The real difference in heat pump versus auxillary heat strips

  1. You need 400CFM per ton. A 5 tons would need 2000CFM.
  2. Always check the minimal OD temperature your heatpump is design for. Some newer heatpump can handle -30F. Those are equipped with a crankcase heater to eliminate liquid refrigerant from compressor base (like paul.goodjohn explained) . You mus look in the installation manual of your heatpump. In fact liquid refrigerant always migrate to the coldest location. It heat ( 30W heater usually) the compressor to evaporate liquid refrigerant using this crankcase heater.

My own system is a 2-tons heatpump York YHM. it can run down to -29F. My balance point ( the point where heat loss of the house > heat produce by the heat pump ) is around 7F. below 7F my heat strip pulse (kind of modulation) to maintain a supply temperature of 90F in the supply duct. The heat pump continue to run with the heat strip. This way when it is 5F outside my heat strip run at maybe 5% + heatpump. when it is 0F it runs at maybe 10% + heatpump. My heat strip is only 5KW… It is the first winter i run my system. you wouldn’t believe how much i saved compared to last winter when i was heating with electric baseboard only. I’m in Quebec City, Canada, so winter is extreme here compare to southern climate.


Thanks for the correction @jonhawkes I should have used cable.
Your are way undersized on duct for your return. It’s starving for air and is really hard on the equipment. It’s not just hard on the air handler but more so on your outdoor unit as there has to be a certain amount of air pushed through the indoor coil for effectiveness and efficiency. So whatever Seer rating your unit is, it is not operating at that efficiency rating.
You should really get your unit looked at. Twelve inch duct won’t take care of a 2 ton we’ll. A four ton needs one 20 or a 12 and another return (better to have more). With just one return your filter should be a 24x24. But even if you ha e the correct size filter but behind it the duct is small it’s still choking it off.
I think if you had this fixed that your comfort level would be better and overall energy cost lower. And this is t even going into the years that have already been taken away from the life of your system. Ductwork is usually fairly reasonable price wise as they can send out a tech instead of a licensed HVAC individual.
@HVAC_Marc is right. I ha e 3 ton and run down to 20 degrees.
One other thing is this was the problem at my wife’s store. A five ton with 12” duct. I added anothe return to get the CFM up to 1800 in July. The next month the bill was $100 less and is usually worse. It has been this way for 8 years while we rented and when whether was really hot the temp would not get below 75. Now it goes as low as we set it.



Do you have a reference for this? I believe it, however, my A/C and ducting people think that I have enough return air.



Sure @jonhawkes
this is all really easy stuff to find and the forum I read at and has helped
me most is called I’m not a member, that is how much
good information is there, haven’t had a need to.
There are some very general rules of thumb in HVAC work. One is the
requirement for airflow or CFM. In cooling mode it need about 400 CFM
per ton. At 4 tons, you need 1600. If you have 12’ round duct then that is
only 600 for supply. But we are talking about return and that’s only 450. So
your in the same boat as I was with our 5 ton unit. Keep in mind that both
the return (inlet side) and the supply (outlet side) need to be right. I like
having more return than I need. I’m betting you have enough or too much
supply and way too little return. Another important issue that goes with this
is the size of your return air grill, you need enough square inches. With my
3 ton I need at least a 20x20. What I have is 24x24. Overkill on purpose as
the grill size can be bigger and just adds filter area. The metal slats in the grill
and the filter slow things down.
Shoot, also note this about duct. The CFM numbers assume straight
duct without any turns or other encumbrances. Left side of pdf chart
is round metal and right side is Flex. Flex doesn’t carry near as well
Ask your installer these questions

Did you do manual J and D calculations? (they are separate)
Can I get a copy?
If these were not done then you could have the wrong size unit.
My house is small, 1575 heated and cooled. Three tons is all
I need and is slightly oversized.
I can already tell you he didn’t and I’m sorry for saying this,
You need another HVAC man. Licensed, insured with references.
Pic of CFM by round duct size and flex duct size.
Link is a good basic read on rules of thumb




I know i can find stuff in forums, however, I was looking for something more reliable.
My house is small 1500 sq ft duplex/semi, however, it only has a little insulation in the attic (none in the walls as far as I can tell). I am fixing the attic and was going to get the walls fixed but my wife vetoed the work.
I knew the return duct was too small and will chase up the ducting guy to get it fixed while it is cold and he doesn’t have much work on. I am even willing to pay for a rigid duct so that it is done right.

The 4 ton A/C is crappy, mis-matched and 15-20 years old. I know I have to upgrade to a non-R22 model in the next few years. If I get the house insulated then I could probably cut the size down to 3 or even 2.5 ton.


I had just suggested the forums for further reading. The link and picture provided
are reliable. The link has really good information with other information on that
site that is also reliable.
The walls only lose about 10-15% heat. The ceiling is another story. When I bought
the house we had 6", that’s R-19. after 2 years I spent all of $400 to blow another
6" cellulose over top making it around R-30. I got that $400 back in bill savings in
just over 3 months that winter. It took a friend and several hours to do it but Lowes
provides the machine for free if you buy so many bags of insulation.
If you have a mismatch where the air handler and outside are not completely
compatible then you are definitely not getting the Seer rating its supposed to be.
For your walls if you wanted, you could hire a Foam blower. I used to build schools,
jails and other huge block buildings and complexes. We had them come in and they drill
a tiny hold at each void, like 1/4", and ill top to bottom from it. It’s not like can foam and
won’t expand and cause damage. If I were to get a really small buildings walls filled like
maybe a convenience store, the cost was around $800 about 10 years ago. That was
when it was new and way before this stuff they spray in homes came around and got
popular. Its also not got the problems with moisture that the stuff they use in homes
I had my new heat pump put in October of 17. I got an American Standard/Trane unit.
Total cost was $4,700. That was for both units and had to rebuild the plenum for the
new air handler. This came with a 10 year warranty of part AND labor and if the
compressor goes out, the don’t replace just that part, the outside heat pump is
replaced as a whole. I had Rheem before and Like the Trane much better. Uses
less energy but is also a higher Seer.
There is nothing wrong with flex as long as its done properly, like pulling it tight instead of
sagging. I have a 20" flex as part of my return and a total of 13 -6" supplies.
I think your right about downsizing to 2.5 for sure


4 tons seems very oversize to me for 1500 sq.ft. Even if your duct are sized correctly, the system will reach setpoint too fast and you will not get proper dehumidification. 72F@60RH is not comfortable but 72F@45RH is good…

You have good hvac tech, they fix appliance. You have best hvac tech, they fix the whole system.


Thanks Sam. I think the foam people want a lot more money these days. like $5k.
I should still chase them up as we suffer a lot from outside noise (Most of the windows are new). This should help.

I have fixed half of the attic. We store stuff in the attic so I got this to raise the floor level

I think I will order another one to do more of the attic.
The 3.5" cellulose had already sagged. I replaced it with R13/15 bats between the joists and R30/38 running on top, in the opposite direction.We are closer to the R45-60 we should have.


The residential foam is different than what’s used in commercial construction. The residential is very expensive and takes 20 years to see a return on cost. The one I’m talking about is lower density and is designed for larger cavities where the other is sprayed on, usually an inch or so thick.
Think of low density being sprayed “in” for fill and the other bed by sprayed “on” for coverage.
That’s a nice kit. Did you notice it does not include insulation? When I did my attic I did the garage also so it was 2,000 square feet at 6” depth for about $400. They also have fiberglass option of self blowing for about 50% more


The kit only include the plastic brackets and metal runners, plus screws.
I already had a simple plywood wood floor so I just raised it.They were in 2’ by 8’ lengths (that could fit through the attic stair opening).


I installed my heatpump on June 24 2018, Jul/Aug/Sep are cooling month so KWH are higher than last year ( i did not have AC ), Nov/Dec/Jan are high heating month… if i had my heat pump locked out at 32F i would never get result like that. When someone tell me heat pumps don’t worth it i show them my results :wink:

Screenshot is in french, hope you guy will understand


I got your screenshot and my heat pump is effective down to 15. Not completely in its own it sometimes will have help from strips for a minute two if trying to maintain. If the temperature difference is 2 degrees or more then the strips help more than I want.


Surely the manufacturer specifies the temp ranges. Why are installers over ridding the recommendations?


Manufacturers don’t make it quite that easy. There are many variables and measurements that go into coming up with efficiency and effectiveness. Freon, pressures of different types and measurements like superheat to name a few.
What is happening is they are stuck with how things were twenty years ago when a heat pump was useless below 32-40 degrees. That just isn’t the case anymore. It also depends a lot on how well the installer has kept up with the literature and sadly, most have not.
The 5 ton we have is older than the 3 ton so the latter works down to 15 but the former is usually at those temps.
With a very good quality thermostat and outdoor temperature sensor, you can determine where it works to and then set the temperature wherever you want lockout to occur.
In case without, like my home. There is a kit installed for when it gets tool low and it doesn’t lock out until that 15 I talked about.


@jonhawkes It’s not “recommandation” from manufacturers but “operation limits”. The main reason why they don’t allow the heat pump to run to the limit is because they don’t want you to call them back. Homeowners usually forget routine maintenance like clearing the snow/ice under the outdoor unit. Plus, if the outdoor unit is on the ground, after several defrost operation ice will block drip holes under the unit. In that case they don’t want the unit to run below 32. Where i live every heat pump are on a rack 12 to 48" from ground to allow dripping in defrost. In the US (southern states where below 32 is less common), i think some installers put them straight on the ground.


I live in Kentucky and our installer did raise our unit up off the ground for both snow and grass. The last one was directly on the pad and was probably part of the reason for its short lifespan.
The unit I have determines on its own when to lock out the compressor. There is a thermostat that is placed in the lines and monitors the temperature of Freon, when it sees the heat pump can’t keep up then the strips come on. My thermostat will allow me to override this setting and temperature but I’ve been just watching what it does for now. My strips haven’t come on since last Thursday and last night we got down to 16 degrees.
The unit our our store is another story entirely as it is not effective in temperatures this cold.
I have called the HVAC installer about this and if this is how it should work. He said to let it do the job it’s designed to do.
Apparently they are just being made much more effective year to year.
The only complaint I have with mine is having 15kw heat strips and I do not understand the staging yet. It seems when they do come on that it’s all or nothing. My old system was 10kw and first 5kw would help and if it needed more and went to compressor lock out or emergency then it would call on the entire 10kw.


@samwooly1 I’ve never seen a thermostat sensing discharge, liquid or suction temperature of refrigerant being set up to change over from heat pump to electric heat. I’ll believe it when i see the electric diagram of this unit.

For your electric strips my guess is you have heat sequencer for 3 x 5kw or 1 x 5kw + 1 x 10 kw. Time is staging it. but heat sequencer stage real quick…


It’s not the thermostat sensing the lines, it’s an added board at the heat pump itself. It’s a small control board with a temp sensor zip tied to the line. It was an optional add on the installer put on without my knowledge.
My unit does not have sequencers as it’s a Trane/American Standard product. It has contacts instead.
I’ll see if I can find that diagram
I do remember him saying when the sensor detected Lower than 90 degree temp on liquid line is when it determined it couldn’t keep up anymore.