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Keep the heating in a house all day on?

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Depends on windows and temperatures but for example here the legislation now requires windows to have U < 1.0 W/(m^2 K), and low energy windows down to some U = 0.5 are available, this already equals the insulation level of the walls just a few decades ago!

Passive/low energy houses were built here in 2000's with such good materials but fairly low thermal capacity, and with a lot of those energy efficient windows (4-glass, with argon filling on the 3-glass element, with spectrally selective coating) facing South. The result was, houses already overheat in early spring. So need to bypass the ventilation (which obviously has thermal recovery) and vent manually. And then when the night comes, heat again. This causes such houses to show significantly, like 20-30% higher energy consumption that was calculated on paper.

In any case, my model uses a single 1.5m * 1.1m Window facing South-30deg because that what I have in the 1950's house. If I triple that window area (while keeping my approximately 0.55 U value, i.e., the new windows would be the fancy 4-glass version), the benefit of high-C widens (see attached thermo6.png).

OTOH, if we completely remove south-facing windows, then as expected, the low-C solution gets more benefit from the 50% power cutting (see thermo7.png).

So I think I'm right about the fact that low-C and high-C solutions have different advantages. And again, IMHO, the advantages of high-C are higher because even if the energy consumption averages to similar values, high-C is less sensitive about prediction errors in human patterns.

One aspect often overlooked is the layout of the house (or apartment) and the possibility to create zones with different temperatures. Bedrooms, hallways and storage spaces with lower, living rooms and offices with higher temperature. Not having to heat the total volume to comfortable temperature saves a lot of energy.

Technology Connections on youtube did a video a few months ago where he figured out that one room in the house could be made to act as a battery for cool air during the off peak power time and in the evening (when he's home) deliver that cool air to the rest of the house.

Very clever.

It can be complicated. My house has an electric heat pump, and so it has better efficiency when it is 12C than -7C... according to the documentation that's a factor of 2 in efficiency.

Given that I work 8-5, and pay the same rate for electricity at all hours of the day, I have the thermostat scheduled to drop 3C at 7:30 and to turn back up at 4:30.

So if you imagine a day where the morning low is 0C and the afternoon high is 13C, I effectively eliminate running the heat when it is 1.2 or 1.3 efficient, and run it instead at the warmest part of the day when it is 1.9 or 2.0 efficient (all relative to that -7C figure).

If you change the conditions, the answers probably change:
With rooftop solar power, you'd want to to use as much of that as possible, meaning running the heat all day
If the power/fuel has variable pricing through the day, it may be beneficial to run the heat to take advantage of that.
If the house was less well insulated and the temperature fell by 6C during the work day, you probably would want to run the heat during the day.

I wish there was better tooling to help homeowners optimize their energy use/cost.  It seems like some of the "smart thermostats" on the market could provide recommendations, but I'm not aware of any that do so.


--- Quote from: cortex_m0 on October 29, 2021, 12:42:14 pm ---I wish there was better tooling to help homeowners optimize their energy use/cost.  It seems like some of the "smart thermostats" on the market could provide recommendations, but I'm not aware of any that do so.

--- End quote ---

I have noticed exactly the same and think there is a lot to do here with little actual/serious competition. But we'll see.

Due to lack of real controls, I simply let my heatpump do the work between 10am to 4am and let it rest when it's coldest. Also having rooftop solar, the operation starting at 10am coincides pretty well with the production.

But lack of control also means lack of instrumentation. Even if instrumentation is available (for example, you can install output energy meter in a hydronic system (one that uses accurate flow meter and two accurate thermometers to calculate power), how do you verify if the actions have the effect you expect? Run for a year, then change your habits for another year, while hoping the weather patterns are identical year to year which they are not?

Large changes such as replacing COP1 sources with COP3 heatpumps are obviously visible as $$$ saved in bills, but how do you verify what the effect of dynamically changing temperature setpoints is? The expectation on the forums seem to be they hold a potential for significant savings, but there is little to prove that except too simplistic napkin calculations. Expect of some low hanging fruit cases, they are micro-optimizations which are easily lost in noise. As you can see from my simulations posted there is no significant difference in energy consumption in any of the cases.

The only solution I can see is simulation with capable enough simulation models. It helps that thermal energy flows are really analogous to electronic design principles and elements like resistors, capacitors, current and voltage sources can be used with Kirchoff laws.


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