Boiler problem or pipework problem? Help

[UpgradeME]

Central heating output of the boiler is from 9.9 - 28.0kW
Designed temp rise 20 degrees
Minimum flow rate for heating is 360 l/hr

To obtain a ΔT of 20°C between flow and return you will need a flow rate of around 950l/h around your system - With your current setup (21.75kW & 500l/h), there will be 38°C ΔT between flow/return during the first 15 minutes (mathmatically speaking).

The maximum ΔT you would achieve when the boiler modulates down to it's minimum 9.9kW would be 17.4°C (with your 500l/h flow rate).

 

[CAS50]

I've fitted a new pump and it is at the third speed setting and this made no difference. Also new diverter valve.

I presume you thiunk its a blockage of some sort. Isolating all radiators apart from the one 1m from the boiler i still get this low flow. I can then see all pipes to from the boiler and they are not squashed.

Would you tend to think it was a blockage in the boiler. I've also had two new heat exchangers so no blockage there.

 

[Agile]

First we need to know exactly which model of boiler you have! The HE version seems to me to have a minimum power of 14.8 kW and max of 30.2 kW both in condensing mode.

Lets assume its the condensing boiler. I would like to know what the minimum power output of your boiler is. Thats not found in a book but by measurement on your boiler!

There IS a minimum CH heat power output adjustment on the boiler. BUT the makers expect you to set that to a figure above the minimum the boiler could give. But I take the view that until that setting has been checked we have no idea what its actually at.

The boiler is very badly suited for smaller heating loads as its minimum power output is pretty high. Thats a common senario for larger combis chosen for their water output.

Tony

 

[CAS50]

It is the Linea Max i have not the HE condensing version. Not sure where i would find/measure/calculate the actual power output range of the boiler.

It does appear as through the burner only has two modes on full or off. When it comes on it lifts the water temp by 40 degrees. If i turn the maximum CH dial down a bit then it just reaches the cut-off quicker!

I have 17 fairly large rads mostly K2 so there is quite a demand waiting on the boiler to deliver.

 

[UpgradeME]

It is the Linea Max i have not the HE condensing version. Not sure where i would find/measure/calculate the actual power output range of the boiler.

It does appear as through the burner only has two modes on full or off. When it comes on it lifts the water temp by 40 degrees. If i turn the maximum CH dial down a bit then it just reaches the cut-off quicker

The non-HE MI do not mention the 75% for the first 15 minutes, however it may still be the case or another variant of starting high, then modulating lower. The rest of the text I quoted from the HE model appears to be the same so it should modulate up/down at some point.

Based on 28/9.9kW (max/min) the calculations for ΔT of 20°C would be a flow of 920l/h (assuming 21kW starting output (75% of 28kW)).

The max/min ΔT for 500l/h would be 49°C (@28kW) / 17.3°C (@9.9kW) and 36.8°C for 21kW

 

[CAS50]

I am getting a temp difference of around 40 degrees at 500l/hr

From that would you then think i have a flow restriction probably in the boiler.

 

[UpgradeME]

I am getting a temp difference of around 40 degrees at 500l/hr

From that would you then think i have a flow restriction probably in the boiler.

You didn't answer my earlier Q's on who installed/designed the system, are all the rads connected via 22mm tails and if it has always been like this?

The low flow rate could be due to the design of the pipework, a blockage, undersized pump and/or incorrectly setup boiler output (as Agile suggested) could be contributing to the problem.

You might want to have a read of this link to workout the resistance you have on your pipework, which will enable you to check if your pump has enough power (especially if you designed / installed the system yourself).

 

[CAS50]

Sorry

The system was installed by a Corgi approved plumber. The system worked well when installed and rads and rooms were very hot all over.

Main pipework is 22mm and this drops to 15 for the tails to each radiator. (Each 15mm pipe only ever feeds 1 rad)

Problem has been getting gradually worse over last 2-3 years and last winter i realised i really needed to do somethin about it.

I wouldn't think of installing or designing a heating system, that's for the pros but it's me that is left with the problem to sort out as everyone points the finger at everyone elses work.

I will have a look at that link to see about the pipework resistance but the pipework has not been altered since it worked well.

 

[CAS50]

Ok UpgradeME I have now looked at that link you included and it appears that the low flow is low because the pump is pushing a 5m head. This is even width all rads off and the one only 1m away open full.

Looks like a flow restriction in the boiler.

Anybody had this problem with a linea max. Can it be cleared easily. How am I best to get this fixed. I will not be doing it but would like to be able to help someone else that will be doing it.

 

[Steambod]

I am getting a temp difference of around 40 degrees at 500l/hr

From that would you then think i have a flow restriction probably in the boiler.

Motorised valve???

 

[Agile]

If it worked OK for a while then something has changed!

A vast amount of skill/experience/luck is required to deal with blockages/power flushing.

Most of this relates to power flushing and particularly which chemicals to use.

I know that a common cause of your kind of problem is as a result of lime compounds blocking a restriction like a valve.

Lime compounds are not affected by the non acid chemicals like X400/X800. Nor are they affected by the citric acid based compounds like DS40.

They are dissolved by hydrochloric acid but you would not want to put that in a CH system. But they ARE dissolved by sulphonic acid ( DS3 ) when its at 80 ° C.

I am telling you what I have learnt/discovered by experience.

What I cannot tell you is exactly what is causing YOUR problem.

Furthermore your boiler should smoothly modulate from the maximum CH power of 21 kW (?) down to the minimum of 9.9 kW !

However modulation is best identified by either an engineers measure of burner gas pressure or anyone's measure of modulating voltage on those gas valves with a DC activated modulation coil.

Based on your description and taking into account what has been changed and your reported rather low flow rate then my best conclusion ( guess ) is that there is a blockage in your boiler, probably isolating valves, caused by plastic, lime scale, solder blobs etc.

Whilst I would be dead chuffed if you told me that you had found something like that in your boiler but doing a remote internet fault diagnosis is a bit like telling someone in Australia where to look for opals or other precious stones.

No one has thought of mentioning it but the diverter valve would usually be a prime suspect if there is a restricted flow to either CH or DHW ! Has anyone looked at that? Edit 1, Steam bod above has suggested that! Edit 2, the OP said that had been changed!

Following those edits I now have to go back to the expectation that there is a blockage at the boiler isolating valves. BUT I an not in front of the boiler and perhaps 500 miles away!

Tony