I have a strange issue with my central heating. Moved into a new house
3 years ago and heating worked fine for a year, then last winter we had an issue where it wouldn't turn off.
We have a standard Gravity Fed System Setup Potterton Profile (in garage) Electronic Timer (in kitchen) Thermostat (in Hall) Zone Valve which switches between water and Heating (in airing cupboard by water tank).
This is an intermittant problem and sometimes it doesn't happen for weeks then happens everyday for 4 or 5 days its very hit annd miss and unpredictable.
So when the heating reaches the correct temperature or you turn the thermostat down the heating stays on, even if you turn the timer off (for the Heating and Water). It just keeps running and the house gets hotter and hotter. We have to resort to flicking the main swich off so everything is shutdown which is annoying.
I've had several things done to try and fix the problem but none has worked:
replaced the Frost Stat in the garage (also bought a thermometer that shows high and low temperature to see if it goes below the 5 degrees, and it doesn't).
Plumber suggested changeing the Zone value, so we did and this made no difference.
I've had a plumber out and they don't know.
I could obviously replace the Timer and thermostat but then i'd have change every part of the system, except the boiler.
Has anyone seen this issue before and have any suggestions to fix, other than just replacing the boiler?
Got a multimeter? Happy with electrics? check to see if the stat actually does stop calling for heat when it clicks - you can have the contacts welded together but the bimetallic spring still clicking.
Check the frost stat calling as well while you're in there. Beware that they use 240V for the switching, not nice safe logic levels :-(
Am I right in thinking that the system does go off when the timer tells it to, it's just the temperature based switch off that fails?
TBH it sounds like the stat to me, well before I'd have suspected the zone valve.
I had something similar with a different type of heater. I was told by the supplier (but never got around to it so dunno if it would have worked) to surpress(?) the mains 'lead' near where it connects within the heater by wrapping it around a ferrite ring.
1) when you turn down the thermostat
2) and when the Timer/Controller is off (both Water and Heating off)
Thats what seems odd, I'm certainly no plumber but can't see the logic of the boiler staying on and heating the radiators when the thermostat and timer/controller is off.
The only way to turn is off the main on/off switch which shuts everything down.
Are you sure that there's only *one* zone valve - or are there two - one for the HW and one for the CH?[1]
If there are two, you have what is called an S-Plan system - in which the timer and stats drive the valves, and some auxilliary contacts in the valve actuators - which close when the valve opens - drive the boiler and pump. If a zone valve sticks open, these contacts continue to drive the boiler regardless of what the timer and stats are doing. This would seem to fit your symptoms.
[1] If there is only one valve, it will have 3 ports - one input and two outputs. If there are two valves, each will have just two ports - an input and an output, in line with each other.
I take it that the plumber has tried removing one wire from the hall thermostat? If not, try it. Leave it off until you are sure that the boiler is not being told to fire because the 'stat is faulty. If you leave it disconnected, it will not call for heating at all. Once you have tried this, you'll know that the 'stat is not the problem. One down, not many to go now.
Disconnect one wire from the timer / controller, preferably the wire that feeds to the boiler live mains input. Leave it disconnected to make sure it is not the controller that has gone faulty and is still calling for heat, even when you think it is off. Two down, less bits to go.
Make sure that the controller is actually connected properly. The hall thermostat and the zone valve should be connected in a loop, so that when the 'stat opens, the valve has to close to tell the boiler to stop firing up. This is also true with any water storage heating appliance with a thermostat on it. Make sure they are connected in a loop wiring scheme with the zone valve, so the zone is the only thing telling the boiler what to do, and when to do it.
The wiring system should be connected in the following way.
Timer / Controller with live feed and neutral to make it work.
Switching in the controller for the heating system should be connected through the hall thermostat, then on to the central heating connection of the zone valve. From the valve it should be connected to the boiler to call for heating.
Hot water switching in the controller should be connected through the thermostat on the hot water cylinder, then on to the hot water side of the zone valve. The valve switching is then connected to the boiler to call for hot water.
This simple connection scheme makes it easier to diagnose faults with the whole system. If the zone valve occurs a fault on the central heating switching, the heating won't come on. Respectively, if the valve has a fault on the hot water switching, the hot water don't work. If the hall 'stat has fault, the central heating will either stay on continually, because the thermostat has stuck closed, or it won't come on at all because the 'stat has stuck open. Again, this same fault occurrence on the hot water 'stat shows the problem in the same way.
It sounds like you have a stuck thermostat or that two 'stats are fighting with each other for the call to fire the boiler. When you say you have to switch the whole lot off, it occurs to me that an automatic device on the boiler is sticking in the closed (call) position until the power off cycle resets it. So it is something that needs power on, to hold it closed (calling for heat).
In this case, you have something that needs powered down to make it reset, which could be the zone valve because it is wrongly connected into the system. Or the controller switching is sticking closed (calling for heat) because it is connected incorrectly or is being forced into fault condition by some other wrongly connection scheme. It is simple to follow the connections from either side of the controller. Look at where the wiring goes from the controller to each of the devices, writing down how each device is connected into the whole system. Check it against the wiring scheme I have given above to make sure.
Remember that the electrician (whatever) will have used both, or maybe all the cores in the cables to make his loop wiring. So following a brown wire from the controller to the hall thermostat, will return from the hall 'stat as a blue wire (assuming those colours have been used). But you get the idea?
I tried to answer the question last night but eventually gave up through uncertainty (which has not gone away).
I also have a Profile system but mine has several 2 port valves rather than a 3 port and when it started behaving in a like manner earlier this year it certainly was a zone valve microswitch that was sticking. But Stuart says the valve has been replaced so if that includes the actuator head then on the face of it a replacement valve being defective is extremely unlikely. (But I don't know enough about 3 port valves to be sure how faults manifest themselves).
Big could be right and the system really has two 2 port valves rather than a 3 port but with a plumber in the loop is that likely to have been overlooked?
I don't agree with Big that the thermostat is to blame, at least not unless there is a double fault. If the setup is as described it is a simple thermostat controlled by the timer so they would both have to be faulty for the call for heat to get through when the control temperature is exceeded.
For the moment I, like Stuart, remain completely baffled.
Thanks everyobne for the info, I will certainly try some of them.
In answer to some of the comments:
I'm 99% sure we only have 1 zone valve. The plumber has already replaced this so i'm fairly confident that it should be working OK, although there is a chance it has been connecte dup incorrectly.
Great suggestions I will try some. I am concerned that the plumber didn' suggest any of these, although the issue was that you could never get it into this faulty mode when the plumber was there, as i've said it very intermitant (although it has been faulty for the last week). The power off, doesn't actually rectify the problem. It just enables us to trun the heating off. For the last week the heating has been faulty so we have been switching of the main power at night (so its not running all night) and then turn it back on in the morning. The wife has to remeber to switch the main switch off before going to work otherwise there is a chance it will stay on all day.
This is half the battle, if you could initiate the fault at will it would be easier for someone to fault find.
I think I may need to get someone else in who is interested in resolving the issue, whereas the plumber I used seems to shrug his shoulders without giving any ideas etc...
Roger, The Programmer is a HorstMann (Channel Plus). Yes there is a thermostat on the water tank, and oddly I checked the temperature of the water coming out of the taps as it seemed excessively hot (as the heating had been on for a while). The water thermostat is set to about 54 but the temp of the water was about 60 degrees so it appears it is not working.
I agree that if the thermostat was faulty then the Controller would also need to be faulty as i'd expect both thermostats to be deactivated if the controller was off but that is not whats happening.
If the system is wired from the controller to the thermostat and zone valve in a parallel configuration, the hot water cylinder 'stat will keep calling for the boiler to fire. This is wrong, and could be causing the problem you are experiencing.
The wiring needs to be from the controller, through the hot water cylinder 'stat, through the zone valve and on to the boiler. That way, the boiler only fires when both the hot water thermostat and zone valve have activated correctly. If either zone valve or hot water 'stat show fault condition, the boiler is not signalled to fire.
The same wiring configuration should be used for the central heating side of the system. From controller to the hall thermostat, through the zone valve and on to the boiler. The same effect is shown by not calling for heat at the boiler, if either the hall 'stat or the central heating side of the zone valve show fault condition.
It still sounds like a wiring fault, or that one of the switching points on the thermostats of zone valve. You now seem to have found that the hot water thermostat could be continually calling for hot water from the boiler. But, are the zone valve and hot water 'stat being activated directly by the controller, together or individually?
Not familiar with that. I presume it is a 2 channel programmer with separate settings for CH and DHW.
As Big suggests it is not outwith the bounds of possibility that the set-up has been wired up wrongly from inception.
If the tank thermostat had been wired up directly (say in parallel with the frost stat) then you might not have noticed that the timer was having no effect. It would be nowhere near as noticeable as central heating without a timer.
If you alter the temperature setting on the stat you should hear a click as it passes the set point even if there is no volts on the contacts. If you can't hear a click it is very likely that the tank stat is faulty.
How happy are you with meddling with mains wiring. If you disconnect the tank thermostat and the problem goes away then you are on the right track and circumstances should have proved that the timer is either defective or not controlling the DHW.
Opening up the programmer and looking at the wiring should at least show whether or not the domestic hot water channel has been wired up. That might be the more sensible first step (after isolating the mains). That way you might get an instant answer to the question of whether the wiring is wrong.
Before the wiring can be checked sensibly, the OP needs to know what it
*should* be like - which in turn depends on the system configuration.
The OP *thinks* that there is only one zone valve - but hasn't yet confirmed that there *is* only one, and that it has 3 ports.
[The sypmtoms sound to me much more like one of two 2-port valves stuck open, rather than a 3-port valve problem].
Assuming that it *is* a single 3-port valve, it might be a diverter valve (W-Plan) or a mid-position valve (Y-Plan) - and the wiring is substantially different between these two options - and different again for an S-Plan system with two 2-port valves.
May I suggest that the OP has a look at the Honeywell documents at
formatting link
[1] in order to work out what he's actually got, and then reports back here.
[1] Bollocks! Honeywell have changed their site, and the document I wanted to reference ain't there any more. However, the reference I've given appears to offer information about S, W and Y plans - once you've registered
I think you'll find further up the thread, that it has been confirmed that it "is" a 3 port valve on the system. Which has already been replaced by the plumber.
The message from "Roger Mills" contains these words:
That depends on the fault.
If the DHW channel on the programmer is not connected then a faulty tank stat seems almost a certainty given what we have already been told. If it is connected then it still could have been connected wrongly but the investigation gets rather more difficult.
I don't think it's quite as explicit as that. I've just re-read the whole thread - have you?
In the very first post, he only mentions one zone valve "which switches between HW and CH". In a later post, he is "99% sure" there's only one valve. At no time does he say how many ports it's got.
His description certainly *suggests* that it's a 3-port valve - quite likely a diverter valve (W-Plan) rather than a mid-position valve (Y-Plan). However there are other possibilities:
There could be another valve which he hasn't found, or
He *could* have a C-Plan system with gravity HW and pumped CH, with a single 2-port valve which turns off the gravity circulation when the HW is hot enough.
The problem is that none of these have any obvious failure modes which continue to run the boiler when everything is turned off at the programmer - which is why it's very important that we get to the bottom of what he's actually got!
The message from "Roger Mills" contains these words:
I was a bit bothered by the use of 'gravity' in the original post but translated that into open vent as my Profile installation instructions say it is must only be used on "INDIRECT FULLY PUMPED systems". However my Profile is described as a 'Profile Prima' and a vague memory from the past suggests that there was a very similar boiler available which had 2 inlets and 2 outlets which presumably must have been intended for part gravity installations. Unfortunately the advertising leaflet I have also retained doesn't go into such detail so I don't know whether the other boiler was also a Profile. What I do recall is that I originally set out to buy the alternative back in Autumn 1991 but was persuaded to switch by the retailer.
However even if the DHW is gravity circulation a 2 channel programmer would need the 2 channels wired up separately with the programmer set so the CH can't be used on its own.
Incidentally the installation instructions states:
"This appliance must be installed and serviced by a competent person as stated in the Gas Safety (Installation and Use) Regulations 1984."
Funny how memory plays tricks. I would have sworn the regulations only came in force shortly before I installed my boiler in 1991. 7 years is hardly a short period. (Lifetime of a cheap modern boiler?)
In most cases, yes. *However* the C-Plan system is an exception to this because it has a 2-port zone valve which cuts off the gravity circulation so that you *can* have CH on its own.
[There used to be a diagram on the Honeywell site, showing how it works - it's quite cunning! - but unfortunately, Honeywell have re-vamped their site and the best I can find is
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