Sounds like the boiler is cycling due it’s own thermostat being set too low. You could be wasting gas due to this.
You need to set the flow temp to at least 60degC and preferably slightly higher than that to improve radiator output and kill off Legionnaires - and your lounge will also heat up faster.
The flow temperature reaches 50C and at this time of year if the boiler continued to burn the temperature would have to rise above 50C - but you have told it not to. The return flow is probably also getting close to 50C
This time of year your whole house may only have a heat loss of 2kW to but if you have an oversized boiler, say, 25kWh for heating it may only be able to modulate its burner down to 6kWh.
If you have a radiator that was sized for 1kWh at a 70C flow and you have now reduced this to 50C then the maximum output of the radiator falls to possibly below 0.6kwh.
If the system has been designed for a 70C flow temperature then it is a balancing act between radiator sizes and flow temperatures to get the the flow temperature down.
I'm experimenting at the moment having established that my original system was designed for a 70C flow but now with windows changed from single to double glazing and much more insulation that my radiators are around 40% oversized for a 70C flow.
Currently I have a 60C flow with a design aim of 40C return but the return is likely only to be 40C when the weather gets colder. If I have the heating on for say an hour in the morning my boiler will also cycle. To prevent rapid cycling it has a lockout period after the burner has turned off before it can fire again.
As posted a few weeks ago - look at the size of the TWO radiators in this room with a heat loss of 1.7KWh with a flow at 50C
Baxi 105 combi is a 30kw burner machine so should be able to raise the domestic temp up quickly. It seems to be short-cycling i.e. turning off too soon. Are there room thermostats turning off the rads too soon?
The boiler is turning off when the rad circuit gets too high - check whether the rads are heating up as expected.
It sounds as if there is insufficient load at that flow temperature.
The load will be a function of the total radiator area, and the difference between the flow temp and the average house room temp.
If the boiler is operating at minimum power, and the flow temp exceeds the flow temp you have set, then it will be forced to cycle since that is the only way it can reduce its output to less than its minimum output when firing.
This probably indicates that your radiators are undersized for that combination of flow temp, and room temp, and/or the boiler is oversized.
If you increase the flow temp limit a bit, then the rads will be hotter and also able to dissipate heat faster.
It does - but also lower flow temperatures will result in lower rates of heat movement (i.e. power) into the house. So if the total rad load is on 5.5kW at 50 deg into a house at 17 deg, and the boiler output at maximum "turn down" rate (i.e. modulated as low as it can go [1]) is
11kW, then it will need to cycle at a 50% duty rate to achieve an average power output of 5.5kW.
Only *if* it can dump all the heat it is producing.
[1] The Baxi 105HE is very powerful boiler, with a heat output to the CH of between ~11 to 31 kW. That *minimum* output power is very high (many houses would be ok with a maximum output of that much!)
How many rads / rooms is it heating?
If it is less than 30, then I would suggest doing some heat loss calcs:
formatting link
(and/or you could measure all the rads and go and lookup the typical output power based on the size and delta T - ideally the total output of all of them added together needs to be near the upper end of the boiler's max power)
Minor rant: heating installers used to do proper heat loss calcs to work out the required boiler size - since going too large meant lots of cycling and inefficiency, less comfort for the owners since you got larger temperature fluctuations, and not to mention the excess wear and tear on the system resulting in lower reliability and shorter life expectancy of the kit.
Then boilers got all clever and gained the ability to modulate their output - at which point it seems that many installers decided - there was no need to bother sizing it "correctly" any more, just leave it to the modulation to match the demand. Alas that only works if the typical heat demand falls into the output range that the boiler can actually manage.
It seems unlikely that the place will get close to the kind of heating load that would need the full output of the boiler.
Combis are often selected for the power they can deliver to DHW rather than heating. The Baxi seems unusual in that it can output the same power to the CH as it can the DHW (Many combis will provide less heating power to the CH. For example the 35kW Combi installed at my previous house would delivery a max of 24kW to the CH).
A 24kW Combi will supply between 10 and 14 lpm (depending on cold water temperature), which is usually more than adequate for most showers unless it has multiple heads and body jets etc). So 30kW will be over specced most showers (but would manage two moderate ones at once, and be "ok" for bath filling)
Which lowers the maximum power output the house can absorb lower.
FWIW I am running 19 rads on a 24kW system boiler that can modulate down to just under 6kW - that is more than adequate in a not particularly well insulated solid wall property...
It does... it is just a "rule of thumb" that ensures the place can get hot enough in the depths of winter (and is somewhat assuming that the boiler is sized appropriately for the property). It probably cuts down on the tech support calls they get from people who can't get the house hot enough.
Stick a bit of masking tape on each pipe, and point a IR thermometer at the tape (or find a bit of painted pipe) - don't take a reading from bare metal since they emissivity of the copper will not be what the thermometer is assuming.
Ideally you will need to check each rad for balance - with the TRV heads full open to start with. A traditional setup would be balanced for a 11 degree delta. Condensing systems would be balanced for a bigger drop - typically 21 degrees.
I expect you will find that the boiler is significantly oversized all the time - even with -3 degree outside temps. So you may need to run higher flow temps and sacrifice some condensing efficiency.
Installing larger rads could help, or even consider using a thermal store to buffer the heating load (i.e. the CH output connected to a large direct cylinder of water, which is then used to heat the CH via a heat exchanger (plate or coil) and a pump). That way the boiler could be set to fire in bursts of high output to replenish the store, and not need to try and match the significantly lower rate of transfer into the CH.
While the linked video doesn't necessarily cover combi water temperatures it fully explains what you are seeing with your central heating.
The minimum you boiler can output is around 8KWh. Today your house possibly only needs much less than 4kW and this is if you heat the whole house to, say, 19/20C. As John has stated in his post the boiler can only achieve this by i) Modulating down to the minimum output ii) Having a on/off cycle of 50/50%
Further, if you have a radiator of nominally 1kWh designed for 70c flow and 55C return and you then use 50C flow and 30 return it now is a
0.4kWh radiator. You could have reduced the output of all the radiators in your house by 60% by using such a low boiler flow temperature. Also your TVRs may have closed down some radiators as well as the one you have turned off yourself. What this could mean is that your radiators cannot deliver the heat loss of the building at a 50C flow. You may now have a building heat loss of 4KWh, the active radiators only able to achieve a 2KWh output and a boiler that cannot output less than 8KW. The boiler will cycle and be off 75% of the time.
You may/will not be able to achieve return temperatures of 40 or less with an oversize boiler and even if you change the boiler for one with a lower minimum output (which usually means lower maximum output) return temperatures of 40C may/will only be achievable with your radiators perhaps being 3x the current size. One reason the use of "gigantic" radiators (under floor heating) is recommended for air source heat pump systems where the flow is around 50C.
You are probably better off setting the flow temperature first at 70C and see how the CH performs, then reduce it to 65C and then 60C, checking the performance each time. You may/will find as the temperature outside falls you may be getting return temperatures of 55C or less. Maybe for comfort you may have to set the flow up a bit during the coldest few weeks of winter.
Consider how much efficiency you are chasing and have a realistic gaol - don't chase the unobtainable unless you are perhaps going to make expensive changes to your system.
I tried it on full on when it came on at 1630 (house at 17C). Obviously radiators very hot as the output temp was 80C. Checked the flue and quite a big flume so wound back to 2/3rds which gave the 70C you suggest above and hardly any sign of plume.
I'm chasing reduced costs. I don't propose any significant changes to the system, at least not this winter. I want to see how having a thermostat, improving the insulation and turning off the radiator in the large hall/corridor space works out.
Of course at the moment month on month (last year v this year) comparisons are a bit meaningless due the mild weather so far. But at least I have an idea of what I'm trying to do and why.
In the 6yrs we've been here, East Midlands but relatively high up, the boiler has always been able to keep the house warm and I don't think I've turn the dial to much more than 1/2 way. It's a bit more than that now but the room thermostat should stop overheating.
Thanks. I accept the boiler is oversized. Given that it is ~16yrs old I've asked three service engineers in the past few years who have given quotes/estimates without looking at the property and its layout. Eg:
1) Boiler: Worcester 29cdi Condensing Boiler
2) Baxi 800 / Baxi 600 (estimates so no mention of capacity)
An ideal replacement for us would be able to:
1) Provide instant hot water for the shower next to the boiler
2) Provide hot water for a cylinder above the kitchen to avoid the
20m+ run that currently exists. The debate is to whether it is a cost efficient solution rather than a convenience solution. Mainly required in the winter so bleeding off some of the CH supply would be a possibility. None of the boiler service engineers have been enthusiastic about looking at this option.
3) Keep the house warm as needed.
The bath runs from the combi and takes some balancing of the temperature to get right but at two or three baths/year significant expense is not warranted. An electic shower in the main property bathroom (the combi shower is in the utility room alongside the boiler) is not currently functional. If I do get it working it will probably remain electric. We still have a cold water tank in the loft (needs replacing as it is rusting) and my gut reaction is to retain that rather than go mains pressure throughout, though it only supplies the toilet, cold water taps in the bathroom and an adjacent bedroom washbasin.
The property does lend itself for a heat pump in as much as the boiler is already on a back wall. But the work for the rest of the house doesn't. Any piping from the boiler goes through a flat roof extension and then awkwardly into the loft. Fully fitted carpeted floors need to be shifted for pipework which I suspect is mostly too small. Tile flooring lifted. Ugh!!
Surface mount pipes on walls can be done quite neatly. Friends of mine had a complete system fitted in a property with solid floors so pipes run at skirting board level and in the corners of the room. Another option is pipes running inside (hollow) skirting boards.
formatting link
Smaller and neater if the original skirting is removed first allowing pipes to be fitted lower and allowing the covers to be smaller.
What you really need if replacing is properly designed system. Also bear in mind that the heat loss is usually calculated for the expected coldest days of the year and for most of the late autumn/winter/early spring seasons your boiler only needs to work at a quarter to half of its maximum capacity.
may find them cheaper on Ebay/Amazon (from a non-China seller).
Also order some replacement LR44 batteries (2 off per module) as the ones they come with may have been in there some time. Although these modules possibly may only have an accuracy to 2C select the two that give that best match and show the (near) same temperature. With a bit of luck all four will show the same temperature.
Tape one probe to the flow and another to the return pipes somewhere near the boiler. As the radiators are heating from cold you will first see a large difference in temperature but check when all the radiators that are turned on are hot and have been in that state for 15 minutes - and the boiler is still firing.
You can also check the temperature difference between the flow and return pipes on the radiators in the same way BUT checking individual radiators doesn't necessarily give you the same results as the flow and return to the boiler.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.