Unable to balance rads and achieve 55 deg boiler return temperature

[wobbly2]

I recently moved into a mid 50's 2 bedroom bungalow and I'm having issues with the heating and would be grateful for some advice.

I am unable to balance the radiators to give a temperature differential of anything greater than 1 to 5 degrees across the rads and 4 to 8 degrees across the boiler after the system has been running for an hour or so with the boiler output set at 70 degrees and all the TRV's heads removed. The lock shields are open from. The only way I can get the boiler return temperature down to 55 degrees or lower is to turn the boiler heating output down to 60 degrees or lower.

I've read different things on different websites which have left me somewhat confused but have established that the problem could be caused by any or all of the following:

• Blockages/air in system
• Poor balancing of the radiators
• Lock shield open too much
• Pump speed too high
• Boiler output to high

Also

• The heating system has a Glowworm 30cxi boiler with 6 radiators.
• The boiler manual states a maximum of 20 degs boiler differential at maximum heating output of about 82 degs but it doesn't give figures at other temperatures.
• Ideal temperature drop across radiators about 11 degrees
• Ideal heating output temperature about 70 to 75 degrees
• The bungalow is about 724 sqft/67 sqm.
• The external wall cavities are insulated with 75mm blown fibre.
• The windows and external doors are double glazed with 20-25mm gaps

Back in the summer:

• The system was cleaned, flushed and refilled with new inhibitor and there are no cold spots on the radiators
• The heat exchanger, expansion and condensate tanks were also replaced

The boiler manual doesn't say anything about adjusting the pump speed and as to the boiler being too big I would need guidance on that.

There are 3 different makes of lock shield with 3 different number of turns from fully open to fully closed. They are open from 1/2 turn to 5/8 turn.

Any advice would be much appreciated in particular regarding the pump speed and whether the boiler is too big.

I have a drawing detailing building layout, room sizes, windows, pipe runs and sizes etc which I can post if it would help.

 

[Swwils]

You might be fighting against valves that have very little valve authority. Quite alot of them allow so much flow that they would saturate most rads at even just a 1/4 turn open, so you need to do it very slowly.

I wouldn't worry to much about getting massive temp differences across rads - the biggest issue will be the return temp being low enough for condensation (below 55).

That can easily be sorted by weather comp or reducing your output temperature.

Unfortunately not much can be done if your boiler is oversized for the demand - that 30 can only modulate down to 5kw which will probably exceed your required output most of the year.

If you have TRVs, check that the pump is operating in a proportional pressure mode, but I think the cxi is just a single speed pump.

 

[Gasguru]

The Cxi isn't a great boiler, you can't set heating output to suit and the pump is a single speed sized for the boiler hot water performance...so it will always be overkill for the heating which is why the manual mentions adding an overall flow valve for the rad circuit.
Most rad lockshields are garbage...an 1/8 of a turn can be sufficient for some small rads/towell rails.
High efficiency boilers are all designed around a 20 delta to reduce pump losses.
Don't get too hung up about this magic 55 degrees...just adjust the boiler flow temp to suit the weather, cranking to max if necessary when it snows.

 

[wobbly2]

I've researched the boiler quite a lot and they do seem a bit pony 'n' trap but hopefully not for the knacker’s yard just yet.


I'll persevere with lock shield adjustment to try and get a better return temperature but as 3 of the valves are 1&1/4 turns from fully open to fully closed, 2 are 4&3/4 turns and 1 is 3&3/4 turns, judging the amount the valves are open will be more difficult that of they were all the same make/number of turns?


I've noticed that if the valves are closed too much the water flow through them becomes annoyingly noisy. This is most noticeable with the bathroom rad, which is also the heat leak rad (no TRV), any less than 5/8 turn it is noisy.



Is weather compensation something that I could fit and, if so, what system do you suggest, or is it something that requires Gas Safe fitting coz it requires access to the inside of the boiler?

I have been turning the boiler output down to achieve a better return temperature but I would prefer not to keep tweaking it.



With regards to the boiler being oversize, its Commissioning Log suggests its CH Heat Input was at 23.12KW (Spec shows max Heat Input Nett Q =23.12KW), Burner Pressure at 20mbar and DHW Input was 30.15KW (Spec shows max Heat Input Nett Q =30.15KW); can this be adjusted by Gas Safe engineer or is it factory fixed hence Swwils's comment about "Unfortunately not much can be done if your boiler is oversized for the demand"? I recall the boiler's burner at my previous home could be adjusted for input.


The Commissioning Log also shows a Flow Temperature of 81 degrees and Return Temperature of 70 degrees (presumably with the user temperature control set to maximum output). So, when installed in midsummer 2005 its Delta T was only 11 degrees and the boiler was way off condensing or am I missing/mis-understanding something (I realise that in summer there could be very little heat loss through the radiators)?


A 2012 service report says the GAS Rate was 2734KW; if Gas Rate is the same as Input then apart from a missing decimal point the boiler Input seems to have risen if it's for the CH or fallen if it's for the DHW or does this indicate something else? Or again am I missing/mis-understanding something?



As for adding an 'overall flow valve', the manual section 4.3 refers to altering the [Central Heating] flow rate by fitting a 'lockable balancing valve in the flow or return pipes shown as valve "A" in diagram 4.1'. To confuse things, Valve "A" in Diagram 4.1 is labelled 'Flow Control Valve'. Just for clarity can you confirm that the 'overall flow valve', 'balancing valve' and 'Flow Control Valve' are one and the same thing?


Regarding fitting a valve, could I use one of the 1/4 turn isolation valves fitted to the magnetic filter in the return for the purpose of CH flow control, least ways in the short term, and then change it later for a gate valve?
Would a gate valve be suitable or should I use something else?

Many thanks