lower temp when HW and CH on together

[bhm1712]

when i have HW on by itself, the boiler heats the water up to 72 degrees and the HW cylinder heats up to full temp in about 50-60 mins. If the CH is on by itself, the rads heat up to 72 degrees. However, when CH and HW are on together, the boiler only heats the water up to 62 degrees and so the HW takes about 20 mins longer to heat up to full.

is this a faulty stat, or purely because the boiler is having to heat up a larger volume of water? if this was the case i would have thought it would still heat up to 72 degrees, but it would take longer to achieve 72.

The system is a Honeywell C plan
Glow worm space saver 40Br

 

[bhm1712]

any ideas gents?

 

[D_Hailsham]

It's because it is a C Plan. This means it has pumped central heating and "gravity", i.e. un-pumped hot water.

When just the heating is on all the heat from the boiler goes through the radiators; the same is true when just the HW is on. When you have both on together most of the water will be circulating through the rads; the HW circuit gets what's left over, so it will take longer to heat up.

There is also another "radiator" (the cylinder) to heat up, so more heat will be lost and the return temperature will be lower. If the boiler cannot produce the extra heat required the flow temperature will be lower.

The solution is to change it to a fully pumped system, e.g Y Plan or S Plan.

 

[bhm1712]

it makes sense now!

easy solution....until i change to a proper fully pumped, just dont have HW and CH on together!

i have changed the times already!