Worcester Combi CH problems

[RegEdit]

Can you help me please? It's frosty at home!

Worcester Hi-Flow 4.5 combi boiler. Approx 10 years old
Slight leakfrom a Radiator lock shield - to be resolved.
Pump replaced on weekend, resolved ketteling and boiler over heat protection cutting in. Pump is not a direct replacement but an equivilent, runs at higher RPM though, seems to make radiators hotter.

PRV cuts in at approx 2 bar, is this too low? Water pressure then drops below 1 bar. Not all rads heat up when the pressure is low, I think a new PRV would solve this. Intermittently the boiler cuts out, not sure why, have not been able to see a pattern yet, although I noticed this morning that it was out following a shower being used.

I think I have several problems all at once here.

 

[DP]

I would suspect pump change has been futile as pump may not be the culprit. Always instal like for like. You or the helpful man down the street will not know the consequences of fitting strange parts in place of makers specified components. Higher speed may not mean correct quantity of water flow. Pump change, according to you post, has not resolved pilot outage/ overheat situation.

PRV should lift at 3bar. Has someone been tikering with this component? Boiler should function adequately at 1/ 1.25 bar all the way up to 3

 

[RegEdit]

Thanks DP for your reply.

Pump change, according to you post, has not resolved pilot outage/ overheat situation.

The pump change has resolved the issue with the overheat sensor cutting the gas supply soon after ignition of the burner. I was also concerned about the non standard replacement, but on the box it was listed as an equivilent to the Grundfos 15/60, (from memory). May be I should have been more persistent in locating the correct pump.

PRV should lift at 3bar. Has someone been tikering with this component? Boiler should function adequately at 1/ 1.25 bar all the way up to 3

No the PRV has not been touched, but it has cut in previously, and now seems to have become more sensitive in it's operation.

However, the bungalow has been extended, now has an upstairs, and it seems that there is a restriction / air pocket that prevents some rads from working unless the pressure is at about 2 bars. When the extension took place, I was advised that the boiler would be doing very well to cope with the length of runs and 14 radiators.

Now that the PRV has allowed the pressure to fall so low, and given that the boiler is 10yrs old, should I be concerned about the pressure vessel? I'd like to check it anyway, but do not recall seeing a schrader valve inside the boiler, I'm not even sure where the pressure vessel is located, there's not much room inside the boiler casing at all.

Thanks.

 

[COG]

The PRV should kick in at 3 bar, but the pressure guage could be at fault, changed plenty ,drain down and check it settles at 0, the pressure vess sh8ould be checked while drained down and should have .5 to .75 bar with NO water present. The pump should be ok if its a replacement for the 15/60 it is flowing in the correct way ? arrow towards the three way valve

 

[RegEdit]

Thanks for your reply Cog, I fitted a new PRV, and it is working as I would expect, replaced the pilot light thermocouple and the leaking lock shield valve, the system stays lit and no longer loses pressure. I had thought that was the end of my problems, there were no issues with the boiler over Christmas and new year, however since then, the boiler has gone out 2 or 3 times. I can't see a pattern. It failed last night, in the night for example. I had been wondering if the overtemp sensor was tripping the gas valve, but it is so intermittent. As an aside, is it possible to remove lime scale within the hot water part of the boiler, I would imagine that after 10 years in our area, the heat exchanger would be much less efficient than when new. It certainly feels that way. I have been thinking about removing the mains water supply, plumbing in an old cylinder and circulating a flushing chemical through the heat exchanger for a while, can't quite make the leap from theory to practise though! I would love to read others thoughts on this.

 

[COG]

If you have lime scale in the area run the tank upto temp the n run a hot tap the two outlets on top of the tank should remain the same temp if they dont then then one of the two coils ,poss just the flow restricters may be blocked depending on how bad te scale is. Descaled plenty of highflows no problem .dont think your diy descaler would work the reversable pump required would be costly on its own ,you can hire one quite cheap.PS I would replace the interupter stat if i were you as they are not reliable after a period of overheating plus double check that new pump of yours if you put a 15/50 on a highflow it will overheat, you said it was compatable with a 15/60 but to what degree

 

[DP]

When you say the boiler switched off during the night, what does that tell you. Heating is off during the night (well most would be), and the HW stat fires the boiler to maintain store temperature. Look at the circuit wiring- you will note that the pump overruns to take heat away from the heat exchanger.

Without changing ANY parts (that included the pump) one has to narrow the fault location to a component. Is the heat exchanger temperature getting too high, is the heat buildup in the HE controlled at the end of burn, is pump overrun relay coming in, is the pump (even though is is new) sticking?

Get to know your boiler, then you will be able to locate the fault.

 

[marios_kin]

Another thought. Was the prv used to drain the boiler for the pump change? If so and it was already suspect then maybe after a 10 yr service life, the added use of this component might have been the final straw? maybe?
Any other leaks 'leaking lockshield' should be sorted asap as these will allow a drop in pressure and stop the system anyway.

 

[RegEdit]

Thanks for your reply Marios_Kin.
The PRV had failed, after replacing it, (and incidentally the lock shield valve), I opened it up and found the mechanism inside appeared to be damaged. The new PRV has not let a drop of water through it.

The system is not now losing any pressure, and the CH works fine, however the boiler still intermittently cuts out and there is little hot water available, I am planning to pressure flush the system over the weekend,and would appreciate some guidance to enable me to make the best use of the flushing machine. There seems to be little difference in the HW pressure or flow, (based on a gut feeling), only the temp and quantity available. I feel that yesterdays observations from Cog and DP point to a lime scale encrusted HE insulating the HW from the heat and masking the true temp from the thermostat, thus allowing the thermistor to cut out the gas valve. But then again I have little knowledge of how boilers work, but I'm learning fast!

RegEdit

 

[COG]

You said flushing machine,are you going to de-scale the boiler and then powerflush the system . the 4.5 has no flow switch but depends on the temp dropping at the stat which is atached to the cold inlet to the heat cell, trouble is there are two coils and only one stat so make sure you have even temp across both when testing, .fernox descaleing powder usually cleans one out in about 1 hour reversing the descaler about everry 10 mins

 

[RegEdit]

Thanks for the reply Cog. I didn't differentiate between flushing and de-scaling. I have now done both, the machine seems to have done a good job, if you ignore dumping its contents over my legs, tools and garage floor when the o/p hose blew off, the jubilee clips were all slack! Only issue now seems to be airlocks for 2 rads which were working problem free. Thanks to everyone who has helped me with advice and observations.

RegEdit