# Central heating - thermostat issue.



## SpokeyDokey (11 Jan 2021)

Any heating experts please?

CH system - oil fired.

Thermostat is Honeywell DT90E - keeps firing even though actual temperature on Stat is above the required temperature setting.

It's been doing this for some time so I checked the installer settings and these were not correct so I reset the following:

On/off from 1 to 4

Cycle rate from 6 to 3

Pb from 1.5 to 2

For a week or so it has been working fine (within 0.5C of desired setting) but now it is overshooting again. Grrrr.

Replaced batteries (just in case) and no difference noted.

The unit is not very old btw.

Any advice appreciated.


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## BianchiVirgin (11 Jan 2021)

Temporarily swap it for a basic 'standard' thermostat and see that functionality works. If so, it is likely to be your Honeywell unit acting the maggot. It's a good brand but little electronic gadgets do go tits up ocassionally.


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## Venod (11 Jan 2021)

Do you mean the boiler keeps firing? I know nothing about your thermostat, I don't know what the figures mean, but our system has two diverter valves one for CH one for hot water, when the operate they shut of the water flow but if the micro switch sticks they do not shut of instruction to stop the boiler which will keep firing then shutting down.


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## BianchiVirgin (11 Jan 2021)

Venod is right too and worth checking. The micro switches on motorised valves can stick and keep things running when they shouldn't. Most motorised valve heads are easy to swap. I'd fogotten about that.


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## iwantanewbike (11 Jan 2021)

Either sticky motorised valves (the motors lose their strength but can be replaced for about £25 at Screwfix), or your thermostat is on the same default channel as another one within range, which can be 100m or so. Changing the channel is as simple as flicking some dip switches on the stat and receiver.


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## SpokeyDokey (11 Jan 2021)

Would these units be near the boiler?


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## BianchiVirgin (11 Jan 2021)

Look for something like this:


View: https://www.amazon.co.uk/Honeywell-V4043H-Valve-Normally-Closed/dp/B001ANU6UA/ref=asc_df_B001ANU6UA/?tag=googshopuk-21&linkCode=df0&hvadid=231920353356&hvpos=&hvnetw=g&hvrand=12990805926270884745&hvpone=&hvptwo=&hvqmt=&hvdev=t&hvdvcmdl=&hvlocint=&hvlocphy=1007272&hvtargid=pla-421896607850&psc=1


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## iwantanewbike (11 Jan 2021)

This would be cheaper and more likely to be the specific culprit but be careful as you'd be messing with 240v - hire a professional if you're not sure:
https://www.screwfix.com/p/drayton-synchronous-motor/28670

I'd spend some time diagnosing it first before buying any parts. There should be a manual slider on the unit - operate that back and forth a few times and you should hear the on/off click of the microswitch. Then check you can hear the same when someone else operates the thermostat. Finally an impedance measurement of the motor can usually tell whether it's lost its ability. Still worth checking the channel though, they're usually installed on a default channel and is not unheard of for neighbors stat to trigger your boiler. But that should be obvious if it's happening as there'll likely be a green light on the transceiver despite your stat being below temperature.


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## SpokeyDokey (11 Jan 2021)

iwantanewbike said:


> This would be cheaper and more likely to be the specific culprit but be careful as you'd be messing with 240v - hire a professional if you're not sure:
> https://www.screwfix.com/p/drayton-synchronous-motor/28670
> 
> I'd spend some time diagnosing it first before buying any parts. There should be a manual slider on the unit - operate that back and forth a few times and you should hear the on/off click of the microswitch. Then check you can hear the same when someone else operates the thermostat. Finally an impedance measurement of the motor can usually tell whether it's lost its ability. Still worth checking the channel though, they're usually installed on a default channel and is not unheard of for neighbors stat to trigger your boiler. But that should be obvious if it's happening as there'll likely be a green light on the transceiver despite your stat being below temperature.



I don't have a wireless 'stat.

Thanks for the slider advice.


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## SpokeyDokey (11 Jan 2021)

Thanks all - I'll hunt the thing down tomorrow morning.


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## BianchiVirgin (11 Jan 2021)

iwantanewbike said:


> This would be cheaper and more likely to be the specific culprit but be careful as you'd be messing with 240v - hire a professional if you're not sure:
> https://www.screwfix.com/p/drayton-synchronous-motor/28670
> 
> I'd spend some time diagnosing it first before buying any parts. There should be a manual slider on the unit - operate that back and forth a few times and you should hear the on/off click of the microswitch. Then check you can hear the same when someone else operates the thermostat. Finally an impedance measurement of the motor can usually tell whether it's lost its ability. Still worth checking the channel though, they're usually installed on a default channel and is not unheard of for neighbors stat to trigger your boiler. But that should be obvious if it's happening as there'll likely be a green light on the transceiver despite your stat being below temperature.


I wasn’t suggesting buying parts. The picture was only to identify a motorised valve.


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## bruce1530 (11 Jan 2021)

Other thing worth considering. It’s been very cold recently. It’s an oil-fired boiler, so i’m presuming it may be in an outhouse or garage?

It may have a frost stat, to stop your pipes freezing. Look for a thing that looks like a thermostat mounted on the wall beside the boiler, and another sensor on the return water flow beside the boiler.

Typically that will give the CH circuit a kick when the ambient temp at the boiler drops below about 5 degrees, and it’ll stay on until the return flow is above 30.

As someone said above, my money would be on the motorised valve. There may be 2 of them - one for the CH and one HW. They’ll be near your water tank probably. You should be able to feel the little lever on the CH valve move when the thermostat tells the system to fire.

In general, the thermostat & timer will open the motorised valves. They’re not connected directly to the boiler.
The boiler is switched on by either of the valves opening - there’s a little switch inside each one that closes when the valve opens, and tells the boiler to fire.
The boiler controls the pump - this comes on when the boiler fires, and may continue for a bit after the boiler stops.


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## Venod (12 Jan 2021)

If it is the synchronous motor thats faulty don't be tempted by the cheaper ones available on ebay, the one recommended above from Screwfix is the best bet. The micro switch is available on ebay for £2.49 but be prepared to desolder the old one and solder the new one.


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## SpokeyDokey (12 Jan 2021)

bruce1530 said:


> Other thing worth considering. It’s been very cold recently. It’s an oil-fired boiler, so i’m presuming it may be in an outhouse or garage?
> 
> It may have a frost stat, to stop your pipes freezing. Look for a thing that looks like a thermostat mounted on the wall beside the boiler, and another sensor on the return water flow beside the boiler.
> 
> ...



Thanks very much.

I'll check that out. The boiler is a great big semi-industrial Italian job down in the garage.


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## SpokeyDokey (12 Jan 2021)

Can't find a motorised valve anywhere!

Checked garage, attic, airing cupboard and under the house (bungalow with illuminated access to all areas underneath living area).


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