacity isn't needed.

I wonder if maybe a two speed system might be a good compromise between simplicity and flexibility, where the high speed can be used for high demand situations, and low can be used for temperature maintainance once the desired temperature has been reached? I think I'll try to find reliability data (CR?) for single, dual, and multi-speed heat pumps.

 

[Time2]

I suspect you don't have the same humidity problem we have in Florida.
Here over sizing an air conditioner means it runs less and doesn't remove enough moisture from the air. Humidity control makes a huge difference in comfort to me. If I can get humidity down to 40% or less I can raise the temperature 3 or 4*F and still have the same comfort level.
I have run a separate dehumidifier to help lower humidity and it was great except for one fact, the area where the dehumidifier was running got heated up by the dehumidifier.