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General Vintage Technology Discussions For general discussions about vintage radio and other vintage electronics etc. |
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1st Jan 2016, 6:30 pm | #21 |
Dekatron
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Location: Whittlesey, near Peterborough, Cambridgeshire, UK.
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Re: Room Thermostat problem
These are the two Honeywell motorised valves.
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1st Jan 2016, 7:23 pm | #22 |
Pentode
Join Date: Oct 2005
Location: Barnet, London, UK.
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Re: Room Thermostat problem
Hi I have used a R-C contact suppressor across the contacts to stop this problem. This is a 0.1uF capacitor and a 100 ohm resistor connected in series.
This will suppress the arcing and reduce RFI. You can get them from R.S and Maplin etc. I hope this helps. Keith |
1st Jan 2016, 7:28 pm | #23 |
Dekatron
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Re: Room Thermostat problem
I had just this problem with room thermostat and Honeywell V4073 mid-position valve (bearing in mind Lucien's comments)- a brief, coarse rasping on radios as the room 'stat closed, sometimes accompanied by a quick chatter of gas solenoid (very straightforward old naturally-aspirated balanced flue boiler, NO electronics, just solenoid, cut-outs and pump!)- this was traced to one of the microswitches in the motorised valve going poor contact. The actuator head is straightforwardly removed for working on, with a little more dexterity, the microswitch cover could be finger-nailed off and the contacts cleaned with fine glass-paper. Reassemble, and be contented for around 18 months..... and repeat. I got fed up in the end and replaced the 3A rated PCB microswitches with a different make 5A type. Problem gone for good.
Of course, I ought to mention the non-manufacturer parts disclaimer bit. |
1st Jan 2016, 11:50 pm | #24 | |
Rest in Peace
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Re: Room Thermostat problem
You seem to have two 2-port valves, probably normal V4043 spring return type. If this is the case, and they are wired and plumbed in the normal S-plan configuration, and one of them is the valve for the heating zone controlled by the room stat, then the stat should only be controlling the ~6W valve motor. In this case it is difficult to see why the stat contacts would be failing unless there is a mistake in the wiring. It might be necessary to start tracing and measuring.
E2A The significance of this bit now suggests a line of enquiry: Quote:
Last edited by Lucien Nunes; 2nd Jan 2016 at 12:08 am. |
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4th Jan 2016, 11:36 am | #25 |
Dekatron
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Re: Room Thermostat problem
Thanks to all who have replied. As many posts seem to think it's a wiring fault, I've got a central heating engineer coming tomorrow. Will post results.
Barry |
5th Jan 2016, 4:13 pm | #26 |
Dekatron
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Location: Whittlesey, near Peterborough, Cambridgeshire, UK.
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Re: Room Thermostat problem
Well, the engineer has checked all the wiring in the thermostat and programmer, and says everything is wired correctly. I may have posted the wrong information, however, in that the thermostat arcs when it's switching the boiler off, not on. Sorry if that makes a difference.
He has now fitted another thermostat, and reckons all will be fine. I remain sceptical, but time will tell. Thanks to all who contributed advice and help. |
6th Jan 2016, 12:04 am | #27 |
Dekatron
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Re: Room Thermostat problem
Arcing when contacts open is typical if the load is inductive. Just what the CR mentioned above is good for suppressing.
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6th Jan 2016, 12:21 am | #28 |
Rest in Peace
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Re: Room Thermostat problem
As far as we can tell, the stat is only switching the valve motor, basically a large clock motor of around 6W. I have had clock motors run successfuly on the few mA passing through a conventional 0.1uF/120R suppressor, e.g. where a contact was supposed to stop a timer when open but the motor carried on running. If you fit an RC, I'd check that the valve runs back smartly and doesn't tend to stick open, and/or choose a small value of C.
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6th Jan 2016, 1:00 am | #29 |
Dekatron
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Re: Room Thermostat problem
My previous electro-mechanical 'stat used to arc just before the contacts opened. I cleaned the contacts every couple of years. By rights, good quality bi-metallic thermostats should have a magnet acting against the closing force of the bi-metal element, thus ensuring a quick clean 'snap' action in both directions. I have a modern electronic (wireless) control system now, which brings its own problems (but not arcing contacts!)
Let's hope your new thermostat does the trick long term. As Jim says, if you have an Avometer with an AC current range it would be very enlightening to measure the current actually flowing when the contacts are closed.
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14th Jan 2016, 10:11 pm | #30 |
Dekatron
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Re: Room Thermostat problem
Suppressing a light load by fitting the RC across the contact certainly can cause malfunctions due to leakage. It's valid to put the RC across the load instead with such things. The RC will still do its job (i.e. reduce DV/DT).
The perils of overcomplicated thermostats are illustrated here (BBC website link) http://www.bbc.co.uk/news/technology-35311447
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Kevin |
15th Jan 2016, 7:47 am | #31 |
Octode
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Re: Room Thermostat problem
Room thermostats these days have very small clearance gaps when the contacts are open as they are designed to carry a few mA simply to control the boilers logic.
back in the day they had massive clearance gaps, were designed to carry more current and built more robustly, this problem never arose. I'm still willing to bet that the CH room stat is wired in a manner that it is carrying the current for the boiler and pump, (it's a no brainer that the load is very inductive because of the arcing that takes place when the contacts break) I know that the system has been verified by a heating engineer but I have seen plentiful wiring errors made by such people, in my experience the electrical side of the system mystifies some of these engineers and they often prefer to use a regular electrician to "wire up" once the plumbing is installed. We had a case recently at work where we had a brand new system installed to a property and the engineers made such a mess of the electrical system that it did not work from the start! We paid them a discounted fee, sent them on their way and my team rewired the system correctly. Just my two penneth worth! Cheers
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Lee |
15th Jan 2016, 1:00 pm | #32 |
Dekatron
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Re: Room Thermostat problem
A 240V-rated relay might solve the problem, if you have somewhere to mount it safely. Only "might" mind you: I use three octal-base relays to control my shed light remotely via pushbutton switches to switch two 12V relays that control the load-carrying 240V relay. Although the load is only 160W resistive, (two GLS lamps, one 100W and one 60W), and the contacts are rated 10A, I had to replace the 240V relay last year due to the contacts failing.
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15th Jan 2016, 9:37 pm | #33 |
Dekatron
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Re: Room Thermostat problem
My thermostat 'buzzes' softly for a few seconds when the set (temperature) point is about to change one way or the other. But in over twenty years it's never failed. I suspect that 'whatever is being switched' by the thermostat is electrically too hefty for the rating of the switch over contacts in the stat - hence they only last a year. Find out what is actually being switched by those contacts and check specs to see if they are compatible. Looks like they aren't.
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15th Jan 2016, 9:49 pm | #34 |
Dekatron
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Re: Room Thermostat problem
Personally, I'd see fitting relays to 'fix' what is obviously a mis-wiring-related overload problem to be a horrible bodge.
Get a properly-qualified [not just a '"gas safe' box-ticker-assessed tradesman] HVAC technician to investigate!! As a first step, you could stick a meter on the wiring to the thermostat and see what current it's handling. Anything more than a few milliamps means you're switching more than the expected "call-for-heat" signal. |
15th Jan 2016, 10:13 pm | #35 |
Dekatron
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Re: Room Thermostat problem
Just a quick note to say fiddling around inside gas boilers is a criminal offence, so I was told. Gas is way off topic, but if it's an electrical problem (sort of on topic) inside a gas boiler unit it still could be an offence to go poking around, property insurance etc instantly void.
Just had a gas safe registered engineer to another property to do a landlords certificate for me, that's what he said. Stay safe. Lawrence. |
15th Jan 2016, 10:25 pm | #36 |
Rest in Peace
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Re: Room Thermostat problem
I think we're going round in pumped circulation without the current measurement. The system appears to be S-plan and if so, the stat should only be switching the valve motor. If it also switched the supply to the valve microswitch that feeds the boiler and pump (there's no electronics in this boiler, we've established, so their full load should be switched by the valve), then you would get a latch-up condition when both heating and water are on. The HW valve microswitch would backfeed the CH valve motor through the CH microswitch and the heating would stick on until the water went off. Barry doesn't report any actual system misoperation so pending current measurement the indication is that the stat is failing just by switching the 6w motor in the Honeywell valve.
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15th Jan 2016, 10:34 pm | #37 |
Dekatron
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Re: Room Thermostat problem
In my previous house the original thermostat was quite large and the switch was an over centre snap switch that did switch the pump and boiler. It was an old boiler (1980 ish) with no electronic parts, just a setable timer with mechanical switches. We changed the stat to a more modern design and that arced as it switched off. I fitted a relay and that cured the problem. When we changed to boiler to one with electronic controls the stat worked well without arcing and no extra relay as it now only switched a control voltage.
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15th Jan 2016, 11:10 pm | #38 |
Dekatron
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Re: Room Thermostat problem
My room stat controls the water pump directly, its been in since 1978. Your average modern 240V room stat available for £20 or so will be rated anywhere between 8-16A and is designed for that job. If your stat of choice only has logic-rated contacts...its the wrong choice!
Gas talk is OT for the forum, but electrics wise your connections will only be under a decorative cover which is perfectly acceptable to remove as a user.
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Kevin |
16th Jan 2016, 1:59 am | #39 |
Dekatron
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Re: Room Thermostat problem
Anything inside a gas boiler, or indeed any gas appliance, IS off topic for this Forum. An external thermostat, however, could be deemed to be part of domestic wiring, as illustrated in the installation instructions left with the user. What you are comfortable with personally is just that, and best not broadcast, just in case!
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16th Jan 2016, 10:42 am | #40 |
Dekatron
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Re: Room Thermostat problem
Yes, what I should have said is an electrical device associated with a system, The rest is self explanatory, apologies for any confusion.
Lawrence. |