A Simple Way to Calculate Heat Pump Balance Point

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At the balance point, a heat pump can supply the same amount of heat as a house needs

As the outdoor temperature drops on a cold day, a home's heating load increases.  At the same time, decreasing temperature means less heat in the air for a heat pump to pump indoors.  And at one special temperature, the capacity of a heat pump equals the heating load in the house.  That temperature is called the balance point.  In my recent article about the three types of heat associated with heat pumps, I mentioned balance point but didn't explain how you might go about finding it.  Let's do that today.

I'll keep it simple.  All we need are three numbers: 

  1. The heating load of the house at the outdoor 99% design temperature
  2. The heating capacity of the heat pump at 17 °F
  3. The heating capacity of the heat pump at 47 °F

Then we'll make an assumption about a fourth number:  the temperature at which the heating load of the house is zero.  We'll take that to occur at 65 °F.

So now we have four numbers, two for heating load of the house and two for heating capacity of the heat pump.  We can plot the two load numbers on a graph of load versus outdoor temperature.  We'll then put the two capacity numbers on the same graph. 

Next, we simplify things and assume the relationship between load and temperature is linear and that the same linear relationship holds for capacity and temperature.  Let's see what such a graph might look like.

A New Jersey example

We recently did a load calculation for a home in New Jersey that works well for illustrating how to find balance point with this method.  The outdoor 99% design temperature for their location is 17 °F.  The heating load for the house (~1,800 square feet) is a little over 15,000 BTU/hr.  For this exercise, let's look at a single-stage, fixed capacity heat pump with a nominal capacity of 18,000 BTU/hr.

There are two ways to find the capacities of this heat pump at the temperatures of 17 °F and 47 °F that I mentioned earlier.  You can get them from the manufacturer's data or the AHRI Directory.  In this case, I used the AHRI Directory and found the capacities to 10,700 BTU/hr and 17,800 BTU/hr at 17 °F and 47 °F, respectively.

Plotting those two linear relationships for load and capacity yields the following graph.

Heating load and capacity vs. outdoor temperature for a 1.5 ton heat pump

The orange line above is the heating load.  As the outdoor temperature drops, the house needs more and more heat and the orange line rises.  The blue line is the heating capacity of the heat pump.  As the outdoor temperature drops, the capacity of the heat pump drops because there's less heat to extract and pump indoors. 

Where the two lines cross is the balance point.  At that temperature, the heat pump is just able to keep up with the how much heat the house needs.  The balance point temperature is about 25 °F in this case.

The significance of balance point

What I've done here is shown a simple way to find an estimate of the balance point based on the heating load calculated for the house and the specifications for the heat pump installed or being considered.  The actual balance point is probably a bit different. 

But what it tells you is that when the outdoor temperature goes below the balance point, you'll need some kind of supplementary heat because the heat pump by itself won't be able to keep up.  If your supplementary heat is the common and expensive electric resistance strip heat, you want a low balance point so you don't have to use the supplementary heat as much. 

Now, you've also got to balance your heating capacity with your cooling needs, and especially in a humid climate, cooling loads generally determine the size of heat pump you install and thus where the balance point will be. 

In my next article, I'll look at two ways to change the balance point.  In the meantime, let's start the discussion of finding the balance point in the comments below.

 

Related Articles

The 3 Types of Heat From Heat Pumps

Finding Balance - Heat Pump Heating Load vs. Capacity

We Are the 99% — Design Temperatures & Oversized HVAC Systems

 

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Comments

Is there an assumed indoor temperature?

Allison
Bailes

Bob, yes, both the capacity and the load depend on the indoor temperature setting.  The indoor design temperature for the ACCA manuals is 70 °F.  If you use 65 °F, the heating load goes down and the heat pump capacity goes up.  At 75 °F, it's the reverse.  Another factor that affects the heat pump capacity is the air flow setting.  The 1.5 ton heat pump in the above example has three air flow settings:  525, 600, and 675 cubic feet per minute (cfm).   As the air flow increases, so does the capacity.

The question of balance point may be a bit more nuanced:

1) Cold weather is often accompanied by above average wind speed, increasing heat loss
2) Heat pump heat capacity may be non-linear between the two rating points - 47 and 17*F. Below 40 degrees, depending also on outdoor dewpoint, heat pumps must periodically defrost themselves - briefly reversing into cooling mode to melt accumulated ice

3) Somewhat related to #2 - cold rainy weather can put a real hurt on a heat pump, particularly if its design and installation allow raindrops to strike the coil fins Some designs handle this better than others.

I'm not sure why I'm in the CAPTCHA dog house, but obviously I can't compose comments and then get booted off.

Curt

Allison
Bailes

Curt, yes, you're absolutely right.  It's definitely more nuanced for the reasons you mentioned plus what Dana Dorsett and David Butler have added below.  I mentioned actual balance points being different and will go into more in a future article.

The AHRI pages don't show the maximum capacity of most mini-splits at either +47F or +17F, only the "rated" capacity at which the efficiency was tested at those temperatures.

For instance the MXZ-5C42NAHZ can deliver 48,000 BTU/hr @ +17F at maximum speed (which happens to be the level at which it was tested at +47F as well as it's maximum at +47F), but the capacity at +17F listed on the AHRI sheet is 36,600 BTU/hr, and it can STILL deliver 48,000 BTU/hr @ +5F at maximum speed. It is by it's design and controls limited to 48KBTU/hr from +5F through +47F and higher, and a linear estimate based on the AHRI sheet would be dead-rong.

See the AHRI sheet:

https://www.ahridirectory.org/Home/Download?ReferenceId=sUzbLVC3/aFB9wkj...

And the submittal sheet:

http://meus1.mylinkdrive.com/files/MXZ-5C42NAHZ_Submittal.pdf

This is not unique to Mitsubishi- most mini-splits (ducted or ductless) are tested at something other than the maximum capacity at either +47F and/or +17F.

Allison
Bailes

Dana, thanks for raising the issue of mini-splits.  I'm going to come back to the balance point and supplemental heat for that type of heat pump in a future article.

Two things that can move the load curve rather significantly:

First, a home's heat load according to Manual J is generally overstated -- more so for homes built to today's energy code and even more overstated for 'beyond code' homes. There are several reasons for this (fodder for further discussion) but it's not uncommon for the true load to be significantly less than 90% of what MJ would have us believe, even as little as half. A home's thermal mass plays a role.

Second, the 'zero load' point for reasonably tight homes is typically a lot lower than 65F. I refer to this as the home's thermal balance point.

In an existing home, the easiest way to determine these balance points is empirically (i.e., through observation), noting that these are moving targets with variations largely due to wind speed (as Curt noted), solar gain, and how much heat is stored in the home's mass prior to the observation.

Case in point: several years back, we in Southeast Arizona experienced a two-night record-crashing cold snap. The first night, which followed several relatively mild sunny days, my AUX heat didn't kick in until the outside temp reached 14F on it's way to 6F. The second night, the AUX kicked in at 18F, since virtually all the heat stored in my home's mass had been depleted the night before. Had it been windy (it wasn't), the AUX heat would likely have kicked in at 19F or 20F.

What about the other end of the load curve? Over the 8 years I lived in that house, I observed that the heat pump never cut on when overnight lows had been at least 45F. Sometimes the heat didn't come when it was even colder than 45, especially early in the heating season (as low as 39F as I recall). Again, that's largely due to residual heat stored in the home's mass, and the absence of heat robbing infiltration.

Armed with these two data points (heat pump balance point @ 18F and the home's thermal balance point @ 45F), combined with the heat pump's capacity curve (better determined from the expanded performance tables rather than the two official rating points, see Curt's point #2), I can plot my home's true load curve, or at least a better approximation than what modeling can provide. So I would argue that these balance points, determined empirically, inform the home's heat load curve rather than the other way around.

Allison
Bailes

All good points, David, as usual.  Do you happen to know how your 18 °F heat pump balance point compares to an analysis done as I've described in the article?

Using the MJ8 load and my heat pump's expanded performance data to plot the balance point, the curves cross around 37F. But I should point out that the home's empirical load curve is only 42% of the MJ load, which is not at all typical (there are several reasons for this). Here's a link to the graph: https://bit.ly/2aEogRH

The dip in the heat pump's performance curve between 40F and 35F indicates defrost ops.

An idea for a future article. What is your opinion of passive solar and the idea of free energy from the sun.

Architects love glass walls and will accept any excuse to justify the excessive glass. Customers love the word free. I see HVAC systems getting oversized because of this glass.

Allison
Bailes

Bill, that's a good topic.  I've mentioned it a bit here and there but I agree with Martin Holladay on two points:

  1. Superinsulation won out over passive solar back in the 1980s.
  2. Solar thermal doesn't make sense economically anymore.

Here's a link to Martin's take on the superinsulation vs. solar:

GBA Prime Sneak Peek: Reassessing Passive Solar Design Principles

And here are Martin's articles on solar thermal for anyone who wants to check them out:  "...Martin Holladay at Green Building Advisor wrote a couple of articles earlier this decade about how even "solar thermal is dead" (2012) and "really, really dead" (2014)."

I've read that article and love. I just don't think it has spread to the architects yet. And in PassiveHouse forums I still keep seeing people wanting to design Passive Solar and get free energy.

Solar thermal is dead for a reason. Solar thermal done right is very expensive, and when we build a relatively tight, well-insulated enclosure, the annual heating loads become too small to justify the cost. It's not even close. (The same applies to ground source heat pumps, but that's another story.)

OTOH, Martin's premise that 'superinsulation won out over passive solar' is only correct because that is in fact what happened. Passive solar adds very little cost, so why did it die out? It happened because of a lack of knowledge among practitioners, a lack of trainers, and a lack of modeling tools. Moreover, many passive solar homes built back in the day were DIY endeavors. As Martin points out, there are plenty of horror stories of passive solar homes with severe temperature imbalance and overheating, often with funky architecture that has little mass market appeal. In short, passive solar got a bad rep.

The truth is, passive solar design is a wonderful thing when done correctly, and the home doesn't have to look that much different than otherwise. When we combine passive solar elements with beyond-code insulation and air sealing, annual heating loads can be ratcheted down to 'Passive House' levels, without the high cost.

I routinely advise clients how to incorporate passive solar design elements during design phase. But the first step is to buy a lot with optimal orientation. Typically that means facing north. For turnkey passive solar design, I recommend architect Debra Rucker Coleman @ sunplans.com.

Can I put a thermometer in the heat register to determine when i should switch the heat pump off? Please explain how to do this.
Thank you for this informative site.

@Jim, when you have a heat pump with electric supplemental heat, the only reason to ever switch off the heat pump is if it were to fail. This is accomplished by switching the thermostat to 'emergency heat' mode. This disables the heat pump and operates the electric strips as first stage.

Keep in mind that at colder outdoor temperatures, even though the heat pump may no longer be able to handle the full load, the heat it does provide is much more efficient than electric strips. For example, at 0F, a typical heat pump produces around 40% of its full capacity at twice the efficiency of electric strip heat. So during normal operations, you want the electric strips to supplement, not replace, the heat pump.

If by chance you have a dual fuel system (heat pump + gas furnace), things work somewhat differently. In that case, the heat pump MUST shut down when the heat pump requires supplemental heat. That's because with a furnace, the refrigeration coil is mounted downstream from the furnace (i.e., at the supply outlet) to protect the heat exchanger from condensation in cooling mode. If the heat pump were allowed to run while the furnace is firing, then the heated supply air would prevent the refrigerant from condensing. Bottom line, with a dual fuel system, the furnace must handle 100% of the load below the balance point.

Older dual fuel systems rely on an outdoor thermostat to control the switch-over point, typically set to 30F or 35F. Much better is to use a dual fuel compatible stat that energizes the furnace based on demand, as a 2nd stage. Either way, you should never need to concern yourself with supply air temps to manage supplemental heat calls.

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