New CH system underpowered/ wrong layout?

[Centralheatking]

30 kw on 9 rads should mean the place is steaming hot - I run my place with a 30kw boiler - 5 bedrooms, 3 bathrooms and hot water cylinder and never have to go past 3/4 heat - thats 16 rads and none are small. r

The OP has a boiler which modulates between 9.4kW and 30kW. He has 9 rads, which at, say, 1.5kW each is 13.5kW total load. Even if they were 2kw, which is doubtful, that would still be only 18kW. So whichever way you look at it the boiler is oversized for the heating load.

There's a good chance that the flow water temperature rises to 75, or whatever is set, but the return temperature keeps on rising, meaning the boiler then cuts out, waits and then restarts. So the mean water temperature is well below the required value, which means the rads are not giving off the correct amount of heat.

The OP should use Whole House Boiler Size Calculator to find out the heating requirement and Stelrad Elite Catalogue, first table to estimate the output of his radiators.

 

[doitmyself]

Is it not possible with these to down rate or limit the heating output to say 15-18kW, as with the Vaillants?

According to the Installation Manual, the Max output can be adjusted but, unhelpfully, they don't tell you how to do it.

Not so sure the OP should have to do any calculations to confirm one way or the other

It won't do any harm to have some figures to back you up. "I feel cold; and this is why!"

 

[vern]

I have said it before but here goes COMBI'S ARE EXEMPT FROM WHOLE HOUSE HEAT CALCS!.......
due to many reasons but the main one is that their outputs are based upon hot water production, obviously this should be matched to incoming mains water pressure and gas runs from a responsible installer but often are not!

To get round this good manufactures get round this by allowing range rating.

Worcester bosch get around this on the junior and Si with a code plug that down rates from 24kw to 12 kw, the CDi can be adjusted from 30kw to around 9 kw on 38 and 42 CDi and 30 kw to 7 kw on 29 and 34 Cdi (this can only be accessed via installer menus, I am not posting how as the menu contains all stuff that should not be touched unless you are trained by WB, pump maps, turbine delays, and much more!

the 38 Cdi is actually 40 kw on DHW NOT 38 marketing bluff as not to pull sales from 42 cdi

i actually look after some trendy apartments that have 42cdi's and 2 rads and a towel rail! 42 kw for hot water due to silly sized baths!

 

[AWheating]

go on the worcester course and you will get taught how to range rate the cdi. Or phone the tech helpline and get talked through it.

 

[doitmyself]

I have said it before but here goes COMBI'S ARE EXEMPT FROM WHOLE HOUSE HEAT CALCS!.......
due to many reasons but the main one is that their outputs are based upon hot water production, obviously this should be matched to incoming mains water pressure and gas runs from a responsible installer but often are not!

I agree that Hot Water production is the overriding factor when sizing a combi boiler, but the specifier has to take the Central heating requirement into account. If he doesn't you can get the situation where the minimum CH output is above the CH requirement of the house. For example a house with an 8kW heating requirement which has a Vaillant 837 or 937 installed (12-28kW CH output)

 

[AWheating]

if the OP uses a online calculator he/she will get a decent idea if rads are sized well.

 

[stevetheplumber]

possibly the ops best course of action may be to get wb in to see whats wrong if he calls it in as a boiler fault they will then tell him if its an installation fault they may charge him if theres no boiler fault but then he will have an idea of whats wrong and can claim against the installer if there remedial work needed
the pipework sounds ok in that theres 2 pipes going every where and in 22 to within one meter of every rad sounds over kill

 

[chris watkins]

To add to doitmyself post above the problem is not always the down rating or modulation of these lager boilers the one thing that the engineers changes can't change (as far as I know) is the CPU's electric control routines which run the start-ups firings, modulations & anti cycling etc all of which remain pre-set for the largest outputs & can still effect the boilers performance even when down rated.

 

[vern]

I agree that Hot Water production is the overriding factor when sizing a combi boiler, but the specifier has to take the Central heating requirement into account. If he doesn't you can get the situation where the minimum CH output is above the CH requirement of the house. For example a house with an 8kW heating requirement which has a Vaillant 837 or 937 installed (12-28kW CH output)

The technical reason are turn down ratios.....but even the 42 CDI's i look after don't have this problem, with 3 rads if set up right, min output, anti cycle delay adjusted from standard 3 mins, for reference the Cdi does not have a return sensor, just flow with is mapped via PCB software to keep flow temp to within 5 dregs of set point on boiler stat, this can also be adjusted in menus but not if you don't know what you are doing, any GSR can fit a boiler ( commissioning and maintenance is altogether a much tougher job)

 

[vern]

To add to doitmyself post above the problem is not always the down rating or modulation of these lager boilers the one thing that the engineers changes can't change (as far as I know) is the CPU's electric control routines which run the start-ups firings, modulations & anti cycling etc all of which remain pre-set for the largest outputs & can still effect the boilers performance even when down rated.

Incorrect Chris on WB anyhow, I was typing the above whilst you replied