Boiler not getting hot enough

[doitmyself]

We moved into our current home in June. The heating is supplied by a 10 year old WB Greenstar CDi 37kkW combi boiler. I had the boiler checked by a local GSR engineer who has serviced my son's boiler for many years. He said it was working correctly.

So far it has worked OK, getting the house up to temperature each morning. A few days ago, however, when it began to get much colder outside, we noticed that the room temperature was much lower, even though the room stat was still set to 21C. I turned the stat right up to 24C but the room temperature only rose half a degree.

I decided to watch the boiler and found that it was heating up to about 55C (set to approx 75C) then switching off. Pump overrun would start and the temperature would drop to about 35C before restarting. So the cycle continued. I checked parameter 9C (heat output) and it was only getting up to 50%.

Why is this happening and what's the solution?

 

[John.g]

Has anyone tried one of these??

https://www.caleffi.com/sites/default/files/file/01166_09en.pdf

 

[doitmyself]

If you do (range rate) the output to say 12kw, I wonder does the 50% fan start up then mean that the output is limited to 6.0 kw or is it 30/2 15.0 kw or 12kw?

The fan speed at start up doesn't change. It needs to be at a certain speed to ensure reliable ignition. Once the burner is properly lit and the flame well established, modulation can take place. Of course it would be really helpful if range rating a boiler afftected both the maximum and minimum as I could then set my boiler to 4kW -12kW instead of the current 10kW to 30kW (standard) nor 10kW to 12kW (top end rated down).

The Caleffi device looks interesting, but it's really designed for balancing of heating systems. In any case you need to have a lot of data available (flow rate and differential pressure) so the correct size devices can be selected.
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Good to see you understand that this is a problem.

It's not to bad a problem as the boiler's pump can be remapped in the settings to act as a fixed speed pump with six (yes, 6) speed settings.

Range Rating
According to the 'Engineer' Service Booklet' for CDi Greenstar boilers the 37 and 42 combi versions cannot be range rated, other versions can. Can anyone confirm this and/or does know why this is?

 

[John.g]

The 10 minute delay to "full" firing, what determines this?

 

[doitmyself]

The 10 minute delay to "full" firing, what determines this?

Where do you get the 10 minutes from?

 

[John.g]

I may have been looking at the CDI classic which says.....Gas Flow rate- max 10 minutes after lighting.

It says the same for the CDI 35 model 47 311 94 & the CDI 40 model 47 311 95

 

[doitmyself]

I may have been looking at the CDI classic which says...Gas Flow rate- max 10 minutes after lighting.

It says the same for the CDI 35 model 47 311 94 & the CDI 40 model 47 311 95

That's not saying that the boiler needs 10 minutes to reach max output. It's just telling the engineer to wait 10 minutes before carrying out a gas flow rate test. It's similar, but not exactly comparable, to letting a car warm up before doing an emissions test.

 

[king of pipes]

That's not saying that the boiler needs 10 minutes to reach max output. It's just telling the engineer to wait 10 minutes before carrying out a gas flow rate test. It's similar, but not exactly comparable, to letting a car warm up before doing an emissions test.

You have quite a good understanding of your boiler sir , but its far to big for the 9kw of radiators connected to it , any good GSI should be able to down rate that boiler this will not affect the hotwater . Kop

 

[doitmyself]

You have quite a good understanding of your boiler sir , but its far to big for the 9kw of radiators connected to it , any good GSI should be able to down rate that boiler this will not affect the hotwater . Kop

I have read that in the installation manual, but the Greenstar CDi Service booklet says this:

I have taken that to mean that the 37 and 42 versions are not adjustable; but now I'm not so sure. The current value is 82% and it doesn't change to 100% if I reset the boiler using function 8E. (I made a note of all parameters before resetting, in case any had been changed by the installer. They hadn't.)

So does the "not on 37 and 42 combis" just mean that they are not adjustable above the default setting? Looking at the data tables for the CDi range, I'm beginning to think it does. Very confusing. There's only one way to find out ...
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The current value is 82%

That was quoting from memory; it's actually 84%.

Using heat input figures (output isn't suitable as it varies with condensing), the maximum input is 37 kW for HW and 30.9 kW for CH. So the CH output is 83.5% of the boiler's max output, which is near enough to 84% when the display only shows two digits.

I have down-rated the CH to 35%, which is approx 13kW. It will be interesting to see what effect it has.

 

[doitmyself]

Having lived with this problem for nearly two months, I decided it was time to call in a pro. His initial diagnosis was a faulty WB DT10 prog stat - the transmitter was losing contact with the receiver in the boiler. I suggested that this could be checked by moving the transmitter closer to the boiler, which he did. It made no difference! The engineer then decided to check the heat exchanger, so he took the inspection cover off. The hex was blocked almost solid. 10 minutes later, after wielding his special cleaning brush, the channels were clear of muck and cover replaced. Boiler lit - and stayed lit. Water temperature rose to nearly 70C without the boiler off. Room stat did its job. Everything working as it should.

For future reference:
what causes the hex to get blocked?
what is the actual material causing the blocking?
does it mean that the hex will soon need replacement (or a new boiler)?

 

[Gasmk1]

it is a natural process the flue gases are mildly acidic would not have happened if boiler serviced regularly

 

[doitmyself]

it is a natural process the flue gases are mildly acidic would not have happened if boiler serviced regularly

Thanks.

So are the flue gases eating into the hex, which will eventually become perforated?

 

[Gasmk1]

Thanks.

So are the flue gases eating into the hex, which will eventually become perforated?

not before the boiler is defunct, and not if you have it serviced regularly.