[frereubu]

I've read that it's always more efficient to turn heating off when you're not home and then turn it back on when you return. Is the reason for it being on 22-24 hours here that it takes a very long time to get back to the desired temperature, meaning you'd actually be cold for quite a while as it returned to the desired temperature?

[leoedin]

The hidden factor here is that condensing boilers and heat pumps have non-linear efficiency vs flow temperature curves. Heat pumps in particular show high increase in coefficient of performance (CoP) as flow temperature drops.

The other variable is how well controlled your heating is. A lower flow temperature means less overshoot of the target set point - and as losses scale linearly with temperature delta, that can mean higher energy losses (depending on the characteristics of the controller of course).

Whether or not you care about losses in unheated spaces depends on your system topology. Personally, all my heating pipes are within the thermal envelope of my house, so flow temperature has no bearing on those losses at all.

If you had a resistive electric boiler, flow temperature would have absolutely no effect on efficiency. You'd be completely right, that running heating only when you needed it would be more energy efficient.

[looofooo0]

You missing ISO7730, it is a system for humans and not air temperature control. (tl;dr heating your home 24/7 allows you to lower air temperature for the same comfort. )

[frereubu]

Thanks for the explanation.

[rsynnott]

> I've read that it's always more efficient to turn heating off when you're not home and then turn it back on when you return.

50 years ago this was _always_ the case, but condensing boilers and especially heat pumps muddy the waters a little. Condensing boilers can be close to 100% efficient (vs ~70-80% for ye olde gas boilers), but generally only at a fairly specific operating temperature, which may be lower than you'd need to get a rapid rise in temperature. Heatpumps are >100% efficient (that is for every joule of electricity you put in they move more than one joule), but are even more fussy about operating temperature.

The answer now is going to be a solid 'it depends', based on behaviour of the heating system, outside temp, desired inside temp, insulation...

[lucb1e]

> Condensing boilers can be close to 100% efficient (vs ~70-80% for ye olde gas boilers)

So you save up to 30% of the gas while heating your home nearly 24 hours a day, instead of saving 67% of the gas by using it only for the ~8 hours that you're home and not under a duvet?

The math might work out for those who work from home, but I mean in the standard case with an hour's commute (round-trip), an 8-hour work day, and a 30-minute lunch break (9.5h gone, 7h sleep -> 7.5h during which the apartment should be warm if you run no errands). Of course, you'd schedule it to start before you get home, but it can also stop a bit before going to bed

I've been hearing both arguments for years and while it's exceedingly convenient to believe the condensing boiler story and just heat 24/7 to always come home in luxurious warmth, nobody ever does the math. You're one of the few people who even mention what the alleged savings are in the first place

We have a condensing boiler, chosen by my landlord so I'm no expert but I looked into it because we pay the bills in the end. The device's manual lists the efficiency as 88% ƞ4 at 60°C return water temperature, called high-temperature operation, and 98% ƞ4 at 30°C return temperature. It also gets tested yearly by a professional (Schornsteinfeger I think they call it here) and produces two efficiency measurements. Just looked up the record again: the mechanic handwrote "min" and "max" with them, so I presume that the "max" one is where the system operates at maximum capacity (minimum efficiency, then?), where the efficiency is 98%. At the "min" setting, the efficiency is shown as 106% (iirc some older measurement techniques don't include the condensation efficiency gain in the percentage, that's how it goes above 100%, or so I read when I looked it up a few years ago). For that difference, please correct me if I'm overlooking something but using a low heat for 24h/day makes no mathematical sense to me

[rsynnott]

Yeah, this only works with quite well-insulated houses, where they're very little heat loss (the system will then spend most of its time off _anyway_, as it has reached desired temperature).

[looofooo0]

ISO 7730 to the help. Just keep low overall temperatures and heat 24/7 in a reasonably insulated home.

[sokoloff]

I work entirely remote so, other than travel, there are not many long periods when the house is unoccupied.

I target the long run time to maximize efficiency. A 160°F pipe will lose more heat to the part of the building that I don’t want to heat as well as more heat to the wall right behind the radiators. It also results in the house going micro too-hot, too-cold, too-hot, too-cold as it cycles. Mine is constantly trickling in just enough heat to replace the heat lost instead of cycling between adding way more than needed then none for a while.

Another large effect is that low return water temperatures into the boiler allow for greater condensation of exhaust gas energy to be used in the building instead of sent outside. Walking by my house on a cold day, you’ll see minimal steam plume during operation. All that steam I see my neighbors emitting is energy they paid for and delivered to the outside… (They paid a lot for a boiler with a 95% or 98% sticker and run it at 80% efficiency.)

https://kw-engineering.com/how-to-optimize-condensing-boiler...

[amluto]

> Another large effect is that low return water temperatures into the boiler allow for greater condensation of exhaust gas energy to be used in the building instead of sent outside.

Correct.

> Walking by my house on a cold day, you’ll see minimal steam plume during operation. All that steam I see my neighbors emitting is energy they paid for and delivered to the outside… (They paid a lot for a boiler with a 95% or 98% sticker and run it at 80% efficiency.)

Please check your assumptions.

A boiler operating in condensing mode will produce a trickle of liquid condensate (that may well be drained somewhere that you can’t see [0]), teeny tiny drops of condensate suspended in gas (colloquially “steam”, but it’s more like fog), and some residual water vapor mixed with the exhaust gasses. You can see the “steam”, but you cannot see that residual vapor except to the extent that it condenses further as the exhaust stream cools after it exits, much as you can see some of your own exhaled water vapor on a cold day as it condenses outside your nose or mouth. The exhaust gas is saturated: it has maximum humidity and is at its own dewpoint, so there is a lot of visible fog. The droplets that form inside the boiler and escape with the flue gas do not represent wasted heat: their heat of fusion has been captured.

A boiler operating in non-condensing mode will produce no liquid condensate, and its exhaust will be well above its own dewpoint. It will contain far more water vapor than a condensing boiler, but you cannot see that vapor except insofar as the flue gas has a different index of refraction than the surrounding air and distorts the background a bit. Depending on weather, a bit of it may condense later. All of it is wasted energy.

[0] This liquid condensate is nasty stuff: it’s basically carbonated distilled water plus some impurities but not usefully buffered, and it’s rather acidic. It will quickly corrode many metals, including copper and many common copper alloys, non-stainless steel, galvanized steel, etc. Non-condensing furnaces and boilers are generally carefully engineered to avoid condensation, because the condensation would damage them. If your plumber is unaware of the degree to which boiler condensate is corrosive and uses copper pipes or metallic fittings (push-to-fit in the style commonly sold as “Sharkbite”), the system will fail. Use plastic pipes (PVC or PEX) and plastic (or maybe stainless steel) fittings such as ordinary solvent-cement PVC fittings, “engineered plastic” PEX fittings, or push-to-connect fittings with plastic wetted surfaces. John Guest makes these, and there is also the somewhat bizarre ProLock brand, which seems to be some sort of joint offering from John Guest and Sharkbite.

[sokoloff]

I’m imagining that what I see in my neighbor’s exhaust is the subsequent condensation as their exhaust gas cools to where the dew point is met and visible moisture becomes apparent.

I can see a clear difference between running my own boiler at 25°F OAT (lots of “steam”) versus 40°F OAT (almost none) while I see my cross-street neighbor showing large plumes on both. I’m not sure if I mistyped above or I’m actually thinking about it wrong, but I don’t think my observations are incorrect.

Having that water condense outside the building (giving up heat to the neighborhood) is less efficient than having that water give up its heat into the incoming (return) water.

[Retric]

> A 160°F pipe will lose more heat to the part of the building that I don’t want to heat as well as more heat to the wall right behind the radiators.

You’ve got the first part of that backwards, it’s the walls near your radiators that are your problem and need more insulation.

[sokoloff]

Indeed the building is 100 years old and impractical to retrofit insulation in any cost-efficient way (structural brick, lathe and plaster walls with about 1” of air space in the original parts of the building).

[eru]

Btw, a heated blanket would be a lot more efficient, as it warms just your butt.

[sevensor]

Heated blankets are ok, but you have to arrange them just so, and then you can’t move without fussing with the cord. It’s the last resort after layering warm clothes before bumping up the thermostat.

[pests]

I use a heated blanket as a bottom layer sometimes. Lets you move around and do whatever you want with the blanket fixed in place and the cord not in the way. I have larger heated blanket that has independent power/settings for each half. I turn one half on max and leave the other off and can roll and find the perfect direct heat and if using another regular blanket on top all that is captured too.

[frereubu]

Thanks for the explanation.