boiler / rads flow and return temps

[jackecole]

flow and return water temps from the boiler / each rad are usually 80 and 70 degrees c on a conventional boiler, but are they lower on a condensing boiler?? if so should larger rads be used preferbaly???

 

[D_Hailsham]

flow and return water temps from the boiler / each rad are usually 80 and 70 degrees c on a conventional boiler, but are they lower on a condensing boiler?? if so should larger rads be used preferbaly???

Rad outputs are measured with a flow temp of 75C, return temp of 65C and room temp of 20C.

Boilers need to have their return temp below 55C for condensing to take place. So if you have a flow temp of 75C, that means a temp difference of 20C instead of the usual 10C.

The output of a radiator with flow 75C, return 55C and room temp 20C will be about 15% less than stated, so you would need to install rads which are nearly 20% larger than the heat loss calculators say, to take full advantage of the increased efficiency due to condensing.

Theoretically, the flow temperature should be adjusted to take account of the outside temperature, as the amount of heat required to maintain a temp of 21C when the outside temp is 10C is half that required when the outside temp is -1C. This is called weather compensation and can be found on the better boilers.

If the house has been well insulated, after the heating system was installed. you may find that the existing radiators are already oversized.

 

[jackecole]

Hi D_Hailsham, tank you for replying.

WHat I am concerned is, more so when heating is also on!! is that if the return temp for my rads is not set to below 55 degrees, does it mean that I am wasting energy or not getting the most from my condensing boiler? How do i make sure the return temperature is below 55 degrees? does this also require the flow to be redued and how do i do that?

 

[bengasman]

In a nutshell, larger rads are betten on a steamer because it is easier to realise a 20 degree drop on a 3kw rad than on a 500Watt model.
It also allows you to keep the flow temperature lower which means the boiler runs more efficiently.
Although there is a rather steep difference around the 55 C mark, going even higher reduces the efficiency further, and when you keep it less warm the efficiency keeps going up for a while; it's a sliding scale rather than a switching point.

I wouldn't worry about it too much as there is very little a householder can do about it, if you need a new boiler it has to be condensing anyway, a steamer will always be more efficient than a non-condensing model, and at the end of the day, the installer will advise you.
Provided you find yourself a proper one of course.