No Central Heating only - fine on HW and CH or HW only

[Wildeman]

The pipework under the floor is 28mm reduced to 10mm close to the radiators where the 10mm tails run through the walls to the rads.

We seem to have eliminated the valve as a cause so the only thing left is circulation.

The maximum boiler output is 17.58 kW (50k BTU) but it can be reduced to a minimum of 14.65 kW. When both HW and CH are running the rads and cylinder can handle the boiler output; but when only the CH is on the rads cannot absorb the output, so the system overheats and the boiler cuts out.

Use the data in the Stelrad Catalogue (page 44) to calculate the output of your rads.


PS the temperature 20 degree difference you mention is degrees F. This is important as heating engineers now work in degrees C and 20C is 36F!. Modern condensing boilers are designed to work with a difference of 20C, but not your boiler where an 11C difference is expected.

The maximum watts of all the radiators in the house is 8548watts or 9kW - the DHW Cylinder is 1050mm x 450mm - 140 litres so would that take around 3kW to heat?

How can I tell if the heat of the boiler has been reduced or does that happen automatically. I see what you mean about when the CH and DHW are running together but what when it is DHW only as this works ok.

Given these figures what can I try next?

 

[Wildeman]

The pipework under the floor is 28mm reduced to 10mm close to the radiators where the 10mm tails run through the walls to the rads.

We seem to have eliminated the valve as a cause so the only thing left is circulation.

The maximum boiler output is 17.58 kW (50k BTU) but it can be reduced to a minimum of 14.65 kW. When both HW and CH are running the rads and cylinder can handle the boiler output; but when only the CH is on the rads cannot absorb the output, so the system overheats and the boiler cuts out.

Use the data in the Stelrad Catalogue (page 44) to calculate the output of your rads.


PS the temperature 20 degree difference you mention is degrees F. This is important as heating engineers now work in degrees C and 20C is 36F!. Modern condensing boilers are designed to work with a difference of 20C, but not your boiler where an 11C difference is expected.

The maximum watts of all the radiators in the house is 8548watts or 9kW - the DHW Cylinder is 1050mm x 450mm - 140 litres so would that take around 3kW to heat?

How can I tell if the heat of the boiler has been reduced or does that happen automatically. I see what you mean about when the CH and DHW are running together but what when it is DHW only as this works ok.

Given these figures what can I try next?

I have just been looking at the data label on the boiler and it shows all the figures for MIN - MID - MAX (output, gas burner pressure etc) and it should have an arrow to indicate which it is set to but there is no arrow - do you think it could be set too high say MAX and needs adjusting down?

 

[Agile]

Look at the FAQ and that will tell you how to measure the heat input at the gas meter.

 

[Agile]

Only an engineer should be setting the power output.

 

[Wildeman]

Only an engineer should be setting the power output.

Thanks I wouldn't mess with anything Gas related myself

Do you think though we are getting close to the issue that the boiler output is set incorrectly?

 

[dilalio]

Only yesterday, my colleague and I swapped out a commercial Vaillant boiler for a domestic one on a house install which had been troublesome for years!

The old boiler was under-gassed and over-powered for the property... Heat was not getting away from the boiler and it was constantly shutting down... The pump was so big I think it could handle a reservoir!

Could be you're having similar issues with heat 'not getting away'!

Do all your rads have TRV's?
Are they shutting down so as to even further reduce flow round the heating circuit? Or short circuiting it! Try removing them all and see if any difference in length of time before boiler shuts down!

Also, on flow and return to boiler, how quickly does return get hot when in CH mode only? Does it get 'as hot' as flow in a short space of time?

 

[D_Hailsham]

I have just been looking at the data label on the boiler and it shows all the figures for MIN - MID - MAX (output, gas burner pressure etc) and it should have an arrow to indicate which it is set to but there is no arrow - do you think it could be set too high say MAX and needs adjusting down?

If there is no arrow, I think we can safely assume the boiler is set to Max, the default when it leaves the factory.

This FAQ tells you how to measure gas consumption, which will tell you how many kWh the boiler is using.

You will need a Gassafe engineer to adjust the output.

In the meanwhile, set the pump to max. This will increase the flow rate and reduce the flow/return difference.

 

[Wildeman]

I have just been looking at the data label on the boiler and it shows all the figures for MIN - MID - MAX (output, gas burner pressure etc) and it should have an arrow to indicate which it is set to but there is no arrow - do you think it could be set too high say MAX and needs adjusting down?

If there is no arrow, I think we can safely assume the boiler is set to Max, the default when it leaves the factory.

This FAQ tells you how to measure gas consumption, which will tell you how many kWh the boiler is using.

You will need a Gassafe engineer to adjust the output.

In the meanwhile, set the pump to max. This will increase the flow rate and reduce the flow/return difference.

Thanks I feel we may be getting close to the root of the issue - I will measure the gas usage of the boiler at the meter tonight when I get home from work to see if it is indeed set to MAX.

I have set the pump to Speed 3 (max) and it has made no difference - reducing the flow/return difference is that the boilers flow/return or the individual radiators? Can you advise how best to do this apart from running the pump on Max. Just to recap I have TRV's on all but two radiators which have dual lock shields on them.

I have uploaded my calculations on the radiator sizes if it helps and you can see the radiators without the TRV's

 

[dilalio]

Remove the TRVs to the rads and make sure the pins aren't stuck (push them up and down gently with the upturned TRV head.
You could also fully open all the lockshields and count the number of turns on each one and note this down for later resetting.

This is all to help get heat away from the boiler!

Feel the flow and return pipes at the boiler - there should be a noticeable difference if the radiators are pumping out their heat!

 

[D_Hailsham]

Remove the TRVs to the rads and make sure the pins aren't stuck (push them up and down gently with the upturned TRV head.
You could also fully open all the lockshields and count the number of turns on each one and note this down for later resetting.

This is all to help get heat away from the boiler!

Feel the flow and return pipes at the boiler - there should be a noticeable difference if the radiators are pumping out their heat!

Beat me to it! I think it's better to check how far open the LS valves are as this gives a better clue as to whether the system has been properly balanced. Anything more than one turn open and serious doubts arise.

I also noticed that the Lounge TRV is only set to 2 - I would have expected 4. Any particular reason?

Is there a noticeable difference between flow and return temps at the boiler? If not, turn the pump down until you get a difference - but check that all rads still get hot.

The rad outputs look correct (I only checked a couple, assuming you can read a catalogue as well as me.)

The reason you get no problems when HW is being heated is that water can absorb considerably more heat than air.

 

[Wildeman]

Remove the TRVs to the rads and make sure the pins aren't stuck (push them up and down gently with the upturned TRV head.
You could also fully open all the lockshields and count the number of turns on each one and note this down for later resetting.

This is all to help get heat away from the boiler!

Feel the flow and return pipes at the boiler - there should be a noticeable difference if the radiators are pumping out their heat!

Thanks Dilalio and D Hailsham for all your advice seems we may be finally getting close to indentifying and fixing the issue!

Beat me to it! I think it's better to check how far open the LS valves are as this gives a better clue as to whether the system has been properly balanced. Anything more than one turn open and serious doubts arise.

I can tell you for sure that the system is not balanced as when we moved in most of the LSV's were either shut off or full on or broken hence the reasoning why TRV's were fitted!

Is it worth me removing the TRV heads (or turn them to MAX) and opening all LSV's on ALL rad's and see if I can get it running in CH only mode (Heat output from the boiler dependent)? If I can how is balancing achieved?

I also noticed that the Lounge TRV is only set to 2 - I would have expected 4. Any particular reason?

If the lounge TRV is any higher than 2 the room gets too hot and the Wife complains - Why would you have expected 4?

Is there a noticeable difference between flow and return temps at the boiler? If not, turn the pump down until you get a difference - but check that all rads still get hot. - not checked this but will check later as long as the boiler keeps running in CH only mode to chck

The rad outputs look correct (I only checked a couple, assuming you can read a catalogue as well as me.)

The reason you get no problems when HW is being heated is that water can absorb considerably more heat than air - Understood.

 

[dilalio]

You're gonna need to let the house get "hot" to see if that resolves the CH issue! Then you can focus on turning the boiler output down or other means of it getting heat away!

 

[Agile]

We really need to know the flow and return temps at the boiler. They indicate flow through it.

The power output is not going to be your problem but it should be ideally 11 kW ( 9 kW plus 2 kW for the hot water ).

Tony

 

[Wildeman]

Ok, this is the test plan I have put together for later - is this the best way to troubleshoot further?

1. Measure the gas usage at the meter with no other gas appliance running - only have a gas hob and fire in the lounge (pilot light out) to ascertain the setting of the boiler Min / Mid or Max.

2. Remove the TRV heads from ALL the rad's and fully open all of the LSV's to ensure there is a fully open path to and through ALL the rad's.

3. Set the Pump speed to MAX

4. Turn the boiler thermostat to MAX

5. Turn on the system in CH only mode on the programmer and see if the boiler fires and stays lit and if the rad's get hot.
Or would you first run the heating in CH and DHW mode with both stats turned right up to check ALL rad's get hot and measure the flow and return temps and then turn off the DHW at the programmer or turn down the Cylinder stat?

6. Whilst this is happening measure the flow and return pipe temps at the boiler using an infrared thermometer

If this works in CH only mode how would I then go about balancing the system to adjust the pump speed and LSV settings?

 

[D_Hailsham]

1. Yes
2. Also check that TRV pin can be pressed down and jumps up when released. Opening the LS valve will upset the balance so some rads may not get as hot as others.
3. Yes - as a starting point; it can be adjusted later.
4. Max is approx 82C; 75C should be all you need.
5. Worth checking all three options work.
6. Be careful using an IR thermometer on copper pipe as it may under-read by a considerable amount. It's best to wrap the pipe with black insulating tape and hold the thermometer right against the tape.

Let us know how you get on; we can discuss balancing later. You did remember to note the LS valve positions?!!