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%.

 

[fezster]

The return temperature will dictate the amount of condensing which occurs. The lower the return temp, the more it condenses (45 degrees for 90%+ condensing, 55 degrees will be less than that). However, even if the boiler is setup for 80 degrees flow temps, and assuming the system has been setup for a 20 degree drop, the boiler will still condense as it ramps up in temperature. Only once it gets to 75 degrees+ flow temp will the condensing efficiency drop significantly (I believe even then, a certain percentage of the heat exchanger will continue to condense), until no condensing occurs when the entire hex return temp is > 55 degrees.

The flip side of this is that older houses may have radiators sized for a non-condensing boiler, with an 11 degree flow/return temp drop, which means that at 20 degrees, the radiators will output less heat. Hence, in the depths of winter, the flow temp may need to be cranked up to 80 to compensate.

The other consideration is a DHW cylinder, which will be set to 60 - 65 degrees to prevent legionella. This means the flow temp of the boiler cannot be lowered too much, otherwise the cylinder stat will never be satisfied.

This is where smarter controls can help, eg. Vaillant controls will ramp up the flow temp to 80 for DHW mode and use weather compensation to adjust the flow temp dependent on outside temperatures and the heating curve adjusted to suit your setup.

 

[John.g]

One of the problems associated with OT control or Opentherm? is that if the boiler cycles due to its min output being < heating demand is that the boiler can have great difficulty in modulating down fast enough on re firing to avoid tripping out on flow temperature, there are numerous posts on here re that problem.

 

[fezster]

I had that exact problem with my Vaillant 438 many years ago John. Despite d.0 range rating, the boiler would first fire at X% of it's total output. A later revision of the PCB improved on this (although, it continued to be a problem to an extent).

 

[doitmyself]

One of the problems associated with OT control or Opentherm? is that if the boiler cycles due to its min output being < heating demand is that the boiler can have great difficulty in modulating down fast enough on re firing to avoid tripping out on flow temperature, there are numerous posts on here re that problem.

I think you meant to wiite:

"if the boiler cycles due to its min output being > (greater than) heating demand.

Yes, that can be a problem, but only with boilers which restart then automatically go to max output. More intelligent boilers will restart then go to the same output as when they turned off.

 

[John.g]

Surely its not a big contol deal to increase the SP temperature temporarily to allow the boilers to get away, some Vaillant's hold the firing at 60/70% for 1 minute before releasing the contols to modulate. Oil fired boilers spend their entire lives cycling without problems.