Inevitably I'll have to replace my heat pump, and when that time comes I'd like to have the calculations on the size of the replacement to account for dehumidifying in addition to heating and cooling. With high humidity being a problem I'd like to at least try to calculate how to optimize some balance on comfort and efficiency while maintaining safety, with "safety" being defined by building codes. What is considered "comfortable" is a matter of opinion, and my opinion will vary from what others believe, but building codes will define minimums and maximums on safety regardless of what I'm willing to put up with to save on costs. How should such calculations be done? Are there guides for this somewhere but I've missed them in my searches so far?

Edit to add: The source of any charts or computations should be some organization building inspectors would trust. The goal is to prove my calculations meet minimums required by building code so using a physics textbook is good but charts from a government agency responsible for safety of humans is ideal.

What is wrong with using the existing guides for specifying the size of an air conditioner to pick out a heat pump? The guides I've seen are for maintaining a comfortable temperature during the hottest days of the year, such guides can mean the heat pump is too large for humidity control as it runs cycles too short for humidity control as it blasts out cold air for the rest of the cooling season. I find this a bit odd since I'll see warnings on not over sizing the air conditioning as that can lead to high humidity. If that is a concern then should there not be a guide on what is too big? If there is such a guide then that may be what I'm looking for. A two-stage heat pump is likely to mitigate on getting too little cooling on hot days and too little dehumidifying on merely warm days but I'd still need calculations to prove to myself and any inspectors that everything is safe.

What is wrong with whole house or portable dehumidifiers? Dehumidifiers add heating load to the house which will be an inefficient use of power most of the time as it is unlikely to see high humidity as a problem without also needing some heating or cooling. Heating lowers indoor relative humidity, often to an extreme that humidifiers are needed for safe and comfortable humidity. Cooling lowers indoor relative humidity as well but there's a balance that must be maintained on cooling power through the year or the humidity can get beyond what is safe and comfortable. I'd like to at least try running some numbers to find that balance but I'd need to know how to equate heat pump cooling power (in tons, BUT/h, or watts) into something useful for indicating dehumidifying power (which is apparently measured in pints per day or gallons per hour).

Aren't there heat pumps with a "dehumidify mode" to use? I've seen these and perhaps these are three stage heat pumps but I doubt it, if I'm mistaken then I'd like to know. One way heat pumps get a "dehumidify mode" is to run heating and cooling simultaneously. This is quite inefficient but perhaps necessary for some to maintain comfort and safety with minimal excess capital cost, at least on smaller scales. For centralized HVAC a more efficient option is likely adding some capability to pull in outdoor air. One other tactic I've seen to dehumidify with a heat pump is to run cooling at high power but with low circulation fan speed. This means the indoor air has more time to cool down and "squeeze" more moisture out but with the lower air flow the cooling effect is minimal. I've not seen this done on any central HVAC system and doing so may be considered a "hack" and could violate any warranties on expensive HVAC equipment.

Wouldn't a heat pump optimized for dehumidifying be too small to provide heating to building code specifications? I expect so. I've yet to see a heat pump around here that does not have back up heat. I can run the numbers on what size of heat pump I'd want to make sure, and again I'd need to know how to do those calculations on dehumidifying power of heat pumps to do this, but I expect to find that I'd need a furnace regardless. I could likely find a large enough heat pump but that could mean running into electric supply limits, compromising on cooling and dehumidifying efficiency, or something. Like with the other examples a two stage heat pump would likely mitigate some of this but I'd need to run numbers to be sure.

Could the house be split into zones to help in efficient humidity and temperature control? That has been considered, or I would not pose the question as a rhetorical device. I could sacrifice comfortable temperatures in certain zones of the house for 1%, or even 10%, of the year (but still keep them safe) so that I have comfortable temperature and humidity in the rest of the house. This could be very efficient, but that still means calculating what I'd need in a heat pump to accuracy sufficient to satisfy myself and any inspectors that what I proposed is safe.

I'm still figuring out how to ask questions here to keep the answers best focused. I thought I'd try this FAQ format as this is roughly the thought process I went through on considering options. I'll see how this goes.

  • Dealing with humidity by itself is something for dehumidifiers. When considered as part of a complete HVAC system, which makes a lot of sense, you have to consider the overall climate. So either provide an approximate location (so we can lookup typical high/low/average temperature and humidity for all seasons, or provide those actual numbers yourself. Right now we're just guessing. We know you're not in Antarctica (cold and low humidity year round) but beyond that there is quite a range of possible climates and the solutions will vary quite a bit. Aug 28 at 0:37
  • "Dealing with humidity by itself is something for dehumidifiers." Smart thermostats are smart enough to run air conditioning for dehumidifying. I would tell the thermostat to allow for dehumidifying by air conditioner then set limits on max humidity and max temp. As an example I could set max humidity at 45%, max temp at 95F, and thermostat would be smart enough on minimum temp so there's no attempt to heat and cool simultaneously. With humidity set at 45% the house would likely maintain a comfortable temp around 75F, but I'd allow for much higher on hot days to keep electric bill low.
    – MacGuffin
    Aug 28 at 1:04
  • My point is not that you can't use regular HVAC, used appropriately, to dehumidify. You absolutely can do that. My point is that determining what to get will be largely based on the anticipated weather conditions. And providing either those conditions or a location (which effectively states the weather conditions) is key. Otherwise this is an academic exercise not likely to produce actionable information. Aug 28 at 1:08
  • "So either provide an approximate location (so we can lookup typical high/low/average temperature and humidity for all seasons, or provide those actual numbers yourself." I want to do the calculations myself, not have them done for me. I'd like to consider options on how much space I'd have dehumidified, how the calculations might change in the future based on anticipated improvements, what size heat pumps I can find and their prices, and other cases. The solution I'm seeking is a chart, graph, equation, or "rule of thumb" on how to optimize a heat pump size for humidity than cooling.
    – MacGuffin
    Aug 28 at 1:14
  • "Otherwise this is an academic exercise not likely to produce actionable information." The actionable information is how to do the calculations myself. I can appreciate the enthusiasm and tenacity of those posting here to come up with an answer like, "two tons, you should get a two ton heat pump" but that's not telling me how this was calculated. And I will want to know how it is done in case any inspector has questions and so I can discuss options with HVAC installers if there's some change in plans or the answer calculated is for a heat pump I can't find.
    – MacGuffin
    Aug 28 at 1:22

2 Answers 2


ACCA Manuals J and S are your friend

The official, Code-sanctioned way to size HVAC equipment comes from ACCA Manuals J on load calculation and S on equipment selection. Manual J in particular is important because it goes through all the ins and outs of the various ways a house can gain or lose heat and giving procedures for putting numbers to each of them. It is a complex calculation, though, so folks who do it for a living often use software for the job.

With a load calculation in hand, one then turns to Manual S to determine the actual rating of the equipment needed to handle the calculated loads, both sensible (heat) and latent (humidity). (Note that the sensible/latent split on most air conditioners is mostly fixed, but can be varied a bit by changing fan taps, at least.)


The latent heat of vaporization (or enthaply) is the BTUs (or more generally heat) required to evaporate water to the air.

Flip the sign (remove heat rather than add heat) and it's the "BTUs (or more generally cooling) required to condense water from the air"

In "Handy for things speced in BTUs" units, that's 970.4 BTUs/pound of water. One pound of water is 15.34 fluid ounces of water, or a bit less than a pint.

So, one ton of refrigeration (12,000 BTU/hr) will condense 12.36 lbs of water per hour, or 11.85 pints per hour. 1.48 gallons per hour, 284.5 pints per day. In theory. In practice there may be a degree of re-evaporation before the water gets all the way out of the airstream into the drain, but that's going to be in the right ballpark, pretty close to reality.

How much, if any, dehumdifing you need will depend on local dew point. It's generally recommended to keep that below 60°F in the conditioned space. It will also depend on air leakage or exchange, whether your method of air exchange (if not just leaks) transfers only heat (HRV) or heat and humidity, (ERV) and sources of moisture inside the conditioned space other than humid air from outside leaking in.

  • While that fills in some details on what I'd need to calculate you are showing a bit too much of your work by starting with first principles. I think we'd have to dial that detail back a bit for the inspectors to give me a permit. That's too much information.
    – MacGuffin
    Aug 28 at 7:31

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