Setting central heating flow temperature

[Matt0029]

What temperature is best to set the flow too on a central heating boiler? Setting it at a slightly lower value can make the boiler condense more?

 

[ShaunCorbs]

Depends on system design temperature but these days maximum of 65 dc

 

[Matt0029]

Depends on system design temperature but these days maximum of 65 dc

Thats so the return should come back at a low enough temperature to condense? Have noticed when serving some domestic boilers they hardly condense at all after initial start up.

 

[ShaunCorbs]

Correct ideally you want it below 50 all the time

 

[John.g]

Thats so the return should come back at a low enough temperature to condense? Have noticed when serving some domestic boilers they hardly condense at all after initial start up.

There are a few methods that will give meaningful condensing with return temps of 35/45C, including TRVs and outside temperature compensation. TRVs can give a very rapid house heat up from cold and then return very low return temps but they achieve this by throttling the water flow which can result in very high heat exchanger deltaTs which will trip most gas boilers if > 30C. Outside temperature (weather) compensation can also give very low return temps depending on the curve setting but depending on the temperature setting which may only be 40/45C can give a much slower house heat up.
Most people want very rapid house heating and don't really care about condensing, one reason IMO why you can frequently see little or none.

 

[fronty]

Most people want very rapid house heating and don't really care about condensing, one reason IMO why you can frequently see little or none.

As a consumer I completely agree, I personally couldn't care less if they condense or not (for the sake of a few more % efficiency), I just want the house to be warm so I don't get complaints from the wife!

 

[Matt0029]

So if people are just setting boilers to max 80c and only getting say a 8 degree drop on the return. The boiler will hardly condensate. So will the boiler be anymore efficient then the old boilers that weren't condensating boilers?

 

[SJB060685]

If you have a flow temp of 80°c and a delta t of 8°c then it will never condense. Mains gas dew point is about 55-57°c, kerosene (oil) about 47°c.
There are many advantages to condensing, not just added efficiency.

 

[Matt0029]

Thanks yes but most boilers I go to are just set on max, so must hardly condensate.

 

[John.g]

Can't comment on gas fired boilers but OF condensing boilers have a extra heat exchanger which will reduce the flue gas temps even if never in condensing mode which most never are IMO, my SE (non condensing) Firebird has a flue gas temp of 230C and would expect ~ 100/120C if a HE (condensing) unit with a efficiency gain of at least ~ 5 to 7%.