Heat Exchanger on old non-condensing boiler lukewarm DHW but hot radiators, how?

[JimboMarimbo]

My 21 year-old Gloworm 24cl needs replacing, (I know, I'm saving up), but the engineer said the fault of lukewarm water and warm/cold intermittent showering water was due to the the heat exchanger needing replacement: however the central heating works fine with radiators almost too hot to touch. How can this be? - it seems counter to logic. Any expert out there who could spare a moment to clue me up, would be much appreciated.

 

[ianmcd]

I Think you mean a 24 Ci and not cl, it could be the plate heat exchanger needs clean ing but could also be a divereter valve problem. turn the CH off and let it cool down then run a hot tap and feel the two laregest bipes below the boiler, CH flow and return and see if one gets hot when the HW only is on

 

[JimboMarimbo]

Thanks for that - si it's a 24ci then, that would explain a lot of confusion regarding spare parts availability! Of course, its 20 years old as well. "can't get the parts, mate".
I'll try your test - so if the ch pipes get hot when ch is off and the dhw running, it's the diverter not doing its job.
Incidentally, I've heard that the plate heat exchanger is so difficult to remove for cleaning that you might as well put a new one in, what with skilled labour costs being what they are.

 

[Johntheo5]

In any combi PHEX as the water flow increases the boiler starts firing harder and the primary flow temperature keeps increasing, if the PHEX is fouled then this flow temperatue will reach the burner trip point of ~ 85C, it will then refire once the primary flow temp reduces.
If you can watch the boiler flowtemp then that will give a good idea of whats happening.

 

[JimboMarimbo]

Jt5, that makes perfect sense - thanks, I had noted that with the kitchen hot tap (next to boiler) running full on, I can hear the flame constantly re-igniting - proof of bunged up 20 years of gunge in the PHEX. That removes any niggling doubt about replacing the exchanger, only to find it was not necessary - the spare part for this model retails at an eye-watering £500ish plus VAT - ouch! plus labour...

Heating Spare Parts

Glowworm part no. 2000801881 - Heat exchanger ,24KW DIN

www.heatingspareparts.com

Still cheaper than a new boiler though, quoted at £3K professionally fitted

 

[ianmcd]

you can remove and clean the existing Plate heat exchanger, it needs some strong chemical cleaner though so be carefull and follow the instructions, I use 1 shot from screwfix but it is very aggressive dont under estimate how corrosive it is, others use brick acid, there are other chemicals that can be used, once cleaned it will be fine

 

[Johntheo5]

I took some readings from someone here (I think) that had a fouled PHEX on 36CDI. It was during last summer so thats why the mains is ~ 19C.
Fouled PHEX
Boiler Flow/Return/dT/Flowrate 71.5C/42.8C/28.7C/8.2LPM
DHW Flow/Return/dT/Flowrate 19.0C/44.5C/25.5C/9.2LPM Boiler output: 16.4kw
NEW PHEX
Boiler Flow/Return/dT/Flowrate 64.5C/49.5C/15.0C/23.0LPM
DHW Flow/Return/dT/Flowrate 19.6C/54.5C/34.9C/9.9LPM Boiler output: 24.1kw

note the huge difference in the primary flowrates, 8.2LPM vs 23.0LPM

 

[JimboMarimbo]

Wow! That's some big improvement -

I took some readings from someone here (I think) that had a fouled PHEX on 36CDI. It was during last summer so thats why the mains is ~ 19C.
Fouled PHEX
Boiler Flow/Return/dT/Flowrate 71.5C/42.8C/28.7C/8.2LPM
DHW Flow/Return/dT/Flowrate 19.0C/44.5C/25.5C/9.2LPM Boiler output: 16.4kw
NEW PHEX
Boiler Flow/Return/dT/Flowrate 64.5C/49.5C/15.0C/23.0LPM
DHW Flow/Return/dT/Flowrate 19.6C/54.5C/34.9C/9.9LPM Boiler output: 24.1kw

note the huge difference in the primary flowrates, 8.2LPM vs 23.0LPM

tap temperature up 10 degrees and 50% increase in kW output: it's going to be money well spent methinks.