System boiler flow temperature fluctuates wildly

[molgrips]

Hi all. I've been reading up on my heating system to try and optimise it. It was bought from the builders in 2007 and I don't think they did any kind of setup or tuning of the system. It's an S-plan system with an Ideal Icos HE-12, in a three storey well insulated house in a mild part of the country. I've had a look at turning the flow temperature down.

According to the manual the flow temperature can be between 30 and 70C. The boiler modulates, but not very much - the minimum is 9kW and the max 12kW, so I'd expect the boiler to cycle a bit once the set temperature has been reached. Aside from wether or not this is desirable, it doesn't actually behave this way. I've attached thermocouples to the output and return pipes just after they leave the boiler so I can monitor them. The CH is off now so I'm looking at how it works with the HW and here's the problem:

When I have the flow temperature set fairly cool, so it should be about 50C or so (yes I know this would be too low) the flow temperature still climbs up to 62C or so before the burner shuts off and the pump continues to run. Ok, fine, but then the burner doesn't come back on for ages, like 20 minutes, at which point the temperature has dropped to 42C or so which must be the temperature at the bottom of the tank. It just keeps pumping water around cooling the tank down. If I turn the flow temperature up higher, the flow temp goes up to 70C or so and then it still shuts off for ages and the temp drops back down to mid 40s.

I can't figure out how the boiler is responding to what the flow temperature should be. I thought maybe a faulty flow control thermistor but I've replaced that and it's the same.

 

[oilhead]

What temperature is your cylinder thermostat set at?
With a cylinder calling for 60C and a flow temperature set at 50C, even if it does rise to 62C, you're never going to satisfy demand, so the pump will run and the boiler will keep cycling around the 50C set point.(12C above and below your setting. If you want water at 60C, you need a flow of about 70C to cope with cylinder and pipework losses.

 

[fixitflav]

Hi all. I've been reading up on my heating system to try and optimise it. It was bought from the builders in 2007 and I don't think they did any kind of setup or tuning of the system. It's an S-plan system with an Ideal Icos HE-12, in a three storey well insulated house in a mild part of the country. I've had a look at turning the flow temperature down.

According to the manual the flow temperature can be between 30 and 70C. The boiler modulates, but not very much - the minimum is 9kW and the max 12kW, so I'd expect the boiler to cycle a bit once the set temperature has been reached. Aside from wether or not this is desirable, it doesn't actually behave this way. I've attached thermocouples to the output and return pipes just after they leave the boiler so I can monitor them. The CH is off now so I'm looking at how it works with the HW and here's the problem:

When I have the flow temperature set fairly cool, so it should be about 50C or so (yes I know this would be too low) the flow temperature still climbs up to 62C or so before the burner shuts off and the pump continues to run. Ok, fine, but then the burner doesn't come back on for ages, like 20 minutes, at which point the temperature has dropped to 42C or so which must be the temperature at the bottom of the tank. It just keeps pumping water around cooling the tank down. If I turn the flow temperature up higher, the flow temp goes up to 70C or so and then it still shuts off for ages and the temp drops back down to mid 40s.

I can't figure out how the boiler is responding to what the flow temperature should be. I thought maybe a faulty flow control thermistor but I've replaced that and it's the same.

Has it always been like that, or is it a new problem? It sounds like the boiler control-stat has a high differential between opening on rising temperature and closing on falling. It might be possible to adjust it.
Some years ago my control-stat started to feel graunchy when turning the knob. I removed it and gave it some oil or WD-40 (or something, can't remember what). That improved the action, I think it worked better differential-wise as well.

 

[molgrips]

Ah this is interesting @oilhead 12 degrees above and belowy setting could explain what I am seeing. 62 down to 38 would suggest I've got it set to 50. With the pump running the water would never go down to 38 as the water in the cylinder is warmer than that.

Is the +/- 12 degrees figure universal or for this particular boiler? It seems as though I can't run both the hot water and central heating efficiently with this boiler. I think maybe that when I turn on the heating in a few weeks I could run the hot water and heating at the same time.

Or just give up on gas hot water. As luck would have it I have an electric car tariff with cheap electricity overnight for the next year or so, so I think I'll just put a timer on my immersion heater.

@fixitflav I am 99% sure the knob is electronic, but I'll check.

 

[JonathanM]

I think maybe that when I turn on the heating in a few weeks.

Just how mild is it where you are?

My heating has been on for a few weeks, just to keep the house at 17C.

 

[ericmark]

My heating has been on for a few weeks, just to keep the house at 17C.

Interesting, we have had one or two cooler days where the heating has cut in, but here in a Mid Wales valley the heating has not been on much yet. A bit misleading as it shows how long the boiler has been switched on for, not how long it has run for, it tends to run for 20 minutes to get the circulating water hot, but then starts to cycle. My boiler does not modulate.

I have at last found my thermometer, just need so thermocouples now, but I have a 18 kW boiler, but unlikely at this time of year that many of the TRV's are open. I hear the TRV motors run from time to time, dinning room 15ºC hall at 16ºC at radiator but 18.5ºC at wall thermostat, living room 18ºC and 19ºC and this room my bedroom at 22ºC and the problem is down to these widely different temperatures, when cold outside the heating works well, but when mild the hall is not cold enough for the main thermostat to fire up the boiler.

So I end up manually turning up the wall thermostat as one room which I want to use is cold. Programmable TRV's do nothing if the boiler is not running.

Late mothers old house worked spot on with a modulating boiler 7 - 25 kW the programmable TRV's worked very well, once winter arrived the boiler kept each room at temperature set on the TRV within a degree, only the sun on the bay windows caused problems over heating living room, and only by a couple of degrees.

My last house was open plan, and the fan assisted Myson radiator worked well, but this house built into the hill side, cheap to heat, but control is some what lacking. However having new patio doors fitted, so not worth doing anything until I can monitor how this changes the way the house heats up.

Your 9.8 kW to 12.2 kW is going to be like my oil fired system where the boiler cycles off/on rather than modulating. I find the DHW helps, as it keeps the boiler running slightly longer for each burn. But C Plan is not ideal.

 

[JonathanM]

I've had my heating on most days for the past three weeks, but only for short periods. I think on the coldest day I burnt 25KWh of gas on heating. Usually it's been between 5kWh and 15KWh. It's a big house and a very old boiler.

 

[ericmark]

I have no real way to measure how much oil I am using, but a tank full a year seems to do it OK.

 

[JonathanM]

I have no real way to measure how much oil I am using, but a tank full a year seems to do it OK.

Yes, of course, hadn't thought of that.

Is it a lot cheaper than gas at the moment?

 

[Johntheo5]

Hi all. I've been reading up on my heating system to try and optimise it. It was bought from the builders in 2007 and I don't think they did any kind of setup or tuning of the system. It's an S-plan system with an Ideal Icos HE-12, in a three storey well insulated house in a mild part of the country. I've had a look at turning the flow temperature down.

According to the manual the flow temperature can be between 30 and 70C. The boiler modulates, but not very much - the minimum is 9kW and the max 12kW, so I'd expect the boiler to cycle a bit once the set temperature has been reached. Aside from wether or not this is desirable, it doesn't actually behave this way. I've attached thermocouples to the output and return pipes just after they leave the boiler so I can monitor them. The CH is off now so I'm looking at how it works with the HW and here's the problem:

When I have the flow temperature set fairly cool, so it should be about 50C or so (yes I know this would be too low) the flow temperature still climbs up to 62C or so before the burner shuts off and the pump continues to run. Ok, fine, but then the burner doesn't come back on for ages, like 20 minutes, at which point the temperature has dropped to 42C or so which must be the temperature at the bottom of the tank. It just keeps pumping water around cooling the tank down. If I turn the flow temperature up higher, the flow temp goes up to 70C or so and then it still shuts off for ages and the temp drops back down to mid 40s.

I can't figure out how the boiler is responding to what the flow temperature should be. I thought maybe a faulty flow control thermistor but I've replaced that and it's the same.

Thats a modulating boiler,so the burner shoud definitely cut out at your setoint temperature, SP plus 5C, it should be enabled again at SP-5C but, depending on the recycle time the temperature may drop way below this before refiring. Get someone to service/check out flow sensor etc.

 

[molgrips]

I'm in South East Wales at low elevation. It was 18C in the hallway this morning, without the heating on, and near 20C upstairs where I am now. It's a well insulated house and we've got thick curtains up. Also blocked a few of the gaps and holes the builders left.

@Johntheo5 is it meant to be set point +5 or +12? 5 makes more sense. I've had it serviced but the local guy (who was recommended) didn't do a lot.

It's possible my thermocouples aren't placed correctly, I may take the front panel off and attach them to the pipes inside to be sure.

 

[Johntheo5]

It wouldn't make any sense to have it at +12C, the only boilers that have a hysteresis of 12C are oil fired boilers and probably old non modulating gas boilers, these boilers cycle on/off all the time and the burner cuts out at the setpoint
temperature, it will then refire when the temperature falls by 10/12C (hysteresis), A modulating gas boiler can't control where the SP and burner cut out are the same, hence, the burner will cut out only when the flow temperature rises to SP+5C when its output is more than the heating demand, the circ pump continues to run and when the temperature falls to the SP-5C the burner is enabled and will refire tthen when the re cycle time has elapsed.
There are a few boilers out there now where you can change the "5C" but not many.
If you can lay hands on the manual for your boiler it might state what this setting is.

 

[molgrips]

That is how I would expect it to work @Johntheo5. But the flow temps seem all over the place so either it's faulty or my thermocouples are mis-reading.

My manual is here https://www.freeboilermanuals.com/assets/pdf/ideal/203318-inst.pdf and it doesn't mention hysteresis or anything relevant that I can see - it just says that the flow temperature can be varied via the knob on the front.

 

[Johntheo5]

Are both flow/return temperatures fluctuating?, when the boiler is up to temperature just just switch it off and monitor the temperatures for 5 minutes or so as the circulating pump will stay on for a few minutes and if the readings then arn't steady its your thermocouples/monitor, are batteries OK?.

 

[molgrips]

The flow and return are about 8C apart but I can change this by restricting the flow. Once the burner shuts off then they drop gradually as you'd expect.

I checked the thermocouples when I got them in boiling water and they seemed ok.