Shower runs hot then cold then hot......

[Johntheo5]

Full bore allways IMO.

 

[Johntheo5]

Have you tried setting the boiler DHW temp to 40C (actual 45C) and then reducing the shower setting to 32/35C?

 

[NigelCollins]

Have you tried setting the boiler DHW temp to 40C (actual 45C) and then reducing the shower setting to 32/35C?

Hi - Yes we did this this morning. I didn't measure the temperature of shower but it seems to be working like this although there is a cut-off when I reduce the shower temp too low the boiler starts cycling again

 

[Johntheo5]

It would be interesting to know if a combi DHW temperature control works similar to the CH side, ie burner off at boiler flow temp SP+5C and/or DHW temp SP+5C.
If so and if one accepts that the lowest DHW SP temp on your boiler (40C) is actually 45C then the burner might cut off at > 50C, so 12.8kw min output would mean at a HW flow of < 6.22LPM, if the DHW control doesn't cut off the burner until say 65C then the flow rate should be able to be reduced to < 4.12LPM before burner cut off.
Sometime, when you get a few minutes you might carry out a few tests re above but with a HW tap only, no mixer.

 

[old duffer]

The temperature on combi's represent a max output temp so it is a relation to flow rate. High flow rate mean lower output temp and visa versa. Continuos water heaters operate by temperature set point and adjust the flow rate to match the desired output.
replace the thermistors (do both while your at it)

 

[Bod ...]

Just read a bit of this post. I'd say your plate heat exchanger is blocked on the boiler.

 

[NigelCollins]

It would be interesting to know if a combi DHW temperature control works similar to the CH side, ie burner off at boiler flow temp SP+5C and/or DHW temp SP+5C.
If so and if one accepts that the lowest DHW SP temp on your boiler (40C) is actually 45C then the burner might cut off at > 50C, so 12.8kw min output would mean at a HW flow of < 6.22LPM, if the DHW control doesn't cut off the burner until say 65C then the flow rate should be able to be reduced to < 4.12LPM before burner cut off.
Sometime, when you get a few minutes you might carry out a few tests re above but with a HW tap only, no mixer.

Thank - ill try next week

 

[NigelCollins]

Just read a bit of this post. I'd say your plate heat exchanger is blocked on the boiler.

Thanks - the heat exchanger was changed 12 months ago

 

[NigelCollins]

It would be interesting to know if a combi DHW temperature control works similar to the CH side, ie burner off at boiler flow temp SP+5C and/or DHW temp SP+5C.
If so and if one accepts that the lowest DHW SP temp on your boiler (40C) is actually 45C then the burner might cut off at > 50C, so 12.8kw min output would mean at a HW flow of < 6.22LPM, if the DHW control doesn't cut off the burner until say 65C then the flow rate should be able to be reduced to < 4.12LPM before burner cut off.
Sometime, when you get a few minutes you might carry out a few tests re above but with a HW tap only, no mixer.

I'm thinking that as my boiler is now 17 years old and the maintenance contract is nearly £40 per month, I'm thinking that I'd be better off buying a new Worcester Bosch with their 10 (or in some cases 12) year warranty that only requires a £100 service per year to stay within warranty terms. I've noticed that new boilers can operate at a much lower output - they seem to modulate to a 5:1 ratio rather than my current one that is nearer to 2:1.

 

[Bod ...]

Full bore allways IMO.

Thanks - the heat exchanger was changed 12 months ago

Did you have your system powerflushed and have you got a good magnetic filter?

 

[Johntheo5]

Did you have your system powerflushed and have you got a good magnetic filter?

Primary and secondary flowrates obviously important to avail of the maximum and minimum boiler outputs, in the above case, because of the boiler minimum output of ~ 12.8kw then a HW tap has to be run at a flowrate of 9.2LPM @ HW temp of 40C, some HW taps will only flow ~ 6LPM so one would have to be satisfied with a minimum temperature of almost 51C in this case. Most modern combi boilers can modulate down to 6kw or so so and a modest flow of 4.3LPM from any HW tap at 40C even from todays mains temp of at least 20C.

 

[NigelCollins]

Did you have your system powerflushed and have you got a good magnetic filter?

Hi, No I didnt have the system powerflushed and I dont have a magnetic filter

 

[NigelCollins]

Primary and secondary flowrates obviously important to avail of the maximum and minimum boiler outputs, in the above case, because of the boiler minimum output of ~ 12.8kw then a HW tap has to be run at a flowrate of 9.2LPM @ HW temp of 40C, some HW taps will only flow ~ 6LPM so one would have to be satisfied with a minimum temperature of almost 51C in this case. Most modern combi boilers can modulate down to 6kw or so so and a modest flow of 4.3LPM from any HW tap at 40C even from todays mains temp of at least 20C.

Thanks - yes it helps to make the (expensive) case for a new boiler

 

[Bod ...]

Why don't you get the system powerflushed before you get a new boiler.

New boiler is the way to go rather than chuck money at an old the old one but powerflush first

 

[Johntheo5]

I think new boiler warranty demands a power flush/chemical clean before installation, from (horror) posts I've read on here, it has to be properly carried out with multiple flushings and water testing so a good idea to do it before removing the old boiler, but will the new boiler warranty be valid if you then run the system with the old boiler for a few days/weeks before new boiler installation?. I presume that the normal method is to do the whole operation in one, ie, remove old boiler, clean the system, then install new boiler. Chemical cleaning is probably "worse".