Lowering your central heating temperatures

Astria

With my central heating temperature at 60c the return from my radiators is 40c, so a 20c difference. If I reduce my central heating temperature to 50c I don't see a return of 30c - It can still hit 40c (in practice more like 38 but close enough), so therefore I'd need to replace my radiators with much larger ones to get the same efficiency, so I've therefore increased the temperature back to 60c as it seems more efficient.

I'd like to hear from anyone else who has reduced their central heating temperature what the outputs of the radiators were before and after, and whether they replaced the radiators with larger ones to match the lower water temperature. Pipework also comes into play, you can get less heat loss through smaller pipes, but they still need to be big enough to feed the supplied radiators.

[Deleted User]

I very much doubt that any boiler efficiency payback would cover the cost of new radiators. If I was going down this particular route, I would want a full heat loss survey and I would want to make sure that what I ended up fitting would work well with a heat pump.

The other things that I would look at are radiator balancing, and the output of the pump. Small bore piping is a known challenge for low flow temperatures.

Mstty

Simples no heating on yet so the input and return are the same lol

The average temp here predicted at 18oC til the start of November.

Looks like we might start heating lightly on the 5th November

FreeBear

Astria said:

With my central heating temperature at 60c the return from my radiators is 40c, so a 20c difference. If I reduce my central heating temperature to 50c I don't see a return of 30c - It can still hit 40c (in practice more like 38 but close enough), so therefore I'd need to replace my radiators with much larger ones to get the same efficiency, so I've therefore increased the temperature back to 60c as it seems more efficient.

I'd like to hear from anyone else who has reduced their central heating temperature what the outputs of the radiators were before and after, and whether they replaced the radiators with larger ones to match the lower water temperature. Pipework also comes into play, you can get less heat loss through smaller pipes, but they still need to be big enough to feed the supplied radiators.

So with a water temperature of 60°C, you are running at a Δt40° (see https://www.stelrad.com/news-events/blog/the-importance-of-delta-t-in-calculating-heating-output/ ). A return temperature of 40°C ensures that your boiler is in the zone to condense effectively. Reducing the return flow temperature isn't going to increase efficiency by much. Maybe 1% or 2%... Assuming your radiators are correctly sized to run at a Δt40°, you are not going to save anywhere near enough on gas consumption to recoup the cost of fitting larger radiators.

On the other hand, if you intend to replace the gas boiler with a heat pump in the foreseeable future, fitting larger radiators now could be a prudent move. To get some idea of how much larger the radiators need to be, you'll need to run the numbers through a calculator (try this one -  https://www.clyderadiators.co.uk/delta-t-conversion ).

Astria

Dolor said:

I very much doubt that any boiler efficiency payback would cover the cost of new radiators. If I was going down this particular route, I would want a full heat loss survey and I would want to make sure that what I ended up fitting would work well with a heat pump.

Agreed, some people do think just turning down the boiler will save money when it can actually lose money if done incorrectly.

I was actually thinking of a heat pump, but it seems that when when I'll need it most (when outside temperature are low) they are at most 2 COP, and considering electric is still 3x higher than gas I'll stick with gas for now. I'd be happy with a GSHP but I don't have the actual space for it.

BUFF

Astria said:

With my central heating temperature at 60c the return from my radiators is 40c, so a 20c difference. If I reduce my central heating temperature to 50c I don't see a return of 30c - It can still hit 40c (in practice more like 38 but close enough), so therefore I'd need to replace my radiators with much larger ones to get the same efficiency, so I've therefore increased the temperature back to 60c as it seems more efficient.

I'd like to hear from anyone else who has reduced their central heating temperature what the outputs of the radiators were before and after, and whether they replaced the radiators with larger ones to match the lower water temperature. Pipework also comes into play, you can get less heat loss through smaller pipes, but they still need to be big enough to feed the supplied radiators.

Delta T does tend to decrease as the temperatures get closer to ambient/room & therefore emission drops. It will be more efficient at the lower return temp even though the differential isn't as large, but not by much. 
&, of course, it will take slightly longer to heat the room. It's up to you as to how to set the balance to suit you/your building/system.

Miser1964

It's not just about reducing the flow temp. If your boiler allows it, you should range rate the CH output to something close to what is needed to maintain target temp the house when it's -2C outside, which is usually in the 6 to 9kW range. Also have weather compensation on the controllers, course.

https://www.youtube.com/watch?v=SbcVKU1QxJ4

QrizB

BUFF said:

Delta T does tend to decrease as the temperatures get closer to ambient/room & therefore emission drops. It will be more efficient at the lower return temp even though the differential isn't as large, but not by much. 
&, of course, it will take slightly longer to heat the room. It's up to you as to how to set the balance to suit you/your building/system.

This is the point I was coming to make.

OP will lose output (and so, all other things being equal, the boiler will run for longer and in cold weather might might not be able to keep the property warm) but boiler combustion efficiency will increase, not decrease.