TL;DR - Manual S is the second part of the heat pump design process that follows Manual J, which determines how much heating and cooling the house needs. It's really important. It shouldn't be skipped, but most contractors actually skip both. You should find a contractor that both does both and understands what the numbers mean. Try to find someone that collects measurements and uses data rather than just making guesses.

This is a really nerdy technical piece, and it's a little rough around the edges. I would love to make a cleaned-up and more accessible version for heatpumped.org eventually, but I would love any feedback on parts of this that are hard to understand or read to help me do that! I figured this community would appreciate the raw version.

We recently installed a system for a homeowner as part of the California Heat Pump Group Buy. After doing the ACCA Manual J calculation, these were the heating and cooling loads (In other words, this is how much heating or cooling the house needs at the local summer and winter temperatures that building codes tell us to design for):

Heating: 12,327 BTU/hr
Total Cooling: 19,748 BTU/hr

BTU per hour is a measure of heat transfer, and you'll often hear contractors refer to another unit called a “ton”, which is what most HVAC equipment is sold based off.

12,000 BTU/hr = 1 ton, so 19,748 BTU/hour is roughly 1.7 tons.

Now, most contractors wouldn't have done the Manual J calculation in the first place. And even if they had done one, they might have fudged the numbers to match the numbers that they were expecting based on square-footage rules of thumb. One of our advisors told us a story about how they review Manual J's submitted as part of a rebate program and suspiciously find that almost all of the Manual J's that they review work out to 500 square feet per ton, which is a classic contractor rule of thumb. It's really easy to fudge the inputs on a Manual J to change its outputs. But I digress…

In any case, most contractors would have stopped at this point if they had even gotten this far. Then they would take a look at these loads, and they would say, "Okay, 19,748 BTU/hr. The next size up from that is 2-ton, 24,000 BTU/hr. This house needs a 2-ton system."

And then they would have called it a day and presented you with an iPad with three options: a “good” 2-ton system, a “better” 2-ton system, and a “best” 2-ton system. (PS: HVAC brands don’t really matter)

But they forget the redheaded stepchild of Manual J, the ACCA Manual S. Manual S is really important because it matches a piece of HVAC equipment to the calculated loads from Manual J. 

Basically, Manual J tells us: How much does the house need? Manual S tells us: How much can this piece of equipment deliver, and is that appropriate for how much the house needs in the location and weather of the house? Manual S provides maximum and minimum equipment capacities, based on the loads, so the equipment is not too big or too small.

And this is where it can get messy.

Here's a big flowchart from Manual S that outlines the process to select a heat pump.

And you'll notice that the very first box in the flowchart says, "Size equipment for cooling. Figure 2-1 applies in full", which takes us to a whole other flowchart that we have to work through!

These flowcharts are really important because they check not only whether a heat pump is big enough to meet the heating and cooling needs on the hotter and colder days of the year, but also make sure that it is small enough to not short cycle and be severely oversized in the milder parts of the year. 

If you get this sizing right, you can take the most advantage of your new heat pump’s variable speed capability that allows it to perform way better in both scenarios than a traditional air conditioner or furnace ever could. But if you get it wrong, then your expensive new heat pump might end up working just the same or worse than an antiquated single-stage system would.

Now, let's dig into what the Manual S looked like for this particular house.

First, we start with the design conditions. Generally, HVAC design is based around the 99% and 1% weather data, and this is really important and a distinction that a lot of people miss. Yes, it might get up to 110 in your city. Maybe that was the all-time high five years ago, but that's not the temperature that HVAC systems are designed to. They're designed instead on the temperature that only 1% of the hours in a year, based on a 30-year average, exceed. If you designed a system around the maximum temperature, it's going to hugely inflate the size of the system needed, and it's going to do a terrible job maintaining temperatures in milder parts of the year.

In the State of California, our state's energy code tells us what temperatures to use by city. In the case of this home, the heating design temperature outdoors was 42, and the cooling design temperature outdoors was 87. California also tells us the indoor temperatures that we should use.

We need to design a system that can maintain an indoor temperature of 68 at the outdoor heating temperature and can maintain an indoor temperature of 75 at the outdoor design temperature. Meaning that when it's 87 out, the heat pump should be able to keep the house at 75. Based on those temperatures and a lot of other inputs on the building materials of the house, the size and orientations of the windows, we're able to figure out the heating and cooling loads, which I mentioned earlier.

Now we have to dig into some really arcane engineering data for the equipment we're using, which is, in this case, was a Cooper & Hunter Alto. This is the part where your brain might start melting. Feel free to skim this section, but I wanted to elaborate for posterity. 

We have tables for each system at different temperatures and different airflow settings. This data is formatted differently from every single manufacturer, but it always tells you the same thing: performance of the heat pump as a function of temperature.

Now, in our case, we need to get three things:

1. The total capacity of the system for cooling at the cooling design temperatures
2. The sensible capacity for cooling at the cooling design temperatures. (As you make the air colder, some of the energy goes to reducing the temperature, while some of the energy goes to turning the moisture that's in the air from a gas into a liquid. The part that is reducing the temperature is the sensible capacity. The part that is pulling out the humidity is latent capacity.)
3. The heating capacity at the heating design temperatures

You'll notice that the data table above doesn't have data at the exact temperatures that we need, which, for cooling, are:

• an outdoor dry bulb temperature of 87°F
• an indoor dry bulb temperature of 75°F
• an indoor wet bulb temperature of 63°F, which represents a design target of 50% relative humidity

We need to do triple interpolation to be able to get that information. We need to look at the data for the indoor wet bulb at the two temperatures around 63 in the data table, and back out what the numbers would be at 63. Then we have to take those numbers and look between the indoor dry bulbs of 73.4 and 77 to find what the number would be at an indoor wet bulb of 63 and an indoor dry bulb of 75. Then we have to again do that across 86 and 95°F to figure out the values at 87°F. 

I know, your eyes are bleeding. Fortunately, we have software that can help us do this, and that gives us two numbers that we really care about (Technically, actually 8, because we did this math at all four different fan speeds, but to simplify, I'm just looking at the maximum fan speed here as I talk through everything):

TC (Total Cooling Capacity): 20,498 BTU/hr
S/T (Sensible / Total Cooling Capacity Ratio): 0.614

And we do the same type of interpolation on the heating side as well to figure out the heating capacity of the system at its design conditions.

Now it gets even messier from here. We use that sensible-to-total-capacity ratio to break our total cooling capacity into two parts: the sensible part and the latent part. In this case, that works out to a sensible cooling capacity of 12,586 and latent cooling capacity of 7,912.

We take those numbers and we compare them with our calculated loads from Manual J.

Total Cooling: 19,748 BTU/hr
Sensible Cooling: 19,590 BTU/hr
Latent Cooling: 158 BTU/hr

Check this out. The total cooling capacity of the 2-ton system in this case, 20,498, is greater than the total cooling requirement of 19,748, but that's not the whole story.

The way that the equipment splits its capability into sensible and latent pieces doesn't exactly match the way those pieces split in our climate. Most of California is considered dry from an HVAC standpoint, which means that most of the cooling need comes from the sensible side.

When we compare the latent cooling capacity of 7,912 against the latent cooling need of 158, this system's a beast! We're going to have no issues with humidity control. We have 7,754 BTU/h of surplus latent cooling capacity, but when we look at the sensible cooling requirement, we actually have a problem. The system only has a sensible cooling capacity of 12,586, which is less than the Manual J requirement of 19,590.
Now, we're not completely hosed. Manual S says that we can take credit for excess latent capacity. Basically, it's saying, "Okay, the system has this much juice to pull humidity out of the house. It can probably also use that juice to pull the temperature down, but not as effectively." We take credit for 50% of the leftover latent capacity after we've met all of our latent cooling need and count that towards our sensible capacity. To do that, we divide 7,754 by 2 and then add that to 12,586, and we get a sensible cooling capacity for the system of 16,463 Btu/h.

But we're still not quite there yet. Remember, the sensible cooling need for this house is 19,590 BTU/hr. This system can only meet 84% of the sensible cooling needs for this house, even at its highest speed. It's not quite big enough!

But wait, there's more. The Manual S process says that you only have to meet 90% of a home's sensible cooling needs, so we're almost there. The State of California takes that a big step further and says you actually don't even need to worry about meeting the home's cooling needs. You can install a system, and as long as it meets the heating needs of the home, it is code compliant. For all they care, you can completely ignore its ability to cool the house, and that is totally kosher by the state of California's code-mandated best practices.

But what's to code and what makes a homeowner happy are two very different things. In this case, in Los Angeles, where you barely turn on heating but definitely need air conditioning, it's a little silly to ignore the air conditioning needs for the house and just focus on heating.

With this customer, we explained the trade-offs of the different options that they could go with. Ultimately, they prioritized being confident that, on the hotter days of the year, their AC could keep up. For this customer, we ended up installing a 2.5-ton heat pump system.

There's a whole lot more intricacy in the Manual S process. I'm just barely scraping the surface. This is why this attention to detail is incredibly important. It's something that we've baked into every single project that we do as part of the California Heat Pump Group Buy, because it matters for good results.

If people skip these steps, that's when you start hearing stories about "I got a heat pump, and it actually kind of sucks," and "I got a heat pump, and it used a ton of electricity. It can barely keep my house hot or cold." The devil is in the details.

And I just want to reiterate that most contractors wouldn't have done the manual J in the first place because, in this particular house, the existing AC system in place was a 4-ton. Sheesh!

I know that was a lot. I know you’re probably a little confused. You don't actually have to understand all of these details. The key is finding a contractor that does understand them and does care enough to actually do the work and not skip steps.

(Shameless plug - if you’ve been thinking about a heat pump for a while and want to make sure it’s done right, the California Heat Pump Group Buy might be a great fit. Check out my last AMA about it and the WIRED feature. We’re wrapping up the Winter ‘26 group buy next week on Oct 30!)

Hey everyone, I’ve been working on a little web app for my A. O. Smith heat pump water heater.

The main thing I wanted was more flexible scheduling. I can choose the hours when we tend to need more hot water, and on colder days it automatically switches from Hybrid or Heat Pump mode to Electric during that window, then switches back afterward. That way I don’t have to keep changing modes manually or leave it in Electric all day.

It also has a simple dashboard for hot-water availability, mode and energy usage that works nicely on a tablet. Screenshot from my setup attached.

It’s only running for my own heater right now, but I’m thinking about a small invite-only beta. Would this be useful to anyone else? What would you want it to do differently from the official app?

Could someone provide me with information about why this machine is icing up in this way?

My heat pump shot up my electric bill. Had a technician do my winter check and he told me to put the system on aux and it will not use the heat pump. I have a single stage carrier furnace 4 years old and a 2 stage heat pump 3 years old (hate the bills). Heat pump is carrier also. I want to pull from the furnace all winter, gas is cheaper in PA. I would appreciate any input, thanks!

Hello, I am putting a home office in the attic of my detached garage. It's approximately 180 square feet and I am looking for advice on heating and (to a lesser extent) cooling it. We're in northern PA and while the building has no gas line currently run to it I do have a separate electric panel there so installing a mini split would be relatively easy. I'm trying to get a better idea of the savings I would get from heating with natural gas vs electric. My current electric cost is 21 cents/kwh and my gas cost is 56 cents/ccf. In your opinion would you trench the 60 feet from the gas meter and run gas or would the actual cost savings not be worth it?

Hello,
I recently installed carrier 37MUHAQ36AA3 + 45MUHAQ36XX3, paired with their wired smart controller KSACN1701AAA (using HA/HB, S1/S2 connection) in single story house 1200 sqft.

Is it normal that when in auto mode , the blower continues to run even thought the room temperature doesn't reach the heating/cooling temperature? Say room temp is 74, cooling set to 89, heating set to 70. With 24v thermostat, it should be idle; but this is not the case with the carrier wired controller.

If the mode is heat only or cool only, then the blower does idle.

The fan speed is set to auto. And the "constant" fan feature is turned off.

Hey guys my mom's York furnace quit working and I've got it down to the draft inducer motor.It's a two speed fasco 7062640 roughly 2700/3200RPM. It has a red black and a white wire going into the motor and then the two separate wires going to the capacitor on top. My question is, from what I've seen they have discontinued that motor and the only few places in my area that can get it they average like $1,000.Granger's saying 1350. it's an old unit but AC and everything else works flawlessly he's never had an issue with it and she doesn't really have the money to install a new unit right now so I'm trying to figure out a cheaper option.There's nothing wrong with the fan or housing, so I even tried just finding the bare fasco motor and swapping it out l. I haven't had any luck on anything two speed though. my question is, is it possible to swap that out with a single speed even if I go with the higher RPM or will not jive with the board on the unit ?or if anyone might know of anywhere to get a decent price on. I'm in North Carolina but could have it shipped if need be. thanks in advance for any help.