House taking ages to heat up

Last June (2019!) we moved to a house, built in the mid 1980's, which has a WB 37CDi combi boiler, installed about 10 years ago. I have estimated, based on the standards set when it was built, that the heat loss is about 10kW. There are about 10kW of radiators. The boiler can only modulate down to 9.5kW so it spends most of its time cycling on and off.

When it started to get very cold (early December) I found that the house would hardly rise above 20C. The boiler seemed to be cycling all the time with the flow temperature only rising to 60-65. When the pump overran the flow would drop below 40C before the boiler restarted.

I down-rated the CH output from 30kW to 13kW and set the room stat to maintain 20C overnight. (At my previous house it was set to 16C - just in case as the house never feill below 18C.) The house is now reasonably comfortable in the morning with a wake-up temperature of 21C. However we find that it will not rise any more. Last night I set the room stat to 24C; two hours later the roomtemperature was still 21C.

I have monitored the boiler, which is set to a CH flow of 75C. However the temperature never reaches this before it cuts out - 65C or thereabouts. The boiler then stays off for several minutes (anti-cycling?) while the water temperature drops about 20C before it relights.

I can't work out what is happening. The rads are all warm, with no cold spots to indicate blockage, but they never get really hot.

I have checked the parameters and they are all correct, according to the Engineer's Service Booklet for the CDi Range (apart from CH output, which I have lowered to 13kW).

1. Why does the boiler never reach set temperature - 75C?
2. Why does the water temperature drop so far before the boiler relights?

1. A first guess would be that the house requires more than 13kW. Try raising it to 15 or 20 and see what happens.
2. Do the radiators have TRVs, and are these set to room desired temperatures?
3. Don't forget the calculations are based on a steady state. Its normal to allow 10% more for the initial heat up period.

Have you checked the room thermostat for correct operation?

Insulation, or lack of, could be a problem. Loft insulation relatively easy to top up. What sort of insulation was used in the 1980s in walls and floors? My parents have a 1980s house (1988 I think) and that seems OK, not as cosy as my extension, but not as bad as my 1930s bungalow!

Although i am pretty sure the heating used to be on all night when I lived there, so guess it does cool down much quicker than modern homes.

I will guess setting of lock shield valves? The return water temperature is important with modulating boilers, so with a TRV and LS valve on every radiator with LS wide open what happens is the easiest room for water to circulate to warms first, as the room heats up then the TRV starts to close, and next room starts to heat, once all rooms are warm it works OK, as the TRV is adjusting flow, but after the heating has been off it takes an age to reheat house one room at a time.

I am told with all TRV heads removed or set to max the temperature drop across each radiator should be around 10°C, in practice I have not found my differential thermometer so use a more Heath Robinson approach, starting at closest to boiler I turn off radiator completely allow to cool, then ¼ turn at a time open it with at least a minute between each adjust, until I feel one pipe get warm. At that point I leave it and go to next, once I have done all radiators, then I monitor rooms, if room cool then another ¼ turn.

TRV's are slow to act, so the flow through the radiator has to be reduced to a level where the TRV does not over shoot, with my TRV heads it shows on the PC or phone target and current temperatures, that makes it easy, once heating has been running for a while is current exceeds target turn lock shield valve down, and if current is under target turn the lock shield open a bit more.

Unfortunately many TRV heads just have *123456 with no °C shown, I am told 20°C is around 3.5 on dial, but not very accurate, I found if the TRV is on return the lock shield setting is far more critical, but I took the electronic head and fitted it to radiator to set lock shield, then replaced with wax head and moved electronic to next radiator until all set.

This house has 9 electronic heads, 4 expensive which show target and current, and 5 cheap which only show target, in °C. Because each room has its own schedule, I can set schedule so bedroom heats first, then few minutes latter before boiler has switched off kitchen start to heat, then dinning room and finally living room. Setting times when main thermostat and TRV heads alter temperature means the boiler has a long burn time, at a reasonably high output. In my case not a modulating boiler, and the thermo syphon flow to heat domestic hot water tends to send hot water back to boiler early so a modulating boiler would also need some extra wiring and motorised valve to turn off DHW heating. However since yours is a combi boiler that should not be a problem.

Electronic TRV heads start at around £10 each so not that expensive, in fact cheaper than some liquid and wax heads. I find over temperature more of a problem to under temperature, design of house, if doors left open, top floor gets too hot. Three floors old double glazing so not that good, three floors and 18 kW oil boiler is ample.

oldbuffer said:

1. A first guess would be that the house requires more than 13kW. Try raising it to 15 or 20 and see what happens.
2. Do the radiators have TRVs, and are these set to room desired temperatures?
3. Don't forget the calculations are based on a steady state. Its normal to allow 10% more for the initial heat up period.

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1. If that was so larger rads would be required.
2. Yes
3. Most calculators allow 10-20% extra for heating up. Mine allowed 20%, which shouldn't really be necessary as the house is always occupied.

The Novice said:

Have you checked the room thermostat for correct operation?

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It gives the correct temperature when checked against a digital thermometer; and the boiler turns on if I put the temperature on. The problem is that the house does not get any warmer.

jonbey said:

Insulation, or lack of, could be a problem.

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Yes insulation does need improving, but that's not an easy job as the house is link detached. In any case I calcuated the heat loss based on what was required when it was built

ericmark said:

I will guess setting of lock shield valves? ...

I am told with all TRV heads removed or set to max the temperature drop across each radiator should be around 10°C

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I don't see why a mis-set LS valve would cause my problem. Modern condensing boilers work best with a 20C differential. It just means that larger radiators need to be installed to allow for the reduced output.

What temperature difference is it when your calculated 10kw of radiators? If it's 50c then you would need to run the boiler at 75c flow rate to get the rated output.
For good efficiency on a condensing boiler you would want flow temperatures as low as possible, say 60c, so with only 30c temperature difference your rads wouldn't be much more than 6kw.
Also with any draughts and thermal bridging your calculated heat loss may not be accurate, depending how you worked it out.
If i were you a quick fix would be boost the flow temperature of the boiler, in the long term better to rejig the rads or add new ones to get a cooler water temperature.
I inherited a very similar heating system to you and went down the route of adding more radiators.

I have been monitoring the boiler activity and learned something I was not expecting.

Turned the roomstat up from 21C to 25C
Flow temp showed 43C, Output showed 32% (set to 35%)
After 5 minutes the flow had risen to 51C, output still 32%

I decided to increase the output (parameter 1A). while doing this I took a look at the range available: it went from 32% to 84%. I was aware of the 84% maximum as the 37CDi only goes up to 30.9 kW for CH compared to 37kW for the HW (30.9/37 = 0.835 or 84%. But I had assumed that the minimum (9.8kW) would be 0%. Wrong! it's 32%. But it doesn't end there. If 32% is 9.8kw then 100% is 30.6kW. But 100% is 37kW. WB obviously have their own way of turning kW output into percentages. I have increase the output from 35%, which I though was 13kW, to 43% hoping that is 13kW. Time will tell.

Central heating is maxed at around 28kW for bigger combis, yours as you indicate you 30.9kw
Higher power is to heat the water only.

What thermostat have you got? How does your hot water perform? As you have access to the service manual one of the settings is for max flow temp, have you checked that

martyndc88 said:

What thermostat have you got? How does your hot water perform? As you have access to the service manual one of the settings is for max flow temp, have you checked that

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The thermostat is a WB DT10RF
Hot water is fine
Flow temp is 75C

Done some more monitoring today.

House is up to temperature (21C).
Boiler is not running
Raised the temp to 24.5C
It takes over a minute for boiler to start
When it starts Parameter 9c (heat output) shows 50 (set by me, approx 13kW)
After two or three seconds param 9C reduces quickly to 33 (min output) then boiler turns off.
A few minutes later the cycle restarts

The water is only 50C, (set to 75C). So why does the boiler modulate down and then turn off?

I tried setting the pump to low proportional pressure (default is high), but it made no significant difference.

Is there anything else I can try before calling a heating engineer?

Check the primary sensor. It may be giving false readings (turning boiler off early)

Dangee said:

Check the primary sensor. It may be giving false readings (turning boiler off early)

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Doesn't that involve opening the boiler, which is a "Gassafe-engineer-only" task?

How would I check the sensor?

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