“What are the pros & cons of dual-fuel heat pumps vs. modern, inverter-based heat pumps?”

Trane makes a unit with a heat pump and a back up gas furnace that they call a “dual-fuel unit”. It does not really use two fuels, it uses electricity to run the heat pump and uses natural gas to run a furnace as a back up for when the heat pump can’t keep the house at the temperature set on the thermostat. This type of unit makes sense in new construction. In existing houses, they already have the furnace so adding the heat pump gives you the same result. In NE you really need this set up. In NC you could probably get away with a cold-climate heat pump and no furnace. If you get one or two days when it is too cold for the cold-climate heat pump you can always use an electric-resistance fan heater to top it up. 

Heat pumps and defrost mode in winter. Heat pumps and a back-up furnace.

These first four answers are provided by Bob Zogg the head of the Heat Smart Alliance:

Q1. Do all ASHPs’ have a defrost mode?  

A1. Yes, unless installed in a climate where it doesn’t get cold enough for the outdoor coil to frost.

Q2. Is it conventional to use the auxiliary heater to defrost the HP, or is it most common to simply reverse the flow using the AC mode to draw heat from the house to defrost the outdoor HP?  

A2. To the best of my knowledge, all ASHPs reverse cycle to perform defrost.  (I assume that’s the easiest and cheapest way to do it.)  Central, ducted heat pumps typically operate the supplemental heating system (furnace or electric resistance coils) during defrost to prevent cold blows.  Ductless heat pumps do not have a supplemental heat source, so they rely on the thermal mass of the indoor units to provide heat during defrost.  (I’ve never figured out how ductless systems get away with this, while it seems that central, ducted cannot.)

Q3. Is the method of defrost left up the installer, the smart thermostat, or the OEM equipment manufacturer?  

A3. In general, it’s probably a combination of all three.  Some installers may leave default settings as they are, and other installers may tweak the defaults.  For my heat pump (central, ducted with supplemental resistance coils), the defrost is based on compressor runtime (adjustable between 30 and 120 minutes), but it does not attempt to operate if the outdoor temperature is sufficiently high that frost won’t form.  The supplemental resistance heat runs full blast during defrost, and this is NOT adjustable through the t-stat.  I didn’t deal with this serious energy waste for many years.  The installer set the defrost time for 30 minutes (or that was the default and they didn’t change it.)  I finally changed it to 120 minutes.  That leads to greater frost build-up on the coil and sometimes longer defrost periods (perhaps 10 to 12 minutes instead or 5 – 6 minutes), but I think lower energy use overall.  

I then opened up my air-handling unit and disconnected the resistance elements, but that led to cold blows (the blower runs at fairly high speed during defrost—don’t ask me why).  So, I convinced an electrician to put switches on each of the 5 kW resistance elements (2 elements totaling 10 kW).  Normally, I leave one 5 kW element on, and the other off.  We don’t have any cold blows.  If we travel, I switch both elements on in case the heat pump fails while we are gone.

Q4. Do you see an advantage to one form of defrost over another?  

A4. If cold blows can be avoided, there’s probably an energy advantage of using no supplemental heat during defrost.  Ideally, the controls would be smart enough to use just enough supplemental heat during defrost to avoid a comfort problem, but no more than that.  This should be easy to do with electric resistance, but, it doesn’t change the rated HSPF, so I guess manufacturers just don’t bother.

I raise these questions because my Bosch/Ecobee system uses the auxiliary heat (oil furnace) to defrost the HP. The furnace comes on for a short cycle (~5 minutes) in defrost mode which can occur multiple times per day. On average the defrost mode uses 4-5 gallons of oil in the winter months. I mentioned this to David Green (Zero Carbon Home) who also has the same 5-ton Bosch ASHP as me but his unit uses indoor air to defrost not his back-up oil furnace, so he says he doesn’t use any oil for defrost.  

You can reach the Heat Smart Alliance’s website here: https://heatsmartalliance.org

Q5. And this is my answer to the last question about how to get the highest efficiency from a heat pump when installed with either back-up electric resistance heating strips or with a back-up fossil fuel furnace.

A5. On the questions: the algorithms in the thermostats/brains of the heat pump are really not very good and, if there is a back-up furnace, often lead to short cycling of both the HP and the furnace in the spring and fall which kills the efficiency of both. So, I do not operate mine this way. I switch off the oil-burner circuit at the electrical breaker panel and leave it off almost all year round. This forces the heat pump to operate on long cycles when it is more efficient. It is almost like the heat pump is my only heating source. Usually, about mid December each year, I come down in the morning and the house is at 65F with the thermostat at 70F. This happens when it is about 20F out. I turn the thermostat up to 75F or even 78F and this keeps the house at 70F until the outdoor temp is about 10F. Below that I turn on the oil-burner breaker and the furnace comes on. I run it for about an hour or two to get the temp up to 70F then turn it off again. This works unless we have several days of arctic cold (say minus 10F, like this February) and for times like this I leave the furnace on. This strategy has reduced my oil use to between 10-30 gallons a year. Before my Fab Four retrofit I was using 3,000 gallons a year and when I let the thermostats run both the hp and furnace it was about 300 gallons a year. Without the furnace, my house would probably get to 60F in arctic-cold weather. If I closed off some rooms and vents I might be able to keep the kitchen at 65F. In this case a small electric fan heater might actually be a better way to do it rather than the furnace but I have the furnace so that works for me. 

My concern with installed electric back up heating elements (like Bob has) it just the cost. They are very expensive to run. And I doubt that this electricity is included in the HSPF calculation, hence the economics of the heat pump with back up electric heating elements is likely not as good as advertised. 

Our Bosch hp does occasionally go into cooling mode in winter (I have noticed it only once in 7 years) and this is how it defrosts itself. It is also how it dehumidifies the air in winter if it needs doing. I set the humidity at 40% year round. It does not use supplemental heat from either the furnace (because it is off) or back up electric-resistance heating elements (we don’t have any).