Boiler output for CH vs HW

[Bargain Bucket]

Not had a chance to take any measurements yet. But the thermistors and PCB were replaced last year (not by me) in an attempt to resolve it.

Every heating engineer who as come out to investigate just wants to rip it out and install a combi. No way on earth am I going to let that happen.

 

[Harry Bloomfield]

This one takes the biscuit. Heard it all now

You are clearly failing to understand what ff has suggested, or you are failing miserably to understand a very basic principle of heat transfer. The hotter the circulated water, the more rapidly and effectively the water will be raised to temperature in the cylinder. If the difference between desired and boiler output temperature is less, the boiler will very obviously involve more cycles to get the cylinder to final temperature. Worst case, is with boiler set to a little below the desired cylinder temperature, when the boiler will cycle for ever more..

 

[fixitflav]

OK

I would have hoped that the boiler would either modulate down much sooner after fire-up or would start low and modulate up.

That's a good way of putting it! It's why I thought a thermistor might be the culprit. Have you bounced it off Glowworm? They ought to be the experts, though the extract below from the manual doesn't fill with confidence. My underlining, that would work for a combi, but not for your heat-only. Also the pressure can be below 20mb and still meet the regs, as discussed on here recently.
11.5 Gas Inlet Pressure
Check the supply pressure at the gas service isolation valve
test point, see diagram 8.1.
The nominal supply pressure should be 20mbar when the
appliance is operating at the maximum heat input. This can
be achieved by turning on several hot water taps.

It's a knotty problem, I can't think of anything else at the moment.

Every heating engineer who as come out to investigate just wants to rip it out and install a combi. No way on earth am I going to let that happen.

Good idea!

 

[DP]

You are clearly failing to understand what ff has suggested, or you are failing miserably to understand a very basic principle of heat transfer. The hotter the circulated water, the more rapidly and effectively the water will be raised to temperature in the cylinder. If the difference between desired and boiler output temperature is less, the boiler will very obviously involve more cycles to get the cylinder to final temperature. Worst case, is with boiler set to a little below the desired cylinder temperature, when the boiler will cycle for ever more..

Read replies 35 and 44. What do you think am I getting at?

Trying to get to the bottom of the issue for the poster whose plight has been sidelined

Then look at at reply 30- boiler is at 65 and cylinder at 60 setting boiler is going to cycle forever at this setting

 

[Harry Bloomfield]

Then look at at reply 30- boiler is at 65 and cylinder at 60 setting boiler is going to cycle forever at this setting

60 and 65 are just barely enough headroom, even supposing both values are completely accurate.