Worcester 28i Junior Combi - Hot Water fault

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Hi

Our boiler a 28i junior (Worcester) has a hot water problem. The water runs hot after a few seconds as normal and then all of a sudden, after approx. 40 seconds runs cold for about a minute approx. and then gets hot again. After serahcing the forum I believe it to be a heat excahnger problem (the boiler has only 1 heat excahnger a crossflo one and the hot water pipe is scaled up). We do live in a hard water zone and the boiler has no scale reducer attacthed to the cold water inlet (has been like this way since we moved in).

The strange thing is: whenever heating is ON and we use hot water, the hot water does NOT have this problem it runs hot for as long as we want (ie to wash up, have a wash, etc). WHY is this, it seems weird?

My original point is, can I replace the heat excahnger myself, or is it very fiddly? I have tools and have done plumbing.

Thank you for reading my post and for your help.

All the best.
 
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depends whether that model has a plate-plate h-exchanger in it?if it has a twin pass h-exchanger in it and hot water is intermittant and works when heating is on then the likely culprit is the domestic h-water flow switch :p
 
The reason why the hot water is seeming OK when heating is on is due to the crossflow Heat Exch. the water is still receiving heat and will not seem cold. I'd say your heat ex is scaled up (happens over time) and either needs power de-scaling or replacing. Probably better to replace and fit an anti scale device on the cold water inlet to the combi. I'm not sure how easy these heat ex's are too fit, have you got the manufcaturer's instructions? (Boiler Manual)?
 
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depends whether that model has a plate-plate h-exchanger in it?if it has a twin pass h-exchanger in it and hot water is intermittant and works when heating is on then the likely culprit is the domestic h-water flow switch :p

Hi

Yes it has a crossflow heat exchanger, 4 pipes from it.

Yes it is ok when heating is on but if not the hot water goes cold after a minute or so and then gets hot again after about 40 seconds or so and this repeats as a cylce.

The flow switch if faulty would surely mean the hot water would be nt coming on at all?

Can I test for this fault with a multimeter?
 
The reason why the hot water is seeming OK when heating is on is due to the crossflow Heat Exch. the water is still receiving heat and will not seem cold. I'd say your heat ex is scaled up (happens over time) and either needs power de-scaling or replacing. Probably better to replace and fit an anti scale device on the cold water inlet to the combi. I'm not sure how easy these heat ex's are too fit, have you got the manufcaturer's instructions? (Boiler Manual)?

Hi

I have the manual.

Have been serahcing and some posts say that a faulty thermistor, or even the incorrect water flow rate through the boiler, or incorrectly gas pressure's can affect this problem. Is this true? Can I test for any of these (apart from the gas problem obviously)??
 
how olds the boiler??????its very rare for a twin pass h-exchanger to have scale issues even more so on a 28 i junior cos there fairly new??if the boiler is modulating on off to a set time then could be thermistor,if its on off to different times then my moneys on flow sensor breaking down
 
the flow switches on these are flow turbines that spin in speed to the relation of the flow rate.it has a magnet inside and they get stuck
 
Thanks for your help - after reading the boiler manual and following it (no fault code shown though) it doesn't say the flow turbine can be a cause though.

The boiler always fires up when a hot tap is opened, it just goes cold after a minute or so.

It isn't condensing ut is not that old it was here when I moed in.

Sorry what do you mean modulating on/off to a set time or different times?

Have heard the cold pipe-leading to hot outlet pipe can get scaled due to fresh water going in all the time?
 
I'll test the turbine then.

I missed that bit from the boiler manual, what page is that on!!?

If it is at fault, are they hard to change?

One other point, could the fault be due to flow rate at boiler incorrect or gas pressure ? Or thermistor?
 
easy to change just o ring and spring clip,it could be thermistor but u would need to chk thermistor readings with a multimeter,cant remember exactly wot they should be.obviously could be gas pressures which is the first part of any gas engineers fault finding but to be fair it rarely is that.my money is definatey flow sensor had exactly the same fault before.scale wouldnt be my first thing,i live in one of the hardestwater areas and ive never had a twin pass h-exch scale,especially as that boiler is hardly any age
 
Ok thanks a lot for your kind help.

I will test tomorrow morning when fresh.

What's the best way to test the thermistor as well, will check for both whilst have the case off.

The gas valve hasn't been touched so it can't be that anyway (unless they can alter on their own)? But I wouldn't knwo how to check that anyway. Is it hard though?

How comes crossflow heat ex's are less likely to be scaled than plate type then? Are they better quality/larger diameter waterways?
 
its very rare a heat exch gets scaled.a plate-plate will get blocked with sludge etc but thats on the heating side,in general if u checked the cold water side it will be clear.similat to twin pass h-exch.to check thermistor u disconnect wires at sensor,set multimeter to resistance then chk readings on sensor.they is a certain range u should be getting depending on temperature,need thos readings off worcester.got to go,hope thatshelped
 

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