It's about 2,200 sq ft, 4 bedrooms, 3 bathrooms, with about 20 rads... My plumber said that Part Load is related to the gas mix - the gas is analysed - he says 18KW is "more than enough"....so now I'm confused as to why I needed a 30KW boiler, if it just gets throttled to close to 50% of its capacity...
Do I need a new boiler? (Possible HEX blockage) (Ed.)
- Thread starter jchasty
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You shouldn't really have to throttle/range rate any boiler but combi boilers especially are generally rated to give a reasonable HW flow which means the CH demand may only be 1/2 or less than the HW demand, you might have a 36kw Combi with only a 18kw CH demand so you might range rate the boiler to 18/20kw, this only restricts the output in CH mode, one reason for doing so is some boilers have difficulty in firing up in "one go" and restricting the output may help to achieve this as it helps to avoid the boiler exceeding the target flow temperature by + 5C and burner trip after the burner is released from ignition settings. Don't know what your plumber means by the gas mix, part load/range rating means exactly what it says, it restricts the output to whatever you have set it to.
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That's biggish, but if it's reasonably well insulated 18 kW should be plenty. I suppose it's a different plumber who says 18 kW.
Now you've knocked it back to 18 kW, do you still have the low temperatures? And if so, is the house warm enough? If it is, maybe there's no problem.
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I recently had a job where the system temp never got above 45 degrees. It was a 38KW heat only glow worm which some idiot builder had specced on the assumption that bigger is always better.
The system pipework couldn't cope with the boiler output and temp rise was too fast so the burner shut down. Anti cycling time was set to maximum so the boiler fired for a couple of minutes then waited for 15-20 minutes before repeating.
I set part load to 18KW and reduced anti cycling time and all was soon toasty.
The system pipework couldn't cope with the boiler output and temp rise was too fast so the burner shut down. Anti cycling time was set to maximum so the boiler fired for a couple of minutes then waited for 15-20 minutes before repeating.
I set part load to 18KW and reduced anti cycling time and all was soon toasty.
For the 3rd time ! is the house warm enough? (also when the weather's cold) The proof of the pudding and all that.
Have you checked the gas flow, to confirm the heat input? Gross CV is about 11 kWh/m3 so you can easily find the power if you know the flow.
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