House taking ages to heat up

Dangee said:

Return the boiler to factory settings. Set to max output (hold button in for 10 secs)
Check temperature of flow & returns simultaneously (no more than 20 degrees difference) if more than 20, it's a system problem, if less a boiler issue

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You can also get a difference greater than 20C if the pump speed is too slow, which is a boiler problem on combi and system boilers.

The boiler does not stay on long enough for there to be a significant difference between flow and return. It might be OK when the boiler starts up first thing in the morning, but that's at 5 am and I'm too old to want to get up then.

I have only 10kW of rads so the boiler will be running at minimum output or in on/off mode most of the time. There is therefore no point setting to max; it will simply modulate down very quickly.

Just done another test:

Set the room stat to 25C
Boiler turns on after about a minute
Flow temperature rises to about 55C, according to the boiler's display, but my digital thermometer only shows 45C when held in contact with the flow pipe below the boiler. The return pipe only registers about 30C.
Boiler turns off after one to two minutes and the displayed temperature drops to about 45C.

I then set the room stat back to normal (22C) and the boiler continued to cycle as the house was only at 21C. So it has taken eight hours to raise the temperature by two degrees (overnight temp set to 20C).

There are two parameters which confuse me - 3b and 3C: 3b is anti-cycle time (default three mins); 3C is anti-cycle flow temperature differential (default 10K). I asume that 3b holds the boiler off for three minutes before relighting; but what does 3C do, and how does it interact with 3b. Can anyone explain this (it's not explained in the Engineer's Service Booklet)?

There is also parameter 9F, pump over-run time. But I believe this only comes into play when the boiler has been turned off by the room stat, not by the boiler's own controls.

D_Hailsham said:

The house is now reasonably comfortable in the morning with a wake-up temperature of 21C.

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Jeez...are you a reptile? How hot do you want your house?...Put some clothes on!

D_Hailsham said:

There is therefore no point setting to max; it will simply modulate down very quickly.

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It was for an experiment not permanently. If your return is 30c (and your flow is 45c) then it won't/shouldn't have modulated down yet.
Sounds like this one is going round in circles, makes sense to diagnose yourself rather than getting someone in to randomly change parts or change the whole boiler. But you really need to narrow down the problem rather than various observations of the whole system which may be interacting.

You are assuming that the boiler only goes off when the flow temperature exceeds that set on the boiler's thermostat (apart from room stat control). But this doesn't appear to be the case. There is no return temperature sensor, so monitoring the differential is not possible.

As another experiment (that's how you learn!) I turned the output up to max. Result (not in the sense of problem solved): Boiler comes on; flow temp rises quickly to 48C in under 30 seconds; boiler turns off; temp drops to about 41C over a few minutes; cycle repeats.

After a few cycles with no significant change in behaviour, I set the boiler to minimum output. There was still no change in behavour; the same cycle occurred.

There are no error codes and the reset light is not on which suggest that the hex and flue overheat thermostats have not come into operation.

All TRVs are set to max and the pins are working. All radiators are warm (about 45C).

What is causing the boiler to turn off before it's up to temperature?

D_Hailsham said:

What is causing the boiler to turn off before it's up to temperature?

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I'd suspect the boiler stat, or the circuitry that interprets its reading if the stat is a thermistor type.

Alec_t said:

I'd suspect the boiler stat, or the circuitry that interprets its reading if the stat is a thermistor type.

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I had decided that you were correct and was going to phone an engineer this morning when I happened to wake up at 5am. I decided this was an ideal time to check the boiler (in airing cupboard opposite bedroom door) as it would have only come on recently. The boiler was on, temperature showing 61C. I watched for several minutes while the temperature rose steadily to 65C wthout the boiler turning off. Went back to bed.

Now this is completely different behaviour from that seen when I put the room stat up from 21C to 24C. Could it be that the thermostat is ignoring any temperature override instructions and just sticking to the programmed ones? To check this out, I changed the room stat from Auto to Manual mode and set the temperature to 24C. 30 miinutes later the room stat still show an actual of 21C and the boiler is off. Rad temperatures are about 45C. So this disproves my theory.

Time for an engineer. Let's hope he's not just a part-changer.

BG Engineer came yesterday afternoon. At first he thought it was the wireless prog stat (DT10RF) losing contact with receiver in the boiler. This was because he put it in "chimney mode" and it ran all the time without stopping. I suggested bringing the wall unit closer to the boiler, which he did, but it made no difference. He then re-paired the transmitter and receiver, which also made no difference. Eventually he decided to check the heat exchanger. It was very blocked so heat was unable to get to the water. A good clean with his special brush and the use of a jug of water and everything was working as it should.

I am surprised that no error code came up.