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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 'Registered Professional Gas Engineer' 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?
 
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 'Registered Professional Gas Engineer' 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?
I have a heat only Gloworm which is very complex re the Boiler thermostat operation and the condensing boilers as will the combies be similar in their technicalities re water temp control. These condensing boilers have a similar PCB to a computer which does not work properly sometimes. The PCB through the inputs from the sensing stats /devices instruct the gas valve and the associated controls to put the appropriate heat into circulation. Many sense the return temp and do not put out the heat that is required for the system to operate properly. Mine in my static caravan has a 'mind of it's own! sometimes it will get right up to the 'target' temperature sometimes it does not. It does not especially when I want it to! When it should 'call for heat' it does not. I have the benefit of fitting the same brand new boiler in a mates house that was seen to behave absolutely perfectly and did not stop once and never has done in about 4 years. When it does start playing up I would wager it will be the PCB that is not behaving as it should but I could be wrong. I have a PCB that has not been fitted 'yet' because while our caravan is satisfactorily warm I'll leave it on top of the boiler in it's packet. Renovated PCB's can be got from International Boiler spares Tyne & Wear for about £50 with a years guarantee. If you are not an expert I would be careful though to check all incoming devices that supply the board with info as that is essential before replacing anything. It may well be the board though but then again it could be the gas valve or even a tuppeny hapeny sensor. Luck does not and never will come into that equation.
I was once asked donkey's years ago what are these new fangled combies like = I said they are all right till they go wrong... Mark my words though in respect that they will go wrong and if you are unlucky enough they stop on Christmas eve!
 

centralheatking

Esteemed
Plumber
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 'Registered Professional Gas Engineer' 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?
doitmyself...repost your post has got mucked up, its very unusual
centralheatking
 
WB DT10RF version 2. It;'s the 7 day version with Optimization, which is turned on. The HW preheat is turned off.
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There's nothing wrong with my post; it's what I wrote.
Did you read my post about how complex the target temperature is on the modern Combies and Heat only Boilers? The PCB is a computer and evaluates the return temp the water temp within the boiler and even with a 'clean' boiler the PCB and associated sensing devices can put out or rather limit the temperature even though the target temperature is set higher than the boiler stops heating the water within the boiler. Some very good engineers are finding htis symptom is a nightmare to rectify because on many occasions it is intermittent. All the sensing devices are checked OK with a good meter & yet the older boards do not get to the target temperature. This subject hjas been discussed by quite a few on the heating forums. It seems to be the most frustrating symptom of all (the mother of all symptoms) You could as I said get a refurbished board from International Boiler spares (they do all boards) £50 approx + £8 next day post. Years guarantee You've got to start somewhere? For starters have you checked the value of each and every sensing device as per the installation manual?
 

oz-plumber

Esteemed
Plumber
Done that, They are all on the default setting.
The reason I said this is because some boiler brands use the same PCB throughout their range.
You sometimes have to alter a parameter to adjust the PCB for the output of the boiler it is being installed into.
Sometimes you have to alter a few parameters to suit the boiler model/ configuration and type of installation.

It may be why the heat parameter is only getting to 50%

The previous owners may have had a board replaced and not adjusted to suit.
 
The reason I said this is because some boiler brands use the same PCB throughout their range.
You sometimes have to alter a parameter to adjust the PCB for the output of the boiler it is being installed into.
Sometimes you have to alter a few parameters to suit the boiler model/ configuration and type of installation.

It may be why the heat parameter is only getting to 50%

The previous owners may have had a board replaced and not adjusted to suit.
You may well be right in your thoughts.... but.... Please see my post re our condensing heat only Gloworm HXi boiler
( because I would not have a combi for a gift with free gas) which is in our static caravan? I firmly believe it is not down to the setting of parimeters in the fault that I have experienced with this particular boiler because the fault is not a continuous fault by any means, it is most definitely intermittent. So I believe that if it was settings of the various inputs it would be a permanent symptom but as said it is not. I have checked and adjusted all the settings as per the instructions & yet the damn boiler still persists in shutting down especially when the burner should continue to its target temp. But most of the time the target and even a few degrees above the target temp is achieved which is acceptable. I bought our boiler from Ebay for £5.50 but had to go to Oxford from Nottingham for it. I often ponder was it because of this frustrating fault the previous owner outed it?
I have played with this boiler for 4 years and have as I said got last year a renovated PCB from the reputable folks at Tyne & Wear. Due to us using the boiler outside of the winter season I have not condescended to fit the renovated PCB. It does produce in minutes 22 gallons of hot water due to my cylinder being two stainless beer barrels welded together. I know the heat exchanger is as new because I always acid every used boiler I lay my hands on. The pump is very adequate and the return arrives back to the boiler well within the 11 c 'drop'. I've cured many a fault on all sorts of brands of boiler and I note that even the experts who work only on certain brands of boiler are into the' change that because I've got one in the van
syndrome'! however that method of approach can and sometimes does save a lot of time and of course saves taking a lot of librium!
 
Did you read my post about how complex the target temperature is on the modern Combies and Heat only Boilers? The PCB is a computer and evaluates the return temp .
I just skimmed over it and decided it was not relevant. My boiler does not have a return temperature sensor, only a flow sensor. So the only control available is to monitor the flow temperature and modulate the boiler down when the flow temperature exceeds the set value by a certain amount. If the boiler has modulated to minimum output and the flow temperature still rises above the set value, it goes into on/off mode with anti-cycling.
 
Someone living a good distance from me had similar problems with a 25 kw WB greenstar I ERP after modulating down to minimum output (~6kw) it would go into cycle/ anti cycle mode with the flow temperature going as low as 30C. After a lot of toing and froing the problem was "solved" by shutting in a external ABV which had been set to 0.2 bar (2M) this boiler has its own internal by pass. the boiler still cycles as one heating zone only demands ~ 3 kw but the lowest flow temperature observed whilst cycling was 58C. Boiler set point is 65C. There is also a very useful menu which shows the various states that the boiler is at, (table 33 in his case). This boiler also only has a flow temperature sensor.
 
Someone living a good distance from me had similar problems with a 25 kw WB greenstar I ERP after modulating down to minimum output (~6kw) it would go into cycle/ anti cycle mode with the flow temperature going as low as 30C. After a lot of toing and froing the problem was "solved" by shutting in a external ABV which had been set to 0.2 bar (2M) this boiler has its own internal by pass. the boiler still cycles as one heating zone only demands ~ 3 kw but the lowest flow temperature observed whilst cycling was 58C. Boiler set point is 65C. There is also a very useful menu which shows the various states that the boiler is at, (table 33 in his case). This boiler also only has a flow temperature sensor.
I am beginning to think that I have a similar problem. I only have about 9kW of rads; but the boiler can only modulate down to 9.5 kW. So the boiler spends almost all it's time running in on/off mode. At start up the fan runs at the default 50% but this quickly reduces as the flow temperature rises very quickly. When it's at minimum and the flow temperature till rises, the boiler cuts off; pump continues; temperature drops as low as 35C and, eventually, the boiler restarts.

I did get the boiler running for longer by turning up the CH flow temperature. (Why is there no way of knowing what you have set it to, apart from guesswork?) but the house does not maintain a steady temperature as the cooling down period is fairly long.

You comment about the bypass is helpful. My boiler does not have a built-in bypass although the installation manual says that an ABV might be necessary to maintain the minimum flow. According to the Benchmark form, the installer decided that a bypass was not necessary! I would have thought that an ABV was essential as the majority of my rads have TRVs, leaving about 2kW permanently open.

Now we have the problem of making an ABV work with a pump set to proportional pressure.
 
If you havn't a external ABV then that's good in one sense as you simply can't have too much flow through it and your 2kw rads by pass is probably giving the required minimum flow.
You probably can reduce the CH max output without affecting the HW max output but you can't reduce the max turndown to 8.5 kw. (on CH) 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? One way or the other it is well worth trying especially if you can do it from a menu.
 
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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?
 
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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
 
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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

Esteemed
Plumber
Gas Engineer
Advent Win
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
 

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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:

37cdi range rating.png

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.
 
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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. :D

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

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

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