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We live in a fairly well-insulated house which uses district heating for the central heating. The individual radiators are controlled by a smart thermostat system. The hot water in the radiators comes directly from the district heating, so the supply temperature to all he radiators is in excess of 70°C. This is obviously unnecessarily high for a well-insulated house.

For the gas-fired central heating typical for my country (the Netherlands), there is a lot of push to lower the water temperature to save on carbon emissions. Obviously, this makes the heater more efficient by being able to cool the exhaust a bit further.

However, I can't seem to find if lowering the temperature* also has additional advantages not specific to a gas heater. For example, right now our radiators are typically very hot in a small area and pretty much cold for the rest, just because that's enough to heat the house. That probably means that we might be losing quite a bit of heat to the outside near the hot spot on the radiator. Also, the crawl space containing some of the pipes gets nice and toasty, although there more insulation might be more cost effective. Finally, I suppose the heat is less evenly distributed through our living room, which might be less comfortable.

I'm billed purely in GJ received, measured as flow times delta T. I'm wondering if I can lower my (billed) energy consumption by lowering the radiator temperature.

*I'm thinking of using a water/water heat exchanger that cools the supply water against the used, cooler water, using a thermostatic mixing valve to bypass the heat exchanger to bring in more hot water if necessary. This way I won't have to use a circulation pump.

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4 Answers 4

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If your walls are well insulated, you'll be losing very little to the outside at that hot spot. If they're not well insulated, there are various products sold for fitting behind radiators to reflect heat and insulate the surface of the wall. These are typically 2mm thick foam with a foil surface. The cheap (e.g. student house) version is corrugated card and kitchen foil.

Insulating pipes in the crawl space can be a good idea, especially if it can lose a lot of heat to outside, or if it passes under rooms you don't fully heat. If the only place for the heat to go from the pipes is into living areas, insulation won't save you much, as the pipes just act as more spread out radiators.

Oversized radiators (once properly bled) can be a problem. They can be hard to regulate down low enough without shutting them off completely, and they hold a lot of hot water even once their (assumed thermostatic) valves close, continuing to heat the room. Radiator covers or even towels over the top are of some benefit in slowing the heat flow into the room, and not over-heating should always save you.

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  • Thanks, this was quite insightful. The constant overshoot is quite annoying, even after regulating the valves to a bare minimum. That's one of the reasons I was looking into the advantages of a lower temperature system.
    – Sanchises
    Nov 29, 2023 at 16:02
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Tl;Dr; Your radiators may be oversized, but it does not matter.

Your radiators have a design temperature and wattage.
The "wattage" is expressed per design temperature, usually for three common values of 75/70/60 C. Notice all radiators always specify multiply temperatures, because water supply temperature is usually something you cannot control - in Your case, it is district who controls it, in case of Gas fired/Coal fired heating units, it depends directly on the unit itself (Coal "prefers" above 60 to prevent calcification, while condenser gas stoves require under 60 for condensing effect to work).

Now, the wattage here means how much heat it can deliver at maximum, not how much is actually delivered.
You need to make an calculation of how much you need for every room. If your house is insulated, you most likely know the energy efficiency class and thus how many watts per square meter you need at maximum in the coldest day. Lets assume your house is in good class and uses 25W/m^2.
If you were buying radiators for the rooms, you would have to calculate the wattage for every room based on its size in square meters, then look up the value in the radiator table for given supply temperature.
I assume you have mixed heat sources - hot (75C) coming from district, and a backup condenser gas stove that prefers 60C (or even 45C). You will have to calculate using the lower of the two. It is not exact calculation, all it matters your radiators are in the right ballpark.

"But", you will ask "the radiator table says at 70C supply the wattage will be twice as when on 60C from my gas heater! Wont the room be too hot then?"
No, because thermostats. In the old days before we had those, we had to size the radiators properly, or the room would be too hot or too cold, there was no way to control that. But with thermostat, the radiator will stop heating when room is warm enough. The only thing that matters is that radiator has enough watts at the given supply temperature to heat the room. It can be twice the size and still work perfectly well, if you want to pay for unused steel.

Your rooms should already heat evenly and properly - if not, make sure the radiators are not obstructed and you are not hanging anything on them. Shorter curtains and such.
"Hot spots" should not matter, because your house is well insulated, so heat should travel across the room, not through the wall and outside (there are behind radiator reflector screens if you are worried about this).

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  • So tl;dr the only reason for promoting a lower temperature is gas heater efficiency which is irrelevant in my case. The radiators are super oversized (I think like 4x the required size) so I guess I could downsize them if I needed the space.
    – Sanchises
    Nov 28, 2023 at 15:36
  • You could, especially if they were installed before house was insulated. But make sure the new ones are big enough to work with modern low-temperature individual heat sources - condenser gas heaters and heat pumps. Heat pumps love low temperatures (floor heating is preferred), but will work fine with your oversized radiators too.
    – Thomas
    Nov 29, 2023 at 9:12
  • Fwiw I don't have mixed heat sources, it's all district heating.
    – Sanchises
    Nov 29, 2023 at 16:04
  • Where does "Coal "prefers" above 60 to prevent calcification" come from? I would expect higher temperatures produce more scale.
    – D Duck
    Nov 29, 2023 at 18:29
  • @DDuck I do not have any english quotable sources. Local automated coal fired furnaces manufacturer states the outlet water temp must be between 65 and 80C. "Common knowledge" from various local forums, blogs and manufacturers dictate 60C is the recommended minimal return temp. This is to prevent condensing & corrosion of internal walls of fireplace from unburned fuel products. You will most likely need automated mixer valve for coal fired, but direct feed is possible.
    – Thomas
    Nov 30, 2023 at 11:04
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For example, right now our radiators are typically very hot in a small area and pretty much cold for the rest, just because that's enough to heat the house.

this means you need to bleed the radiator. There is a large airpocket in the radiator that is preventing the hot water from reaching every part of the radiator.

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  • That's probably true. With oversized radiators, the overshoot is worse if they're properly bled - by overshoot I mean the room gets up to temperature, the valve closes, but the radiator is full of hot water and kicks heat out for another half an hour or so. Turning the valve down a little helps of course.
    – Chris H
    Nov 29, 2023 at 9:38
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It depends how your heat is metered.

In general terms, one litre of fluid that is 40C hotter than the room has the same heating ability as two litres of fluid that are 20C hotter than the room (all other things being equal), so the actual temperature doesn't make a direct difference.

But the system must measure in some way how much energy you take, and that may affect the optimal way to take energy. For example, suppose it only measured flow - in that case you'd want the water as hot as possible to get the most energy out of it for a constant flow. So before you can optimise anything you need to work out how your bill is calculated.

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    The bill is calculated as flow times delta T, so in actual heat delivered.
    – Sanchises
    Nov 29, 2023 at 5:51

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