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Comment by xnx | Hacker News Reader

When solar is abundant, homes should be pre-cooling as low as any occupants can tolerate. Example: 85 degree day, homeowner is at the office, cool the home to 64 degrees when the sun is shining, so that there's less cooling necessary when the owner walks back into the home at 5:30.

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In most parts of California you don't need active cooling at night. Just open all the windows at dusk and let the cool air blow in.

And then well insulated buildings will stay comfortable a good portion of the day too. Even during Sacramento 100+ heat wave I was amazed how cool a modern home can stay without A/C

Works well unless you are a renter (as many in CA are) and do not have screens on your windows thanks to the “landlord’s special” effect.

Screens should come with the rental, but adjustable window screens are easy to come by if this is a problem you have.

Where are these well insulated California homes? Coming from somewhere with real winter, I am aghast at the insulation standards here.

How quickly does heat naturally increase in a house that's been cooled like that?

I've only lived in apartments, but I guess it gets back to equilibrium with outside temperature in a few hours once AC is turned off. I guess a couple hours of cheap cooling in the middle of the day might save an hour of expensive cooling later, if it's timed right. I'm interested if there have been any studies or maths to back it up and quantify the effectiveness.

depends on insulation

In part, but that's not all of it. The bigger driver is the amount of thermal mass.

Using the house as a thermal battery (heat or cold, dealer's choice) works great in my house but my house is so well insulated that we have CO2 problems.

Planning to setup a fresh air intake with an ERV to mitigate this.

I mean that's going to depend heavily on insulation, light insulation, how many device you have generating heat (computers, servers, refrigerator, tv), and how many times you enter/leave your place.

In a well insulated place with airlock-style entrance (even just a mudroom with solid insulation on both doors) where you're blocking out sunlight and not running a ton of electronics, it could easily take hours and hours.

Some research here: https://www.pv-magazine.com/2023/08/14/mitigating-residentia...

> I guess it gets back to equilibrium with outside temperature in a few hours once AC is turned off.

I've been in houses where we shut the windows as soon as possible after sunrise, as temperatures started increasing, and the house never caught up with the outside temperature - it was cooler all afternoon, cooler until sundown. No AC needed.

I think that tactic might work well generally. Just let in the cool air at night, shut the windows and trap it. At least you can save a good part of the day's AC. It's obvious, free, and simple, but I never see it discussed.

My dad has holes under the window for the water to roll off when you clean it. Those holes let in cold and hot air readily. He doesn't even wash the windows. It's a really bad design for what you're saying

Read on about decrement delay and thermal buffering if you're curious.

https://www.greenspec.co.uk/building-design/decrement-delay/

> 85 degree day, homeowner is at the office, cool the home to 64 degrees

That sounds miserable, and realistically no one is going to do that. If you want to leverage that sort of idea, you need new technology (like a big insulated cold water tank or other buffer device).

Our 50 year old CA house is typical---wood framed with a vented crawl space, so no slab and minimal link to the ground mass. It has only 1970s fiberglass insulation in the 2x4 framed walls and ceiling, with none beneath the floors. It also has 1970s single pane glass and many southwest-facing windows. We make effective use of the house's thermal inertia in both summer and winter. We lack central air-conditioning and have an inefficient 1970s gas furnace.

I can definitely feel the radiant temperature of the house and its content around me. This is linked to the thermal mass and contributes to comfort along with actual air temperature. Our biggest improvements have been modern roofing and DIY heat-rejecting window films. Both seem to reduce the radiant connection between the outdoors and interior, helping in both summer and winter.

In summer, nighttime ventilation and daytime closure means our indoor temperature can start low and lag behind the outdoor heat by many hours. At some point in the evening, the falling outdoor temperature meets that indoor and we start ventilating again. During multi-day heat waves, we will see that the house retains heat, increasing both morning lows and evening highs. But, it still lags behind the outdoors and smooths out the peak.

In winter, we heat to a more comfortable target for a few hours in early morning and after dusk. We let it coast with a lower baseline target otherwise. During clear days, we get enough solar gain to either hold it steady or even warm the house above our desired set point. Then, the evening heat cycle either holds or brings it back up as the light fades and the walls start to feel a lack of radiant warmth. By the time the house is feeling chilled, we're snug in our bedding and it coasts lower until the next dawn cycle.

> modern roofing and DIY heat-rejecting window films

What do you use?

> nighttime ventilation and daytime closure

I do that, it works very well (though depending on the house, of course), but I'm surprised I don't see it as standard advice for homeowners. Even better, an input at a cool spot (near shady grass, etc.), and output with an exhaust fan at the top; auto open/close based on thermostats outside and inside.

> In winter, we heat to a more comfortable target for a few hours in early morning and after dusk.

Is the early morning heat for efficiency somehow (or just it's too chilly when you wake up after an unheated night).

Also: Our perception is of it being colder when it's colder outside, despite the thermostat being set to the same temperature - that is, we need to set the thermostat a bit higher to not feel chilled. I wonder if our perception of temperature depends on something like the coldest (micro-)draft more than the ambient average.

It saves energy to let the house get colder overnight when we can rely on bedding to keep us warm. With a lower differential between indoors and outdoors, there is less total energy lost to the outside than if we held a constant target. I'm only dropping the set point about 5 degrees, not disabling heat entirely.

Then the morning heat is to make it easier to get out of bed and start the day. We stop it again to take advantage of the common solar gain. But also, it doesn't take as much heat to feel comfortable once we've eaten and gotten more active for the day.

For the roof, it is just one of the "cool roof" compliant asphalt shingle systems, with full plywood sheathing. We did not retrofit radiant-barrier sheathing, as there is fine print that this could overheat the shingles. As far as I can tell, the same performance to reject solar load in summer also reduces radiant loss in winter. Vaulted ceilings feel less exposed on a cold winter night.

For windows, I applied Gila "heat control" film found at the local Home Depot. It is a modestly cool gray tint that looks only a little bit mirror-like from the outside, not too different from modern low-e glass. It takes a little practice to get satisfying results with the DIY application. I just used a spray bottle filled with tap water with a few drops of dishwashing soap, rather than any proprietary application fluid.

Regarding heat perception, it is air temperature, air movement, and radiant effects. In winter, you may feel colder facing an exterior wall or ceiling than with an extra room or story between you and the outside. Or facing a window versus having heavy drapes pulled shut. This is even if the air in contact with your body is the same temperature.

Thanks! One thing that works for me: I just turn the heat all the way off at night (or down to ~12 C to protect pipes, but I don't think it's ever gotten that cold inside). I just use more bedding if I need it - I'm generating 37 C, much warmer than desireable, so I just need to trap enough of that heat. Mornings are delightful, and my body retains the nighttime heat from the bedding (in its thermal mass, I suppose). Usually I don't turn on the heat until sundown.

You can get a better effect with the phase change materials. CaCl2 hydrate has a transition temperature of 29 C = 84 F. That's good for keeping warm in the winter, but not so great for the summer. On the other side, acetic acid has a transition temperature of 17 C = 62 F. But that will be below the dew point sometimes, so you need a clever way to distribute the cold and stay dry.

64F is 17.7C - what is miserable about that? In Scotland in winter my heating is set to back off to 17C during the night and when I’m not in. Is that an extreme temperature for some?

My apartment rarely goes above 19C in winter, but for many people that is much too cold.

Nearly have my own solar, I'm wondering if there's a way to do a home thermal battery like this (although for me might not be necessary, as I've also got a battery...)

Like - if I put a water tank under the house, can I cool it (summers) / heat it (winters) as both a backup water supply, and thermal sink? And have that also work to help maintain house temperatures?

You are talking about hydronic heating, which is old 1970s tech.

Imo, it would make more sense to store the power in battery form, and cool or heat it on demand.

That's true from an energy perspective but from an economics perspective it is very hard to beat a well insulated tank filled with water.

I get the physics, the question that economics when were talking about installing a well-insulated cistern under a house, and retrofitting it with hydronic Heating.

Disposing of the cistern capacitor chemical (H2O) is also better for the environment than the battery chemicals. Manufacturing is probably better too.

Sure. I have had hydronic heating in several houses. You don't even need storage. Soler thermal is common throughout the world.

But on a price basis, you can get a 5kw battery for 2k. Good luck running water plumbing and radiators throughout your house for 20k

> you can get a 5kw battery for 2k. Good luck running water plumbing and radiators throughout your house for 20k

That's not quite apples-to-apples; the energy from the battery also needs distribution through the house. Of course, every house has wiring, not all already have radiators.

Especially if it's a large tank. You've got the square-cube law on your side here.

Better still would be to use a truly huge cavern as a thermal battery for your whole city.

Using it for both heating and backup water supply is probably a really bad idea: you'd basically be storing water in the ideal temperature range for legionalla growth.

The workaround is to use a heat exchanger so that you're not drinking the stored water, just transferring heat from it on-demand.

They're doing similar things at larger scales in Sweden: https://www.bbvaopenmind.com/en/technology/innovation/underg...

Yes, but OP wanted to use it as both heat storage and emergency drinking water supply. If you're using a heat exchanger you'd still need separate tanks for hot water storage and for emergency drinking water - and that's the very thing they were trying to avoid.

Water under your house is hard to use as a backup water supply. You would need a pump to use it in the house when your utility water had no (or low) pressure, and you'd want to bypass the pump in normal operations, etc: mechanically complex. It might be easier to have the water tank above the house, but only if your structure can manage the extra weight, and you need to be well prepped for leaks. And yeah, microorganisms.

It can be done but it's not that practical since you need a huge insulated tank and if you're also using the water you it's even less practical to store heat in it. One can use a pool as a heat sink during summer though. Water stores 1.16 kWh/m³K. The heat capacity could be increased with phase changing materials.

The air in a house has relatively little mass so for heating it works well but if you switch off the heater the house will get cold relatively quickly compared to water based systems and the reverse is true for cooling. What you could do is to cool down the floors if they are made of concrete, but then you have to be very careful to stay above the condensation point.

The contents of the house (furniture, counters, floors, walls, etc.) also act as thermal mass. I would guess many times the thermal mass of the air.

Large ceramic indoor potted plants are wonderful thermal mass vessels.

Do you mean real plants in a ceramic planter? Why is that better than furniture or rugs, for example? Water content?

Three dimensional volume for one. Ceramic and soil also will hold the heat you get from sunlight and radiate the heat back out into the environment.

Thermal mass is the key and potted plants provide that with the large volume of soil combined with the ceramicpot that if placed into sunlight will absorb the energy and radiate it back out and keep the room warmer.

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