Central heating circulation problems

[chris83]

I had a new Worcester 24Ri fitted to an existing system a few months ago and now the flow temperature never gets above about 50deg.C. ( (measured with pipe thermometer) The Worcester service guy has visited 3 times , replacing the heat cell and the control circuit board. Each time the diagnostics show the boiler reaching max temp. then modulating down. Conclusion - insufficient flow .
The system has been powerflushed twice, demonstrating no excessive resistance to flow. It has a new Grundfoss 15-60 pump.
The system heated up OK with my old Gloworm boiler although I know the new boilers are more sensitive to flow rate.
It's an open vent system with the boiler on the ground floor and the pump on the first floor next to the 2 zone valves & HW cylinder. 11 rads + HW in a 4 bed semi.
Any suggestions?

 

[D_Hailsham]

I had a new Worcester 24Ri fitted to an existing system a few months ago and now the flow temperature never gets above about 50deg.C. ... 11 rads + HW in a 4 bed semi.

That's a very large boiler for only 11 rads! Who sized the boiler for you? Or did you just choose the biggest??

What is the total output of your rads? If you don't know use the Stelrad Elite Catalogue to get a good idea.

To find out what size boiler you should have use the Sedbuk Boiler Calculator

The 24Ri will modulate between 8kW and 24kW. If the boiler is oversized it will start off at max output and quickly modulate down to the minimum. If this is still too much, it will go into on-off mode, like non modulating boilers.

 

[Agile]

Thats a very good response above!

The only thing that I can add is that when you have responded to those questions you could measure the flow and the return temperatures with a contact thermometer.

That will indicate what the flow rate is through the boiler.

When a boiler is seriously oversized it will come on for about 20 seconds, reach set flow temperature and then turn off for the anticycle delay of about 3-4 minutes.

If you used a professional to specify the system and size the boiler then he is going to have to come and replace it with the correct one which might be a 12 Ri or 15Ri.

If you chose the boiler yourself unscientifically then you have only yourself to blame.

Thats of course if there is really not a flow restriction but the temperature measurements will confirm that.

Unlikely not to have been noticed, but is the flow in the right direction through the boiler? Flow will be hotest!

Tony Glazier

 

[chris83]

Thanks for the replies,
I had several quotes for the boiler replacement and only one asked for room details to size the boiler. I gave him dimensioned floor plans. He quoted for a 30CDi Worcester boiler !
I used the boiler calculator as suggested and it came up with 19.3kw.
Most of the radiators are 30+ years old and mostly double. Approximating from the suggested Stelrad info, they total 17kw. Adding 2kw for HW, this would be similar to the calculator.
The flow temperature is generally about 50deg , return 40deg.
The house is a largish extended 1920's semi. It has a loft room 5th bedroom also, however this has no radiators, so I did not include it in the calcs. The previous boiler was 80kBTU.
I understand the comments about oversizing, however, in the summer when HW only is required , technically, all CH boilers are oversized but I would expect them to heat the water to approx 65deg.
This boiler only gets to 50deg for the whole system. Would approx. 20% oversizing make that much difference?

 

[D_Hailsham]

I used the boiler calculator as suggested and it came up with 19.3kw.
Most of the radiators are 30+ years old and mostly double. Approximating from the suggested Stelrad info, they total 17kw. Adding 2kw for HW, this would be similar to the calculator. ... The previous boiler was 80kBTU.

80kBTU= 23.5kW, so the new and old boilers are the same size. Over-sizing by 20-30% should not cause your problems, so we have to look somewhere else.

The flow temperature is generally about 50deg , return 40deg.

These are definitely on the low side.

What do you have the central heating temperature set to on the boiler?
Did the installer balance the system?
What speed is the pump set to?

Turn the room stat up very high, open any TRVs to max and report what happens - do the rads get hotter, is the flow and return temp higher etc?

 

[chris83]

The boiler temp is set to max.
The pump is on the max speed
The installer did not balance the system.
All rads. are fully open with the temps. stated Sometimes the boiler runs reasonably steady between 45/50deg flow, sometimes it cuts out at this temperature. I have had it once at about 55deg by turning off all but one downstairs rad. The flow temp. showing on the pipe thermometer (right after boiler) never gets higher than this.
The pump was pumping vertically down, so I suspected air ingress or cavitation. I rearranged the pipework so it was horizontal - no difference.
I have now changed it to pump vertically up - no difference.
I wondered if converting to a sealed system would improve matters

 

[D_Hailsham]

The boiler temp is set to max.

Good.

The pump is on the max speed

Try it on second speed - this should increase the flow temperature, but you may find the last rads in the chain do not get hot.

The installer did not balance the system.

He probably assumed it was OK. Worth doing.

All rads. are fully open with the temps. stated Sometimes the boiler runs reasonably steady between 45/50deg flow, sometimes it cuts out at this temperature. I have had it once at about 55deg by turning off all but one downstairs rad. The flow temp. showing on the pipe thermometer (right after boiler) never gets higher than this.

Have you checked what happens when you turn the wall stat up to, say 30°C? Does the boiler temperature get any higher (Max=80°C)

 

[Agile]

Balancing the rads will help to heat the house. You can do that yourself with help from the FAQ.

You have too low a temp differential to give efficient operation when the problem is sorted out. But balancing will sort that out.

Did you check to see that the boiler flow pipe is the hottest as I asked?

There is clearly a simple problem which needs to be identified.

What does the installer have to say about all this? Its his responsibility to ensure the new boiler is operating properly. I dont understand how anyone can get [aid for installing a system which does not work!

Tony

 

[chris83]

Selecting the middle pump speed makes little or no differnce. Switching to the lowest pump speed perhaps makes the boiler cut out sooner. The boiler is on constant demand because at the moment the room temp. is never satisfied.
The flow pipe is always hottest, all the radiators get hot but obviously not hot enough to be efficient.
The installer has been back and called in Worcester. Their diagnostics show the heat cell sensor quickly reaching the 88deg limit when the burner shuts or turns down. Prognosis is the boiler is working as expected and there is insufficient flow through it to dissipate the heat.
I am getting the installer back in and will also contact Worcester technical
support.

 

[Agile]

Their diagnostics show the heat cell sensor quickly reaching the 88deg limit when the burner shuts or turns down. Prognosis is the boiler is working as expected and there is insufficient flow through it to dissipate the heat.

I am getting the installer back in and will also contact Worcester technical
support.

What the sensor says and what the actual temperature really is are not necessarily the same thing!

I dont want to rubbish the Worcester engineers because they seem generally very good. But merely looking at a sensor temperature indication is only part of the diagnosis and the actual temperature needs to be measured using a contact thermometer.

If the flow temperature is really 50°C as you say then there is a descrepancy which needs to be explained!

Perhaps you need a proper boiler engineer?

Tony

 

[Agile]

The installer did not balance the system.

Not a very professional installer then!

Was he really CORGI registered?

Tony

 

[alfa.romeo164]

Hi, did you sort this out? I've got a similar issue with a worcester ri24 and would be grateful for any info - previous boiler OK, now only half the rads getting warm and boiler cutting out v quickly, best, M

 

[Agile]

So who fitted this 24 Ri ???

Tony