Flow and return temperature differences

All the rads get hot when the H/W zone valve closes. but the low return temp remains

this is why i said cheak your bypass. if you open this up the return temp would be closer to flow temp hence protecting your heat ex

but if you leave it look on the bright side the blr is going to condense till the cows come home :LOL:
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Rads get hotter when HW satisfied because you have taken a 6 - 10 kw load off the system
As Tamz points out the pipes appear to be undersized.

What times of the day do you have the boiler running?

What room stat temperature setting have you selected?

Im not at the house at the mo. what kind of size pump should it be at a best guess and Il check tomorrow night. sorry if im messing you around I do appreciate you answering me and your time
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would the small pipes cause the heat loss? the upstairs and down stairs are on morning and night and are set at 21* there is on roomstat upstairs and one down stairs
Minimum 15/60, maybe bigger.

would the small pipes cause the heat loss?

No the small pipes restrict the flow around the system. Pipes of all diameters can only carry a certain heat capacity. Smaller the pipe smaller the capacity unless you increase the speed then that brings a whole new set of problems.

You need bigger pipes as the main priority.
not realy an option to increase the pipe sizes wouyld bigger pump do it?
At least increase the size of pipes from boiler through the pump to where it branches to the HW. If boiler is in the same cupboard it won't be difficult, and i know it is only 22 connections off the boiler. That one always amazes me but pipes should be increased. You can fit 28mm x 1" bent male adaptors to the boiler instead of the supplied pipes.

15/60 might help.
What times of the day do you have the boiler running?

What room stat temperature setting have you selected?

Whilst you wait for some one to change some of you pipe work as tamz has suggested it would give an improvement to your perceived problem if you converted you S/Y plan to DHW priority,(W plan) this would allow the boiler to heat the cylinder whilst isolating the heating circuits until the cylinder is heated.

Having said that, if the cylinder and heating zones are on separate time clocks you could stager the start up times to achieve a similar result when starting from cold.

Have had a quick look at the Installation and servicing instructions and it would appear the 40CDI does not have the facility to allow two different boiler flow temperatures for the cylinder and the heating circuits which is a shame as this would allow you to have reduced you gas consumption.


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