New Boiler slow to heat radiators

[Harry Bloomfield]

This would have add complexity.

Not really - with a hot water cylinder, you just need to set things so the water heating, is done when there is no demand for heating. It is sensible really, because the two temperature demands from the boiler, are usually very different. My CH water demand, might be as low as 30C, whereas any demand for water heating, needs to be 70C, to heat the cylinder to 60C.

 

[Johntheo5]

Obviously, there are pros and cons but Combis are almost non existant around here, I have only seen one, in the UK I think I saw that something like 70% of new boilers were combis.

 

[Delta925]

Most boiler makers show the expected flowrate at a dT of 35C which is fair enough IMO because the very coldest mains temp in winter is occasionally 5C maybe, so a average showering temp of 40C, so the published flowrate will be the very lowest.

You can calculate quite easily what flowrate to expect from your 32kw combi in the debths of winter or whenever.
At 35C dT, a 32kw combi will flow, 32*860/60/(40-5), 13.1LPM. (assuming mains at 5C & showering temp oif 40C)
At a more realictic say 8C in winter it will flow, 32*860/60/(40-8), 14.3LPM
For most of the summer at around a average mains temp of 15C, it will flow, 32*860/60/(40-15), 18.4LPM. (If your mains can support that flowrate)

You can do your own sums in the attached simple spreadsheet.

Back in time I had a higher flow but while no means then to know actual rates it was noticeably reduced when the first water meter was fitted.
The boiler DHW demand is set to 50C.
A spot check of boiler readings while running a bath showed the maximum DHW flow rate is 10.5 l/min and the target modulation 71% so 22.7 kW. I later checked with the kitchen tap wide open and again the DHW flow rate was 10.5 l/min.
From this in winter, particularly if unlike recent years a prolonged cold spell, I guess a 25 or 26kW boiler would at best be close to 100% output.

Back to me wrongly presuming the combi could deliver both heating and hot water even if at the lower of the set temperatures at 100% output and maximum DHW flow say 6kW would be available for heating.

 

[Johntheo5]

A spot check of boiler readings while running a bath showed the maximum DHW flow rate is 10.5 l/min and the target modulation 71% so 22.7 kW. I later checked with the kitchen tap wide open and again the DHW flow rate was 10.5 l/min.
From this in winter, particularly if unlike recent years a prolonged cold spell, I guess a 25 or 26kW boiler would at best be close to 100% output.

The mains temp is 14C just now so the boiler output above should be 10.5*60*(50-14)/860, 26.37kw so boiler performing OK in combi mode.
The modulation is a bit of a mystery to me, if the above figure of 71% (22.7kw) is correct then the DHW temp can only have been, 14+(22.7*860/60/10.5), 45C. If the boiler modulation say on CH dropped to 0% does the boiler stay firing, if so then its still outputting ~ 4.5kw at 0%?

If your max flow remains at 10.5LPM then with a boiler output of 100%, 32kw, the mains temp would have to fall below 6.3C before the HW temp would start falling below 50C, so pretty OK for 99% of the time.

 

[Delta925]

The mains temp is 14C just now so the boiler output above should be 10.5*60*(50-14)/860, 26.37kw so boiler performing OK in combi mode.
The modulation is a bit of a mystery to me, if the above figure of 71% (22.7kw) is correct then the DHW temp can only have been, 14+(22.7*860/60/10.5), 45C. If the boiler modulation say on CH dropped to 0% does the boiler stay firing, if so then its still outputting ~ 4.5kw at 0%?

If your max flow remains at 10.5LPM then with a boiler output of 100%, 32kw, the mains temp would have to fall below 6.3C before the HW temp would start falling below 50C, so pretty OK for 99% of the time.

I wonder overall how accurate the sensors are. Temperature ought to be within a degree or so but might the others have a wider tolerance?
If I remember I can borrow a temperature probe from work one evening and put into the running tap outlet. There ought not to be a big loss at 10l/min after running for a few minutes.

I'm uncertain about modulation too plus note the display is Target Modulation not Actual Modulation.
Several websites have similar statements along the lines of
'Boiler modulation is the ability of a boiler to 'turn down' its output. That is to say, if you have a 20kw boiler, but you only require 10kw of heat for the next hour, rather than outputting 20kw for 5 minutes then resting for 5 minutes repeatedly (how the older non-modulating boilers used to work), the boiler will simply turn down its flame by 50%.'

Given for my boiler the minimum output is stated as 3.7kW and is in a D.xxx setting 11% modulation, the lowest I have seen in CH mode, nominally matches this but of course no way to actually measure.

I've not looked after the early days but the boiler also logs what it thinks is the gas input. When I compared to the meter readings it looked to be under recording by a few %.