how many boilers modulate down enough

northernsoul

Hi
I have a Valliant eco plus 837. 28kw output
No matter what boiler flow temperature is set at and no matter what thermostat is set at , and no matter what the outside temperature is , my boiler always cycles when the room is up to temperature. 
I have a Honeywell cm927 thermostat. The boiler cycles 6 times an hour. I believe the Honeywell controls the cycling. The time the gas is on , the time the pump is on and the time the boiler is 'off' varies according to the weather.
I have 8 radiators of varying sizes, all balanced. In total 13kw output.
My boiler has been sized for my rainforest shower.
I would like to know how many peoples boilers manage to modulate down enough,  so effectively, the gas is on all the time, but at a low enough output that matches the output from the radiators.
How many people achieve this narvana ?
Cycling is much talked about but does anyone actually avoid this supposedly inefficient usage.

Thanks   

FreeBear

To take full advantage of modulation, I suspect you need a smarter thermostat that can "talk" to the boiler. That way, as the thermostat approaches its set temperature, it can instruct the boiler to modulate the heat output down rather than cycling on/off.

northernsoul

Hi
Thanks for that.
At times it does 'seem' to modulate down. There is much less flue output and the boiler definitely sounds like its not performing at full chat like at the start.
It is now 12 years old and , according to the specs, has an heating output of 12 - 28 kW. My house is a terrace with about 90m2.
I suspect it cannot modulate down enough to match the required amount.
I think the honeywell is a class 4 thermostat.
I would be interested what output peoples boilers require to 'top up' while modulating, to the required temperature, either from some information on the boiler or an hourly average read from the meter.
At a outside daily temp of between 1C and 5C , my hourly usage over 24 hours, starting at 20C and maintaining 20C for 24 hours consistently, my boiler uses between 3.07kWh and 4.13kWh per hour (this last figure taken as an average while i sleep - i did not check the hourly meter readings while i was asleep ! ).
Edit 1 . A total use of 7.41m3 , or about 80kwh for 24 hours consistently, at 20C, costing now, at 10.24/kWh,  £8.20 for 24 hours.
Edit 2  . Do any boilers modulate down to 3kw ? 
Thanks
 

FreeBear

northernsoul said: Edit 2  . Do any boilers modulate down to 3kw ? 

The WB Greenstar 4000 will go down to 3.2KW, and I'm sure other makes/models will modulate down that low as well. A 1:10 modulation range seems to be pretty standard with most current boilers.

ComicGeek

As the heating demand changes throughout the year you won't get a perfect match between minimum boiler output and actual demand. More modern Vaillant boilers have an anti cycling mode whereby they prevent the boiler from kicking in until a specific amount of time has passed. The updated 835 combi does have modulation down to 3.4 kW, so definitely something that can be resolved when you next replace it.

You can also get weather compensators that will monitor external temperatures and then work out whether the flow temperature can be reduced - much better to run the boiler at a lower temperature than having it turn on and off constantly.

The old rule of thumb was that heating systems generally run most hours at 30% of peak output - so your 13 kW radiators would be 4 kW running output, which seems to fit with what you're seeing. We used to make sure that heating systems would work well at both 100% and 30%, but that isn't really possible with large older style combi boilers like yours.  

northernsoul

Found a Viessmann Vitodens 050-w 3.2 to 35 kw output and a Vokera Linea One 38kw 3.8 - 38kw. So recent boilers do look like they could evade cycling. Maybe less outpi=ut boilers have even less minimum output.

RonRamsgate

Hey, that boiler mentioned is a decent item but often not installed or maintained by manufacture agents or experts.

Yes there not the newest of uk gas boilers many gas£+ industry people consider it in need of exchange but maybe its not set up correctly and cycling on/off/on/off with burner burn.
In radiator heating mode them boilers can be range rated down to the expected KW needed its done during installation, its probably been done already 

If the boiler cannot be range rated down enough, there are alternative idea which can allow the boiler to rest a little more and not cycle on and off so often.

That boiler has an internal adjustable bypass valve, if there is an external bypass valve or the radiator system is not   constricted with  all but one radiator without a trv. Some adjustment can be carried out (engineer).
There are some adjustments- push button to alter cycling and pump over run time but they have to be altered with consideration to the rest of the central heating plumbing including t
Hopefully there are no hot water leaks or other faults in the tap/shower installation that tell the gas boiler to fire up and cycle on off more than necessary (pre heat).

Maybe just physically turn down the room thermostat by 1 degree when the boiler begins to cycle and consider getting in a vail engineer to fine tune it.

Next boiler consider getting open therm heating control. its good but dont believe anything on the internet and youl be ok.

northernsoul

Hi RonRamsgate

Thanks for the tips. 
It was not range rated when it was installed. He made to pipe the condensate such as the trap cannot be unscrewed to clean ! 
I range rated it a few years ago but was unsure whether it made any difference so put it back. Might try again.
I have tried several gas engineers for service and have been disappointed with their skill set , including the Vaillant recommended engineer. All of them seem to do nothing much more than a safety check and gas analysis. Takes 10 minutes.
Boiler needs a graphite boiler seal doing so will have the Vaillant engineer to service it.
Thanks again

BUFF

northernsoul said:

Found a Viessmann Vitodens 050-w 3.2 to 35 kw output and a Vokera Linea One 38kw 3.8 - 38kw. So recent boilers do look like they could evade cycling. Maybe less outpi=ut boilers have even less minimum output.

Vitodens 200 will go down to 2.5kW but it is a premium boiler.