That’s an artifact of how heating is setup inside your home. Which is more efficient depends on where you’re dumping heat inside the home, levels of insulation, etc.
Energy moves from hot to cold linearly with temperature differences. Hypothetically, if the pipe was the same temperatures as the inside of your home all the heat transferred would be outside the envelope. The hotter the pipe the better this ratio becomes. This is true regardless of what percentage of the pipe is inside the envelope.
However, heating along the exterior of the home under windows and such then you’ll heat the exterior walls to higher temperatures than the interior thermostat thus losing more heat to the outside. Radiant heating on the other hand largely avoids this effect.
Look at ISO 7730, a lot of comfort comes from non-cold walls and their radiant heat and small difference of wall temperatures to air temperatures. So having a thoroughly heated home allows you to lower your air temperature. Apart from that modern gas and even more heat pumps greatly gain efficiency by lowering flow water temperatures.
Having read that document, the ISO 7730 model itself depends on stable temperatures. However, I think the key is simply to understand thermal mass; people can be in hotter air, but feel cold due to cold surfaces (e.g., floors or furniture), which heat more slowly (or lose heat more slowly) than the air itself.
Therefore,
1st: Heating/cooling cycles from your HVAC are fighting these objects because they don't mix at the same speed as other objects (e.g., the air itself), so you end up with gradients across objects; people rate this feels unpleasant.
2nd: Mechanical equipment tends to operate more efficiently under constant load compared to constant start and stop cycles.
With #1 and #2, you can just heat constantly to increase both the uniformity of heating across objects and also the efficiency of the mechanical equipment's energy conversion.
There's a 3rd point, which, really, is just a sneaky way of reframing #1 and #2, and that is that you can also lower your setpoint and still have a subjectively superior comfort perception compared to a cyclic system. It drives home the point to say "constant 68F feels more comfortable than intermittent 72F." But it also invites the complaint about constant versus intermittent energy use, right? So I think just detailing #1 and #2 is better.
That spec aligns with my understanding, including the model's dependence on comfort perception. I was, initially, in disbelief about it, but changed my mind after reading thru. The texty reply was to make it more palatable for someone like me to accept. I think we agree.
One thing I missed in summary is the concept of general radiant temperature gradient. It's not only about the gradient for conduction, but for radiation (and convection). So you could probably improve my summary by talking about any gradient between different objects in the environment and their EM, which feels unpleasant (but I think it had value in its reduction of the problem, too).
It’s a deep rabbit hole as condensation, humidity, etc also enter the picture. Efficiently lowering temperatures for sleeping further complicates things.
That said, heat loss is through exterior surfaces so you really want to avoid spot heating of poorly insulated exterior walls. Thus the design of baseboard heaters can make a larger impact than you’d think.
Baseboard heaters need very high temperatures. I would not recommend installing this anywhere. Having big Typ 33 heaters for temperatures below 45°C will greatly increase efficiency of your heating system. Otherwise, a split air con is also an efficient way of heating.
Baseboard heaters are often sized such that very high temps are needed (because that's what cheapest/lowest labor/least space used), but they don't have to be sized that way. In the attic bedroom, we have baseboards around the entire perimeter on two walls and same in the bath. I run the attic zone on the same water temp (outdoor reset controlled to be quite low) as the rest of the house (mostly large cast iron rads, one cast iron convector). Good insulation and air sealing in the attic means that the attic zone calls way less than the downstairs.
My return water temps are 115F (46C) on a P98 design heating day, and obviously cooler on warmer than design days. Cooler is always better, but "baseboards require 180F [82C] water because that's what's on the spec sheet" is a commonly-held but mistaken belief.
Ah this is school knowledge of thermodynamics: the smaller the delta the more efficient the heat pump. For human comfort look at the iso7730. Also the system is self regulatory with such low temps.
Heat pumps and furnaces behave very differently here.
For a furnace you’re talking fractions of a percent difference in efficiency across a wide temperature range so by far the most critical issue is total heat losses to the outside. A heat pump’s efficiency is far more variable making total losses to the outside less important.
First, sooner or later you have to replace your furnace with a heat pump. Second, modern furnaceses are condensing. Return temp should be as low as possible maximise condensing.
We agree return temps should be lower, but to determine how critical this is you need to out numbers on that. In steady state operation at maximum load a difference in return temperatures of 15f is ~1% efficiency. But steady state at maximum load is an extreme situation the average return temperature is well below that theoretical maximum thus an even wider difference is needed for a seasonal difference of 1%.
Redundancy is critical in areas that get really cold. That may eventually mean turning to hydrogen, but a backup gas furnace for a well insulated home really isn’t a major CO2 contributor. More relevant to the discussion it further reduces the impact of minor changes in efficiency or comfort.
As said, at some point you will likely replace your furnace by a heat pump. Then all investment to low temperatures will pay off. A modern gas furnace is no backup for a heat pump, as it also needs electricity. I would recommend a low tech solution like a wood stove for backup. I have one in my cellar. In case of a long power outage I would likely just install this through some open window and do some redneck stuff to make it airtight.
Backup doesn't necessarily mean "in case of grid failure". A gas boiler or a simple electric boiler is a valid backup to a heat pump being inoperative, in the middle of defrost cycle, or otherwise just unable to provide enough BTU/hr into the building due to an extreme cold temperature.
Particularly in temperatures where the heat pump is at/under 2.0 COP for a few dozen hours per year, an electric boiler makes a great deal of sense. It's almost as clean as the heat pump in the extremely rare times when it's needed for supplementation due to extreme cold temps, can allow for a more suitably sized heat pump to cover the 99% of heating hours [meaning greater efficiency there], is extremely reliable due to its simplicity, is fully automatable with existing controls, doesn't require local gas supply network, and is a pretty good match for a heat pump.
Trusting in the power grid for both is obviously not redundant, but it’s cheaper to get independent power for a furnace than heat pumps. Actual redundancy means you want completely independent systems so you shouldn’t then use the same heat delivery loops/pumps/fans either.
Wood stoves require you to physically be home and are kind of a pain to use thus make a poor backup on their own, though a solid 2nd backup.
PS: Most areas are better off with solar thermal + heat pumps. Solar thermal however requires sunlight which not everyone gets in the depth of winter.
I dont see the point in making an emergency option convienient. Forget solar thermal, this is to much hussle for anything. PV plus battery more likly the solution you want.
Less about convenient than being to go on vacation in the winter.
Work out the amount of solar + batteries you need in the winter when the power goes out and your COP is less than 2 because it’s very cold and you need more BTU’s than normal because it’s very cold. Solar thermal is just significantly cheaper on a large home in a cold climate.
No, this does not work. Solar thermal will not work without power. Any battery solution monitoring your home temperatures are enough. Then you can travel home or ask your neighbor.
I moved all my radiators away from under windows (and upgraded the windows to triple glazing) to avoid maximising the temperature differential and energy loss through the wall under the windows, while eliminating the cool drafts that the under-window radiator placement was intended to counter.
My house (built in 1916) was insanely over-provisioned. When we upgraded to a modulating-condensing boiler, we halved the BTUs and are still able to easily keep the house heated to any desired temperature even on the coldest winter days.
The windows all still open, but in winter we have (nearly) enough MHRV (Mechanical Heat Recovery Ventilation) not to need to ventilate directly, eg see:
I'm not sure, but I think that the reason that radiators are placed near windows (at least historically) was to avoid hot/cold spots in rooms.
By placing the radiator near the place that is likely the coldest place in the room, you ensure that the room is an even in temperature as possible. Rather than to counteract 'cool draughts'. I think.
So perhaps people thought that your initial comment was wrong/misleading.
But if you have triple glazing and this mitigates the heat loss, then the coldest wall of your room may no longer be the one with a window, so you may well be doing the right thing for your room(s).
Even if the coldest wall is still the exterior one (it should be, thermodynamically), best maintaining comfort in the room need no longer be by pumping heat out through that wall (or window) to reduce thermal gradients in the rest of the room. Those residual gradients (and, eg, cold drafts down those cooler exterior walls) can be small enough to not need fixing any more.
Energy moves from hot to cold linearly with temperature differences. Hypothetically, if the pipe was the same temperatures as the inside of your home all the heat transferred would be outside the envelope. The hotter the pipe the better this ratio becomes. This is true regardless of what percentage of the pipe is inside the envelope.
However, heating along the exterior of the home under windows and such then you’ll heat the exterior walls to higher temperatures than the interior thermostat thus losing more heat to the outside. Radiant heating on the other hand largely avoids this effect.