Flow temperature for a combi boiler

Dedekind

Hello

I had recently had a Worcester Bosch Greenstar 8000 combi boiler fitted. The fitter set the flow temperature for CH at 59 degrees and DHW at 57. 

For DHW i think it's OK. But for CH the manual says 65 is suitable for most installations. 

Is there any preference? I see that currently I cannot get the house too hot, my thermostat showing that it doesn't go above 22 even if I set it to 25. This can be due to a number of other factors such as location of thermostat and so on. Also I don't want more than 22 so it's not a huge concern. 

But I do want to set the flow temperature at a value where its most efficient. Also reading online says something lime 70 or even 75 is most efficient which sounds counterintuitive to me. 

daveyjp

Hot water start low and increase until you are comfortable.  In summer ours might be as low as 40, its now at 48.  No point using gas to heat water so hot you then need to add cold to cool it down.

Heating circuit similar.  Its been quite mild so far this winter so its at 58 and it gets the lounge to 21 quick enough.  If the outside temp drops it will be increased a few degrees.

Ectophile

For a condensing boiler, low is better.  The return temperature to the boiler (which you can't control directly) should be below about 55C, or the condensing doesn't happen.  Without the condensing, it's no better than an old-fashioned boiler.

Another_Level

Them boilers come factory set up at 65°C for heating (radiators). That can some times be to low depending on other factors.

Dont know why it was set so low, maybe ask the installer again if there is a reason why.

Is the central heating system radiators or under floor heating ? 

If radiators, you can adjust the heating thermostat upwards the instructions are in the user booklet under Adjusting the appliance flow temperature -Heating Key.   There will be videos on youtube 

I would run it at 70°C  it will still be efficient the extra gas burnt is minimal and you will be warmer 

[Deleted User]

Another_Level said:

Them boilers come factory set up at 65°C for heating (radiators). That can some times be to low depending on other factors.

Dont know why it was set so low, maybe ask the installer again if there is a reason why.

Is the central heating system radiators or under floor heating ? 

If radiators, you can adjust the heating thermostat upwards the instructions are in the user booklet under Adjusting the appliance flow temperature -Heating Key.   There will be videos on youtube 

I would run it at 70°C  it will still be efficient the extra gas burnt is minimal and you will be warmer 

Most combi boilers are much bigger than is needed to keep a property at its set temperature. Installers go down this route because homeowners want rapid HW heating. If the heating system has been set up properly then the difference between the flow and the return temperatures should be 20C. Provided that the return temperature is below 55C, then heat will be extracted from condensation. The lower the return temperature the greater the amount of condensing and heating efficiency. Heating efficiency reduces energy costs.

Running a boiler at 70C does not make a house warmer.

Apodemus

Dolor said:

Running a boiler at 70C does not make a house warmer.

Both physics and experience suggest otherwise!

[Deleted User]

If a property has no heating controls, then I would agree; however, that isn’t normally the case. Most homes have thermostats and/or TRVs. Once the set temperature is reached, then the boiler will shutdown. The only difference between having an 80 degree flow temperature to a 60 degree flow temperature is that a property will take longer to reach the set temperature at the lower flow temperature.

https://www.theheatinghub.co.uk/why-our-condensing-boilers-do-not-condense

For what it is worth, my system boiler is set at a flow temperature of 60C. Sadly, I cannot set it any lower as I need to heat a HW cylinder. Our 4 bedroomed house is heated throughout the day to 20C downstairs and 18C upstairs and my gas use is 7000kWhs/year. The reduced flow temperature has the added advantage of reducing the amount of boiler cycling; boiler efficiency is increased and we have no major temperature overruns. The thermostats vary the boiler start up time to achieve the required set temperatures by the programmed time in the morning.

Apodemus

Dolor, in an adequately insulated house, with large enough radiators, what you say is true - time taken for rooms to come up to temperature is the main difference. 

But the vast majority of houses in UK are not in this fortunate position and there will be many properties out there where, despite having TRVs, the temperature in parts of the house are, in practice, limited by the main thermostat and where a higher flow temperature would manage to overcome the thermal loss from the room.  This is the main reason why many of us have to turn up the system temperature in winter.

In the OP's example, with the system as set up, it is not capable of providing enough heat to overcome the thermal loss to take the temperature over 22C but if the flow temperature was higher, the probablility is that the radiators would manage to transfer more heat and the maximum achievable would be higher. 

Many of us are in properties that would reach this point at much lower temperatures!  I was recently looking after a house that just couldn't heat one of the rooms and turning the boiler temperature up resolved the problem.

Bendy_House

Dedekind, that's a top-range boiler. What controls were fitted with it? The more sophisticated type won't turn on the heating when you have asked it to, but will work out when to turn on the heating to get your house to the temp you want, when you want. Ie, this could change from day to day depending on the ambient temp.

And does it have weather compensation - a sensor on the outside of the house to tell the boiler how 'hot' it should go to get the house comfy? Ie, on 'freezing' days, it'll turn up the flow temp to compensate.

Norman_Castle

I set mine at 55 which didn't heat properly. I've raised it to 60 and its now working well and based on the drips from the condensate pipe its condensing helping the efficiency. With yours try setting to 65.

https://www.theheatinghub.co.uk/why-our-condensing-boilers-do-not-condense

BUFF

For a condensing boiler the lower the return temperature the better for efficiency.The next set of Building Standards Part L 2022 will make the maximum designed flow temperature of new/retrofit installation to be 55°C or lower.