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strax
Posted: Wed Jan 11, 2006 10:11 pm Reply with quote
Joined: 06 Dec 2005 Posts: 9
I dont really know much about central heating, although I know it is a baxi back boiler, about 18 years old and still going strong Smile
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JohnT
Posted: Wed Jan 11, 2006 11:47 pm Reply with quote
Joined: 30 Dec 2005 Posts: 6
The most common cause of this problem is the boiler stat, it's quite rare to find a problem from the room stat.

The thermostat in a gas boiler switches the supply to the gas valve to control the maximum temperature of the water in the primary circuit of the system.

The solenoid (electromagnet if you like) that operates the gas valve is a fairly large coil of wire with a corresponding high inductance. When you remove the supply from the coil the magnetic field that was created by the current flow through the coil collapses and produces a high voltage across the coil and so across the thermostat contacts.

As the contacts in a thermostat age they loose their original spring tension and the contacts begin to open more slowly and eventually you reach the point where they open slowly enough for an arc to be drawn across the contacts.

It is this arc that is the cause of the interference you then experience to radio and television reception.

If you stand a portable radio next to a boiler with this problem and listen to the interference you will discover that the interference occurs when the boiler is switching off.

You will hear the interference start and it will stop at the same moment as the gas flame goes out on the boiler. In some severe cases this can be several tens of seconds after you hear the interference start and you may be able to physicaly hear the arcing from within the boiler.

There is usually no more interference when it turns on than when you switch on a light switch.

The cure is to either change the thermostat or to fit a suppresor across the gas valve solenoid.

A suppresor must be fitted across the supply to the load in order to work i.e. one leg to the live on the thermostat switched output and the other leg to neutral.

Its easier to just think of connecting it across the gas valve which will by default be the correct place.

As with anything to do with your boiler you should not attempt to do any work in it unless you are qualified to do so.

Connecting a suppresor across the thermostat will not work and is a common mistake.

Your average gas engineer will not find this problem when carrying out normal servicing work as the thermostat still works correctly in regulating the water temperature and radio interference is not somthing he understands or is required to test for.

Hope that may be of some help.
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Purpler
Posted: Thu Jan 12, 2006 1:43 am Reply with quote
Joined: 10 Jan 2006 Posts: 7
A text book and accurate posting John T


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malc
Posted: Thu Jan 12, 2006 9:52 am Reply with quote
Frequent contributor Joined: 08 May 2005 Posts: 1370 Location: chippenham
JohnT wrote:

Connecting a suppresor across the thermostat will not work and is a common mistake.


Not necessarily. The complete circuit will be something like:

timeclock -> room stat -> boiler stat - gas valve
-> pump

I'd agree that the best place to add a suppressor is across the emi source, the gas valve, but you could also add suppressors across all of the contacts in the chain. That would also work. The electrical energy in the gas valve you are trying to control would be circulated back through the mains but that's not a problem.

You need more suppressors and it is slightly inferior but it might be easier to do, perhaps,

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JohnT
Posted: Thu Jan 12, 2006 11:19 am Reply with quote
Joined: 30 Dec 2005 Posts: 6
The room stat is used to control either the pump or diverter valve or even both but will not be in series with the gas valve.

The water in the primary circuit of the heating system will usually need to be maintained at a temperature which is higher than the required air temperature in the rooms served by the radiators on the system as it will also be providing heat to supply hot water to the premises.

The purpose of the boiler stat is to allow the maximum temperature of the water in the primary circuit to be set.

If the room stat was in series with the gas valve then the water in the primary circuit would not get above room temperature and I doubt that anyone would be satisfied with the hot tap producing water at ambient air temperature!

I acknowledge that this is not the case with combi boilers but a quick check with colleagues agrees with my experience that combi boilers do not seem to be a common interference source whereas we all trace interference to conventional boilers on a regular basis and the problem is resolved 100% of the time with either a stat change or suppressor across the gas valve when the gas engineer attends.

There are many systems out there that do not use a room stat at all and use thermostatic radiator valves to allow the temperature in different rooms to be set independantly of each other.
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malc
Posted: Thu Jan 12, 2006 3:54 pm Reply with quote
Frequent contributor Joined: 08 May 2005 Posts: 1370 Location: chippenham
Most modern systems do not allow the boiler thermostat to be the sole control of the gas (or oil for that matter). In fact the latest guidelines forbid it. Basically any time the pump is on, the boiler is on, unless the boiler thermostat cuts it. Any time the pump is off the boiler is off. Most hot water tanks are pumped.

When the time clock allows and the room or tank stat say heat then the pump and boiler turn on. If the water reaches max temperature then the boiler stat will cut out, then back on again as the water cools. However, the best boilers are modulating are reduce power rather than cut out. Or you could have a large thermal inertia so that the room stat cuts out first. Which is more efficient, especially with condensing boilers.

What you definately don't want is the boiler sitting there turn on and off to keep the water in the boiler warm as it is a waste of gas and that is why they are all now interlocked with the pump demand.

You may have been confused by my diagram as it hasn't come out right. ->pump was meant to be underneath boiler stat. The time clock has a heating and hot water output. The heating goes through the room stat. The hot water through the tank stat. If either of these says heat then the pump will turn on and the boiler

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JohnT
Posted: Fri Jan 13, 2006 9:38 am Reply with quote
Joined: 30 Dec 2005 Posts: 6
We seem to be getting away from the point here as I was attempting to clarify the probable working of the system as described by the original poster and not modern practice.

I, and a colleague who is looking over my shoulder as I write this, have been investigating interference problems on a daily basis as part of our job for the last 25 years or so and there are a huge number of systems out there working in the manner I described.

In fact my colleage is also the proud owner of a Baxi back boiler which was installed in 1979 and had a supressor fitted at about 6 years old when it started to cause interference to his TV.

His system works in exactly the way I described with no room stat and thermostatic rad valves to deal with room temperature as seemes to be common practice at that time.

We have very few problems with modern systems which may be either as a result of them operating in a different way as you are describing or they have more reliable components and have as yet not started to cause problems.
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malc
Posted: Fri Jan 13, 2006 10:35 am Reply with quote
Frequent contributor Joined: 08 May 2005 Posts: 1370 Location: chippenham
I would agree we are getting away from the point. There are many central heating plans and installations that follow no plans. So getting back to point.

1. Suppressing the noise source is best done across the source - the gas valve seems the most likely to be the problem.

2. If that is not possible then an alternative is to suppress EVERY set of contacts in series with the noise source (gas valve).

Option 2 is not quite as good as 1 but will still work as long as every set of contacts is suppressed. The scatter gun approach is to supress all the central heating contacts, not as good as 2 but a nobrainer.

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ryclark
Posted: Fri Jan 13, 2006 12:21 pm Reply with quote
Frequent contributor Joined: 10 Jul 2005 Posts: 1542 Location: Shropshire
Can one place a SSR (solid state relay) between the contacts and the solenoid? Most of these relays switch very fast and on zero crossings of the AC and therefore should produce very little (if any) interference.
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malc
Posted: Fri Jan 13, 2006 5:10 pm Reply with quote
Frequent contributor Joined: 08 May 2005 Posts: 1370 Location: chippenham
Which zero crossing? The problem with inductive loads such as gas valves is that the current crosses zero at a different time to the voltage. Depending on the device this can lead to semicoductor switches using triacs not to switch off at all. A suppressor is all you need anyway, which is just an resistor and capacitor. This catches and dissipated the energy in the coil so that it doesn't reach arcing voltage.

If you had a device where the current and voltage were in phase then it would be a resistor by definition and therefore you wouldn't get any arcing and thus no interferrence.

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