I have had a Vaillant 38kw boiler installed in my largish 4 bedroom house. From the start the boiler was cutting out. A Vaillant engineer said that this was due to insufficient water flow but did not analyse the cause as he was only there to look for faults in the boiler. The pipework to/from the boiler is 28mm. Before the install no-one (out of 3 firms that quoted) suggested that this might not be adequate for the size of boiler. The pump is a new Grundfos 1560 which I am told is top of the range for domestic use. The boiler will only run properly if set to a maximum of 22kw. Any ideas?
New boiler cuts out ? pipework
- Thread starter alanb122
- Start date
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Whilst you have told us quite a bit you have told us nothing about the installer or the system past the pump!
28mm would normally be adequate for a condensing 38 kW boiler. A 15/60 might not be if you really needed 38 kW of heating which I dont believe it does!
So I will ask the questions!
How competent do you rate the installer?
Is he gas registered?
How much did he charge for the labour element of the boiler installation?
How did you choose him?
Is this a 438 ?
Whats are the flow return temperatures as read from d40 and d41 ?
What fault code is shown when it "cuts out".
What does the installer say is wrong?
Has he fitted an auto bypass valve ???
What water treatment did the installer do?
Whats is the heat output of the installed rads?
Whats is the whole house heat loss ?
Whats your typical previous annual gas bills?
With the answers to those questions we would be able to give you a good idea of what the problem is !
Tony
28mm would normally be adequate for a condensing 38 kW boiler. A 15/60 might not be if you really needed 38 kW of heating which I dont believe it does!
So I will ask the questions!
How competent do you rate the installer?
Is he gas registered?
How much did he charge for the labour element of the boiler installation?
How did you choose him?
Is this a 438 ?
Whats are the flow return temperatures as read from d40 and d41 ?
What fault code is shown when it "cuts out".
What does the installer say is wrong?
Has he fitted an auto bypass valve ???
What water treatment did the installer do?
Whats is the heat output of the installed rads?
Whats is the whole house heat loss ?
Whats your typical previous annual gas bills?
With the answers to those questions we would be able to give you a good idea of what the problem is !
Tony
leave it at 22kw then. Prob all you need anyway in a 4bed house.
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Thanks for the reply.
The boiler is a 438.
The installer is a local company which has been in business many years. They installed the whole system 19 years ago, originally with a Potterton 125,000 btu boiler.
There are 17 radiators plus hot water. I don't know the heat loss from the house. It has solid walls, loft insulation and half double glazing, so not very thermally efficient.
The system has been powerflushed. Certainly before that some radiators were not heating up properly.
I don't know whether the system has an auto bypass valve. One has not been installed recently.
Boiler cut out is random as far as I know. It normally comes back on on its own but once cut out with code F23.
Sometimes the radiators do not get fully hot.
At the time of checking just now the outflow temp is 62 and return is 51, but the Vaillant engineer a couple of weeks back found that the temperature difference was more than 20 which I understand is the maximum the boier will work with. With the boiler set to 22 kw max he got the difference down to 21. The boiler is set to 75 C.
The Vaillant engineer said that he would estimate the requirement of the house at 30 kw + so in theory 38 kw is the correct boiler size as the next one down is 28kw. I did consider a 28 kw boilerbut decided to play safe!!
The cost for the whole job was about £3,800. It was not a cowboy job.
The boiler is a 438.
The installer is a local company which has been in business many years. They installed the whole system 19 years ago, originally with a Potterton 125,000 btu boiler.
There are 17 radiators plus hot water. I don't know the heat loss from the house. It has solid walls, loft insulation and half double glazing, so not very thermally efficient.
The system has been powerflushed. Certainly before that some radiators were not heating up properly.
I don't know whether the system has an auto bypass valve. One has not been installed recently.
Boiler cut out is random as far as I know. It normally comes back on on its own but once cut out with code F23.
Sometimes the radiators do not get fully hot.
At the time of checking just now the outflow temp is 62 and return is 51, but the Vaillant engineer a couple of weeks back found that the temperature difference was more than 20 which I understand is the maximum the boier will work with. With the boiler set to 22 kw max he got the difference down to 21. The boiler is set to 75 C.
The Vaillant engineer said that he would estimate the requirement of the house at 30 kw + so in theory 38 kw is the correct boiler size as the next one down is 28kw. I did consider a 28 kw boilerbut decided to play safe!!
The cost for the whole job was about £3,800. It was not a cowboy job.
Forgot to say that annual gas bill with the old boiler was about £1600. In the very cold winter months the cost went up to about £225 per month based on actual meter readings. I have shopped around for the best seal I could get
My intitial comment is that your installer did not set the boiler up properly and the vaillant fellow should not have had to do that.
There is no evidence of a reduced flow as that differential is a bit too low!
The rads probably need balancing.
A 24 kW heating boiler I went to yesterday produced a gas bill of £1400 so in your case I dont think the heat loss is as high as 30 kW.
With only 17 rads I would have expected the total heat loss to be in the region of about 22 kW but that depends on the average size. before choosing a boiler you or the installer should have calculated the heat loss. Further evidence of a questionable installer! but if you had unilateraly decided you wanted a 38 kW boiler the installer may have taken the easy route and just fitted what you wanted without warning you.
I would just not fit a boiler that was oversized as it does nothing for my reputation!
Lets see what the others have to say.
Tony
There is no evidence of a reduced flow as that differential is a bit too low!
The rads probably need balancing.
A 24 kW heating boiler I went to yesterday produced a gas bill of £1400 so in your case I dont think the heat loss is as high as 30 kW.
With only 17 rads I would have expected the total heat loss to be in the region of about 22 kW but that depends on the average size. before choosing a boiler you or the installer should have calculated the heat loss. Further evidence of a questionable installer! but if you had unilateraly decided you wanted a 38 kW boiler the installer may have taken the easy route and just fitted what you wanted without warning you.
I would just not fit a boiler that was oversized as it does nothing for my reputation!
Lets see what the others have to say.
Tony
Thanks again Tony for your reply.
Of the 3 companies that quoted for the job (one national, two local), none did anything so sophisticated as a heat loss calculation. The consensus seemed to be to allow for an average of 1.5 kw per radiator + 3 Kw for hot water. That makes 28.5, so arguably the 28 kw model should just about be adequate. I appreciate that some commentators suggest that significantly less power should be enough. With an outside temperature of 10 degrees I am sure that 22 kw would be fine. But, when you are paying a lot of money for an upgrade you want to at least have the option to keep warm when it falls to minus 2 as it was during a lot of the last winter, even if you can't actually afford the gas!
We have moved forward a bit on this today. A second pump has been installed next to the boiler. The original pump is next to the hot cylinder which is about 13 metres from the boiler and the installers though that the distance might be causing the problem. So we now have pumps at both ends of the system. With this configuration we have set the boiler back to 38 kw maximum. The output temperature is now reaching 75 C as set and the return temp is within the 20 C differential which is said to be the maximum. I will monitor the situation over the weekend.
One thing that surprises me is that there seems to be no ready way to measure what water flow rate is being achieved. If 27 litres per minute is needed it would be helpful to know what we actually have. In general, although the modern boilers are highly sophisticated, designing a system seems to be very much rule-of-thumb.
I have to correct what I said about the automatic bypass valve; one was fitted as part of the job. I have seen it.
One other question if I may: The old boiler had an operating temperature of 82 C. The new one has a factory setting of 75 C maximum but the installation instructions state that the installer can increase this if required. Presumably with 75 C the heat from the radiators will be less than at 82 C and in theory you may need bigger radiators to heat the rooms. Is there any downside to increasing the boiler temperature to 82 or even 85, eg reduced boiler life?
Many thanks again for you advice.
Alan
Of the 3 companies that quoted for the job (one national, two local), none did anything so sophisticated as a heat loss calculation. The consensus seemed to be to allow for an average of 1.5 kw per radiator + 3 Kw for hot water. That makes 28.5, so arguably the 28 kw model should just about be adequate. I appreciate that some commentators suggest that significantly less power should be enough. With an outside temperature of 10 degrees I am sure that 22 kw would be fine. But, when you are paying a lot of money for an upgrade you want to at least have the option to keep warm when it falls to minus 2 as it was during a lot of the last winter, even if you can't actually afford the gas!
We have moved forward a bit on this today. A second pump has been installed next to the boiler. The original pump is next to the hot cylinder which is about 13 metres from the boiler and the installers though that the distance might be causing the problem. So we now have pumps at both ends of the system. With this configuration we have set the boiler back to 38 kw maximum. The output temperature is now reaching 75 C as set and the return temp is within the 20 C differential which is said to be the maximum. I will monitor the situation over the weekend.
One thing that surprises me is that there seems to be no ready way to measure what water flow rate is being achieved. If 27 litres per minute is needed it would be helpful to know what we actually have. In general, although the modern boilers are highly sophisticated, designing a system seems to be very much rule-of-thumb.
I have to correct what I said about the automatic bypass valve; one was fitted as part of the job. I have seen it.
One other question if I may: The old boiler had an operating temperature of 82 C. The new one has a factory setting of 75 C maximum but the installation instructions state that the installer can increase this if required. Presumably with 75 C the heat from the radiators will be less than at 82 C and in theory you may need bigger radiators to heat the rooms. Is there any downside to increasing the boiler temperature to 82 or even 85, eg reduced boiler life?
Many thanks again for you advice.
Alan
I know it's a bit late now, but you might like to use either Sedbuk Boiler Calculator or EST online boiler calculator to find out what size boiler you really need.
Please note:
Dimensions are internal
Room height is floor to ceiling
A mid-terrace has two external walls
A semi has three external walls
A detached has four external walls.
You could also use Stelrad Elite Catalogue to find out the output of your radiators.
You are quite right to think that a rad running at 75C will give of less heat than one running at 82C. The output of a radiator is determined by three temperatures: Flow (water in), Return (water out), Room. For this reason all rad outputs are quoted to a British Standard, which specifies temperatures of 75C/65C/20C.
The principal reason for running boilers at a lower temperature is so the return temperature is below 55C, if it is the boiler will condense and the efficiency will increase; this is compensated by installing oversize radiators. You also use less fuel.
At the temperatures you quote of 62C/51C, the radiator outputs will be about 65% of the manufacturers figures. This is nothing to worry about, it's due to the higher outside temperatures - heat required is directly proportional to the difference between internal and external temperature. The boiler has sensors to measure the flow and return temperatures and adjusts the boiler output accordingly.
Please note:
Dimensions are internal
Room height is floor to ceiling
A mid-terrace has two external walls
A semi has three external walls
A detached has four external walls.
You could also use Stelrad Elite Catalogue to find out the output of your radiators.
You are quite right to think that a rad running at 75C will give of less heat than one running at 82C. The output of a radiator is determined by three temperatures: Flow (water in), Return (water out), Room. For this reason all rad outputs are quoted to a British Standard, which specifies temperatures of 75C/65C/20C.
The principal reason for running boilers at a lower temperature is so the return temperature is below 55C, if it is the boiler will condense and the efficiency will increase; this is compensated by installing oversize radiators. You also use less fuel.
At the temperatures you quote of 62C/51C, the radiator outputs will be about 65% of the manufacturers figures. This is nothing to worry about, it's due to the higher outside temperatures - heat required is directly proportional to the difference between internal and external temperature. The boiler has sensors to measure the flow and return temperatures and adjusts the boiler output accordingly.
The differential you quoted was too low.
Adding more pumps will lower that even further!
The actual performance depends on a lot of factors including the setting of the bypass.
When heating water unless the cylinder heating flow is balanced that can seriously upset the total balance.
It seems you have been the victim of some pretty marginal installers if none of them wanted to measure or calculate the heat loss.
Generally older systems had oversized rads because they were fitted before increases in insulation in the loft and double glazing were fitted so most are conventiently still about right even with a lower mean temperature.
Tony
Adding more pumps will lower that even further!
The actual performance depends on a lot of factors including the setting of the bypass.
When heating water unless the cylinder heating flow is balanced that can seriously upset the total balance.
It seems you have been the victim of some pretty marginal installers if none of them wanted to measure or calculate the heat loss.
Generally older systems had oversized rads because they were fitted before increases in insulation in the loft and double glazing were fitted so most are conventiently still about right even with a lower mean temperature.
Tony
Seems a little bit harsh.
At the end of the day, there are 17 existing radiators which you would assume the OP is happy with there output and performance, so why go to all the bother of heat loss calculations etc when the old appliance, which was lower rated, did the job perfectly well. The new boiler should piiss it.
It's easy to shout 'installer error' in every instance, make the OP lose trust in his installers, and maybe give yourself some 'internet kudos' in the process, but it certainly won't help the situation.
Did the boiler move position?
Could just do with a few hours adjustment and balancing
Sam
At the end of the day, there are 17 existing radiators which you would assume the OP is happy with there output and performance, so why go to all the bother of heat loss calculations etc when the old appliance, which was lower rated, did the job perfectly well. The new boiler should piiss it.
It's easy to shout 'installer error' in every instance, make the OP lose trust in his installers, and maybe give yourself some 'internet kudos' in the process, but it certainly won't help the situation.
Did the boiler move position?
Could just do with a few hours adjustment and balancing
Sam
Quite possibily but thats the job of the installer and should be within his skill range.
IF the owner is totally happy with the heat output of his rads then they can be used as a guide in rating the boiler. Taking into account the lower flow temperature.
However, in this case it seems none of the quoters nor the installer looked into the heat loss of the property OR the installed rads!
Tony
Too many assumptions for me.
Estimating heat loss on someone else's gas bill, without knowing how long either use there heating for, how much water they draw off, the size of the rad's, insulation and layout of piperun's???
How do you know the installer didn't even look at the radiators? Given the fact they installed it 19 years ago, unless they are as skilled as Stevie Wonder, I find that hard to belive?
Installations can be problematic sometimes, give the fitters some credit, they've been back what sounds like a few times to try and rectify the problem.
I've searched ebay, but can't find one of those crystal balls you have Agile, can you send me a link ?
Sam
Estimating heat loss on someone else's gas bill, without knowing how long either use there heating for, how much water they draw off, the size of the rad's, insulation and layout of piperun's???
How do you know the installer didn't even look at the radiators? Given the fact they installed it 19 years ago, unless they are as skilled as Stevie Wonder, I find that hard to belive?
Installations can be problematic sometimes, give the fitters some credit, they've been back what sounds like a few times to try and rectify the problem.
I've searched ebay, but can't find one of those crystal balls you have Agile, can you send me a link ?
Sam
I've searched ebay, but can't find one of those crystal balls you have Agile, can you send me a link ?
Sam
You have not searched Ebay very well ! How about this 7 kg one for example?
http://cgi.ebay.com.sg/Feng-shui-7-kg-Crystal-Ball-/170457129004
The installer had been paid to install the new boiler! Its the installer who should be sorting out any problems not the OP needing to call and pay me. London is a big place and whilst I am getting better at travelling further afield I still try to limit my visits to about 8-10 miles across London although outwards to 30 miles to north or west is no problem.
Any competent installer would first do some assessment of the required boiler size. Usually either whole house or existing rads. I usually do both as well as gas rating the esisting boiler and a bill examination as that can have an implication for the controls.
Tony
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