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Community and Q&A

Capturing Daytime Temps With a Heat Pump

AdamT | Posted in General Questions on

Would it be feasible to run your heat pump harder during the day while temperatures are high(er) and COP efficiency is greater – thereby storing heat in the home’s thermal mass and letting it cool down and coast during the night?

Would setting the thermostat daytime temps 10F higher than evening temps be considered cycling in this scenario and cause heat pump inefficiencies?

This idea was in part inspired by this chart showing daytime highs/lows from:

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  1. maine_tyler | | #1

    Here's my non-expert non-answer:

    It likely depends on the specifics of the system, such as sizing, and how cold its getting at night compared to the output credentials of the unit.

    But my sense is that the beauty of right-sized inverter driven heat pumps is that they can operate efficiently through the varying conditons while maintaining a steady interior temp, unless we're talking about really cold temps that push its capacity, which case I could see real benefit in trying to 'load up' the house before the arctic blast.

    In normal steady temp mode, when it's warmer out during the day, the system is still benefitting from being more efficient. If it gets warm enough though that it starts cycling, increasing output enough to keep it running on low perhaps reduces some inefficiency, assuming that it gets cold enough at night that it can continue to pump within its modulation.

    There's probably a hypothetical setting (optimization curve) based on the unit capacity/efficiency at varying temperatures, the heat demand (over an interval of time, like 24 hours), and the outdoor temperature profile for that interval of time.

    By the way, that graph is of seasonal temperature fluctuations in a ground source well, not diurnal fluctuations. But we get the idea.

  2. Danan_S | | #2

    This is effectively what Harvest Thermal's heat pump system does (, except it is programmed to optimize primarily for electricity prices, not winter outdoor temperatures. They use a water tank for heat storage, though, which is much less leaky than the building mass.

    1. Expert Member
      DCcontrarian | | #4

      I looked at their website, and boy did my pseudo-science detector start buzzing!

      They say their water tank is the same size as a regular hot water tank. That's kind of vague, but let's say 50 gallons. That's about 400 lbs of water. Let's say they get a 50 degree rise in the tank -- I wouldn't expect much more from a heat pump. Without even discussing the practical problems in getting that heat out of the water, that's 20,000 BTU. That might be half an hour's heat in a well-insulated medium-size house on a cold day.

      1. iwatson | | #5

        If their photos are accurate it looks like Sanden's SANCO2 air-to-water heat pump with a 119 gallon tank. Then they're bolting their "smart" controller onto it. So maybe 50,000 BTU?

        1. stevedavis | | #10

          I had a call with them probably 2 years ago and they do use a SANCO2 heat pump. I quickly realized this system would not be good for my house so I didn't store most of what we discussed in my brain but the sales rep basically said the system works best for passivhaus where the loads are very small to begin with. It's a really cool concept but it seems like it needs a super high performing house to work. He also said they had done a few homes in the 40k's range so it is not a cheap system. Heating your home with CO2 based refrigerant is something of a holy grail though.

        2. Expert Member
          DCcontrarian | | #14

          Ah. Typical pseudo-science. Avoid apples to apples comparisons by introducing extraneous variables. How many people know what a SANCO operates like? How can the casual user determine if the "thermal battery" really does anything?

      2. Trevor_Lambert | | #12

        Probably more like 1.5 hours, but certainly not enough to ride out overnight.

        1. iwatson | | #13

          Yeah. Even if you used multiple tanks to have hours of capacity, it raises an interesting academic question regarding OP's original question:

          Where I live, off-peak is from 11pm to 7am. So you'd pay less per unit of power in the night. But it's colder so you'll lose efficiency. Which one wins out? Is it a wash? At the end of a day it's probably a minor enough difference that it's not worth the headache.

  3. Expert Member
    DCcontrarian | | #3

    What you're describing is essentially a setback thermometer. They've largely gone out of favor. Why? Mostly because there's a lot more awareness of the importance of sizing systems properly. If you run your system during fewer hours of the day you're going to need a bigger system.

    It also turns out they don't save that much energy in a typical residence. And the better-sealed and insulated your house is the less effective they are.

    1. Tim_O | | #6

      In theory, you're "right sized" system is already oversized during the day. Speaking for around here, we see a 20ish degree temp swing mid day to mid night I think. The theory is sound, the COP of the heat pump would be better during the day, likely the capacity higher, plus being 20* warmer, you aren't at the design temp.

      The problem is finding a place to store the heat that doesn't make the home uncomfortable. And on top of that, electric rates tend to be cheaper at night.

  4. paul_wiedefeld | | #7

    I say try it for a month!

    1. Expert Member
      DCcontrarian | | #8

      All you need are two back-to-back months with identical weather!

      I say model it. There are two factors that you will need to establish experimentally: the first is the tolerance of residents of your house for swings in the interior temperature. The second is the heat capacity* of your house, how many BTU it takes to move the temperature by one degree. You'll also need a heat loss profile for your house and a performance curve for your heating system. The last two can also be measured experimentally if you don't already have them.

      Basically you model a day in one-hour increments. Find the high and low temperatures for the day, and assume a sine form for the hours between. For each hour decide on a target temperature. Calculate how long your heat has to run given the starting temperature for the hour, the outside temperature, and the parameters of the system, to hit the target temperature. Calculate the energy usage for that amount of run time at that outside temperature.

      I'd do it for an average winter day, and for a design day.

      Fiddle around with different temperature targets. They could even be different for the two days, no reason your programmable calculator couldn't factor in outdoor temperature.

      Then compare to a simulation where the target temperature is kept constant.

      *(This is sometimes called "thermal mass." Longtime readers will know it drives me crazy when that happens.)

      1. Expert Member
        DCcontrarian | | #9

        To measure your house's heat capacity experimentally, on a cold day turn the heat off and monitor the temperature as it drops. Then turn the heat on and monitor it as it rises.

        You can also measure the tolerance of the residents in the same experiment. At some point you'll start getting complaints that the house is cold.

        1. Tim_O | | #11

          It might workout depending how much you like to set back at night as well. I like it 68ish at night. But I want it warmer by 6am when we have to be getting ready... And 6am is actually probably some of the coldest parts of the night in mid winter.

  5. Expert Member
    DCcontrarian | | #15

    Another consideration is that heat pumps tend to have a higher COP when running at less than full capacity. Is it more efficient to run in the part of the day with higher outdoor temperatures but at higher output, or run around the clock at reduced output? I don't know.

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