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Keep the heating in a house all day on?

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Siwastaja:
... and when building from scratch, cost optimization for amount of insulation is quite simple. I did rebuild the upstairs here, completely insulating the attic, and did "napkin" calculation in Excel. I ended up with average 180mm of PIR (polyisocyanourate) sheet because calculated that way, payback time for increasing further 50% from there would be well over 20 years. But going from say 80mm to that 180mm pays for itself in just 10 years.

And the cost of energy source of course affects that payback time. Choices of poor insulation have been always made when energy has been cheap.

Siwastaja:

--- Quote from: Marco on October 28, 2021, 12:02:45 pm ---Thermal mass is not useful storage, you generally want to keep storage separate from your living room, so you can heat it to say 60 degrees Celsius and not die.

--- End quote ---

This is demonstrably just a false claim. Thermal mass is very useful energy storage as proved by myself and many others. Of course the amount of energy that can be stored is limited by low dT range available, but it's still not meaningless. It's in the range of some 5-50 kWh depending on the house and living standards (how much temperature variation is acceptable), and can carry over high-cost hours with practically no further investments at all.

Similar li-ion storage system would cost like some $3000-5000.

Those who have a thick concrete slab with in-floor heating pipes and a heatpump, notice the advantages immediately by making the heatpump run by daytime.

Sure, on paper, given same R, it's best to use as small C as possible, then use external C (like that water tank) for more controllability. But this also costs more to build.

Siwastaja:

--- Quote from: Marco on October 28, 2021, 12:02:45 pm ---
--- Quote from: langwadt on October 27, 2021, 08:22:23 pm ---well, it should also mean that it stays at a comfortable temperature longer

--- End quote ---

When it's not needed.

--- End quote ---

... and if you really want to simulate the leak-the-heat-out behavior of poorly insulated house, nothing prevents you from actively cooling a room with a heat pump, and putting that heat into a storage tank. Obviously such high-tech solution never pays back for itself, but this is just the demonstration of the fact that poorly insulated house leaking out the heat is not an advantage even if it seemingly gives you control "by only having to heat when heat is needed".

Siwastaja:
Simplified,

Heating(t) = k*(t_in - t_out) - sun_through_windows - human_generated_heat - non_heating_electric_power, when t_in > t_out
Cooling(t) = k*(t_out - t_in) + sun_through_windows + human_generated_heat + non_heating_electric_power, when t_out > t_in

Without heat capacity,
energy consumption = integral over time of (COP_h*heating + COP_c*cooling)

Every integration time step will have either heating or cooling.

With heat capacity, heating and cooling needs can be averaged together, and so the integral will be smaller. Ignoring complex math, you get into the ballpark by taking the average during the RC time constant. For example, with 24 hrs:

Heating(t) = k*(t_in_avg_24h - t_out_avg_24h) - sun_through_windows - human_generated_heat - non_heating_electric_power
Cooling(t) = k*(t_out_avg_24h - t_in_avg_24h) + sun_through_windows + human_generated_heat + non_heating_electric_power
energy consumption = integral over time of (COP_h*heating + COP_c*cooling)

With conditions changing between heating and cooling, the latter integral will be significantly smaller.

This is well evidenced in old massive brick houses of pre-WW1 where significant part of the year goes without any heating power yet they are quite comfortable in summer even without cooling, and reportedly measured energy consumption is significantly lower than calculated based on the U-values of the structures.

At the same time, many "passive homes" or "low-energy homes" of 2000's have failed to get even close to the calculated near-zero energy consumption because they have low C, they overheat already early in spring and require constant switching between heating/cooling to be comfortable. But their consumption has been calculated by taking the sun into account as a positive only, but when the calculated heating power goes into negative, it has been just clipped to zero, causing temporary overheating, which wont't be automatically stored due to low C. But people won't accept that so they let the aircon to switch into cooling mode, following what would have been the correct calculation (negative heating power in the integral -> cooling power).

The high-C old brick houses overheat much less due to sunshine, and that little amount of overheating carries long into the night and morning.

The trick here is that the old high-C brick houses also have surprisingly high R for their age. C is not any good if the insulation leaks the power out.

Value of R is most critical, different ranges of C can be worked around and have different advantages but claims that high C is definitely bad are wrong. In my opinion, high C have more advantages than low C, but only if R is properly chosen for the climate. This is an opinion and I might change it seeing different evidence.

Siwastaja:
Attached is a simulation (yes, I'm working on this problem) that fairly well matches my house, simulating May-June 2012*, model includes temperature, wind, solar radiation from different data sources (PV generation simulation is also included for cost analysis, but let's skip that for now), also waste power from electricity used indoors.

*) Why 2012? Because it happened to be a very "average" year here, also containing a balanced mix of extremes, so good for design of systems. Last few years have been warmer.

The top graph shows, in blue, the calculated heating power demand, in Watts, to keep +21degC indoor temperature. Negative values mean cooling is required. Orange line specifically shows the solar irradiance input power.

Mid graph shows outdoor temperature.

The last one is interesting as it is the simulation of the room temperature, just for this discussion I chose to set heating/cooling power to constant zero. Now the blue graph is resulting room temperature if thermal capacity of the house is 1kWh/degC, orange graph when it's 10 kWh/degC. The heating/cooling cost is the same by definition (zero in this simulation). Which one you would prefer to live in?

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