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Comment by retrac | Hacker News Reader

Heat is low-density and it's difficult to work with. And fossil fuels are extremely high-density and easy to work with. So when heat is needed, the general approach has been to burn fossil fuels locally and just dump the waste heat. This has not properly accounted for the cost of fossil fuels.

Low-intensity waste heat is valuable. And it is something that can be captured and used. But that is a different kind of mentality, and a different kind of engineering, one that hasn't been much-developed. For example, here in Canada, like in many countries, we like to take hot showers. We take hot showers even in winter, and there can be an 80 °C difference between the water flowing down the drain and the air outside. Is there no further useful work that heat could do?

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In BC and Ontario (and I believe other places too) the building code includes passive heat capture for drains for new builds. You can recover quite a bit of the heat for very little money.

These sorts of technologies tend to be very region and building dependent. They add complexity for some amount of energy recapture, but often the energy recapture isn’t worth it, or some other optimization would have a better pay off. (If we were talking about software, the law would be similar to “all floating point must be done in unrolled assembly loops”, which sounds reasonable but breaks GPUs, instruction cache, interpreters, etc).

For example, we didn’t insulate our concrete slab floor because our windows provide too much passive heat 10 months out of the year. We’re on the edge of the region where such insulation is recommended (thankfully, it is not required here - in the fall, we have to run the air conditioning when it is in the mid sixties outside! In the summer, we don’t need A/C until the upper seventies because of the angle of the sun and passive cooling from the slab).

Anyway, our hybrid hot water heat pump has a COP of two or three, and was a much cheaper upgrade than passive heat reclamation would have been.

Because we bought an efficient unit, it uses hardly any power (800kwh per year, so an average of 91 watts, or about $100/year, or charging my car from empty to full 24 times), so we’d be better off with some other upgrade on a thing that hasn’t hit diminishing returns.

For instance, we should have used bigger air ducts on our hvac system to improve the air handler’s efficiency.

I’d love to hear more about your design, and retrospective on the decisions. We are about to undertake a design for a new build and energy efficiency using passive technologies is something that matters to us.

We had been planning a highly insulated slab with exposed concrete floors, but your comments on having to run AC seems concerning if I am reading it right. We plan for awnings to keep sun off the windows in the high summer sun.

See sibling comment for awning comments.

We went all electric in an area where PG&E outages are frequent. We have battery and solar, but during extended outages there usually isn’t any sun. Our solution is to get a propane tank and generator that can charge the battery. This is a complicated compatibility problem, so we should have put the generator in at the same time as the battery and solar.

Heat pumps are fine, but the contractor undersized the ducts. Run a system backpressure test yourself (get the service manual) before writing the final check for the install.

Permitting and insurance in California are maliciously bad.

If you want a graywater system, apply for the permit at the same time you start the project. Also, be prepared to pay $20-30K for “engineering” and permits, but only $1-5K for parts and installation. (Or look around more than we did for an ethical designer.)

Get the best possible insulation and if possible pay for plaster. We’ve had a few leaks, and plaster / rockwool is much more tolerant of water than drywall / fiberglass.

Pay for all the smarthome stuff, but insist on offline home assistant compatibility.

Wire multiple outdoor circuits (120v and 240, if possible) so you can charge a car, run a hot tub, run a pool pump, charge a lawnmower, etc, etc without thinking about circuit capacity. You won’t do those things, but they cost basically nothing now, and $10K or more later.

AFCI breakers are a scam (incompatible with appliances that contain motors or switches), so expect nuisance trips unless you replace them with GFCI post-inspection.

The fancier the lamp, the more compatibility and capacitor whine you will encounter. Our most trouble free lamps were under $100 at ikea. I know which ones cost 10x that, but meh.

Under no circumstances purchase doorknobs made by emtek, assa abloy or kwikset. (Two of those brands are $$$).

Keep a bottle of spray air on hand at all times because modern smoke detectors will go off at 2am and the only way to fix it (other than a hatchet) is by spraying off the sensors.

Whatever your plan is for a roof, get a nicer one.

Pick a general contractor that has put up lots of homes in your county.

Oh, and either find someone that has installed like 100 hybrid hot water heaters, or check the installation for wires that rub (heat pumps usually don’t vibrate) and air exhaust / inlet condensation post-install.

One other thing. If the house is passively cooled and heated, you will want to leave the windows open when you are not home. European style tilt and turn windows are perfect for this, since they are secure when open in tilt mode:

https://www.vetrinawindows.com/styles/tilt-and-turn-windows

(I’ve never heard of that brand, but their webpage has a nice animation explaining the product.)

this is such a telling story ..

I am immediately on the side of the owner-builder, but then

> in the fall, we have to run the air conditioning when it is in the mid sixties outside!

what? building design has a lot of parts to it.. "big cement slab" and "some insulation" are not the only parts.. But good quality design and the parts to build it are expensive. There is an underlying theme of saving money to accomplish something otherwise not possible. I am sympathetic to that of course.. Something is not right about the design, but what bad manners to point it out first! Someone put a lot of effort into some upgrade and they likely cannot change the decisions they made, due to time and costs, etc.

good to hear the details and mostly supportive here -- green building has got to get better, starting yesterday IMHO

The problem is that it is sunny in that month. Historically around here, we’d have more cloud cover, and it would be tuned right. The plan was to open the windows if this was a problem, but we’re also getting more flies / biting flies than before during those months.

Of course, the excess sun means excess solar production when most people aren’t running heaters or A/C.

The expensive fix is to increase the amount the roof sticks out by a foot or so to block the sun for an additional month in fall and spring.

The cheap fix (what we are going for) is to put up curtains or blinds that let us manually tune the amount of passive heating we get from the sun.

Either fix would also reduce glare, but only the curtains let us tune the pre/post solstice passive heating. It is colder post-solstice for a given angle of sun, so extending the roof enough for November probably implies paying more for heat in Jan/Feb.

Do your windows not have screens? That would allow you to have the windows open even when there's flies. Now if you have bad air pollution/smells (smoke, livestock, city) then screens won't help.

We have screens, but they are retractable (high wind area), and the flies are somehow drawn to the tiny gaps in the mechanism.

Drain water heat recovery. See: power pipe.

In the winter I always leave my tub full and the bathroom door open after taking a hot bath to allow more of the heat to warm my home.

I paid to heat that water up, so I damn well want to get as much out of it as I can.

Sounds like a great way to encourage mould growth

In winter climates it’s extremely common to run humidifiers to add water to the air. Using a bathtub for the same purpose is pretty common and I’ve never heard of either causing mold in normal operations.

Yeah, given how dry our place gets in winter the added humidity is not a problem.

If I took baths every day then it might become a problem but once a week or so had had no negative effects.

Humidity is usually too low during cold months, so leaving the door open is fine and helps everyone feel warmer (humidity + a bit of extra heat).

Vernacular architecture here was for people to live directly over the barn, as a side effect giving them residual heating.

These days there's heat recovery drains. The idea is generally to first make sure you use a thermostatic tap for the shower and then use the drain water to heat the cold water line going to the tap. The thermostatic tap will adjust to keep water temperature steady as the incoming "cold" water gets warmer over time.

The marketing material claims energy savings of up to 50%, but presumably you'll only reach that amount of savings if you take hour long showers at ridiculous temperatures.

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