How important is combi boiler flow rate?

[JPeck]

I need to replace my combi which is a WB greenstar 28i junior. We have a standard semi with a loft extension and the heating works great but filling a bath in the winter is a pain - fiddling with taps to get the hot water out. Is this a flow rate issue? I've read it's best to match a boiler with your flow rate but my flow rate from taps is 16-17l and any boilers this high are far too high kw for my house? 

 

The boiler was installed in 2007 so could it just be because it's old or should I buy a higher flow rate model? 

 

Thoughts greatly appreciated.

[marshian]

2 minutes ago, JPeck said:

I need to replace my combi which is a WB greenstar 28i junior. We have a standard semi with a loft extension and the heating works great but filling a bath in the winter is a pain - fiddling with taps to get the hot water out. Is this a flow rate issue? I've read it's best to match a boiler with your flow rate but my flow rate from taps is 16-17l and any boilers this high are far too high kw for my house? 

 

The boiler was installed in 2007 so could it just be because it's old or should I buy a higher flow rate model? 

 

Thoughts greatly appreciated.

 

I though modern combi boilers could be range rated for CH requirement and have a decent modulation ratio and left un range rated for HW

 

16 - 17 lpm is a good flow rate for bath taps (compared to my house) can you fit flow restrictor to the hot tap to match the boiler output rate - save fiddling around with taps to balance flow and temp

[TonyT]

Have a look at viesmann boilers they can do a 1:17 modulation I think from memory

[JohnMo]

Flow rate from combi boilers can be rubbish. The figure published is not a real UK figure, as it's based on a summer only rate, where incoming water is warmer. The UK models have a flow restriction added so it can cope with colder winter water incoming to the boiler. This reduces max flow rate all year.

 

There are ways around it. Have a read of the attached. From real experience they work exceptionally well, I have one, as an eBay buy.Canetis-SuperFlow-Product-Sheet-WE-050318.pdfCombi-SuperFlow-White-Paper-v1-2-4.pdf

[Marvin]

I thought the size of the instant hot water boilers which also supply heating were always sized related to the hot water flow requirement for the property!  This almost always means the boiler can easily heat the property as that requires less power? 

 

However the water flow rate into the building needs to be taken into account as well.

 

Good luck

 

M 

 

 

Edited October 20, 2023 by Marvin
changed energy for power

[JohnMo]

3 minutes ago, Marvin said:

thought the size of the instant hot water boilers which also supply heating were always sized related to the hot water flow requirement

Trouble with that rule of the plumbers thumb, the gas boiler is nearly always way too big for the property, in my case it was 11x too big for the max heating load. So it lived in short cycle mode, until I got the first gas bill and nearly fell over. I then added a 160L buffer to keep it happy.

[marshian]

34 minutes ago, JohnMo said:

Trouble with that rule of the plumbers thumb, the gas boiler is nearly always way too big for the property, in my case it was 11x too big for the max heating load. So it lived in short cycle mode, until I got the first gas bill and nearly fell over. I then added a 160L buffer to keep it happy.

 

That's an approach I haven't heard of before but in my head it makes sense.

 

Any chance of some pictures?

[Marvin]

43 minutes ago, JohnMo said:

Trouble with that rule of the plumbers thumb, the gas boiler is nearly always way too big for the property, in my case it was 11x too big for the max heating load. So it lived in short cycle mode, until I got the first gas bill and nearly fell over. I then added a 160L buffer to keep it happy.

 

Yes you are of course right! I am used to smaller properties....