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Tim JA
Tim JA, HVAC Technician
Category: HVAC
Satisfied Customers: 3159
Experience:  Owner at Self-employed
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My old 400bf (non electronic) boiler starts up for no

Customer Question

Hi, my old 400bf (non electronic) boiler starts up for no reason, even when the timer is 'off'. It can turn itself on at anytime, bringing the radiators up to full temp.
JA: Are you getting any error codes or blinking lights on your boiler? How old is it?
Customer: It doesnt have a display. The PCB is very basic, just a couple of relays
JA: How long has this been going on with your boiler? What have you tried so far?
Customer: A few weeks now, it started doing it very occasionally but now, especially with the cost of gas, it is becoming a concern as it is very frequent
JA: Anything else we should know to help you best?
Customer: I don't think so
Submitted: 19 days ago.
Category: HVAC
Expert:  Tim JA replied 19 days ago.

Good afternoon, and welcome to the Technical section in Just Answer, an independent paid question and answer platform that matches customers' questions to experts in various skills. My name is ***** ***** I'll be happy to help you today.

I'm sorry to hear of your heating problem. So that I can give you the best assistance, I need to ask some questions. If you don't know the answers, I can help you to find them.

1. Is this a Worcester gas boiler?
2. Do you have a programmer?
3. Do you have a room thermostat?
4. I expect that you have a hot water cylinder, with either one 3-port MZV (motorised zone valve) or two 2-port MZVs.  Could you confirm that and tell me how many MZVs you have?

Customer: replied 19 days ago.
Hello Tim, It is a Worcester Highflow combi. No room stat. External programmer. No motorised valves. For your info, I am a qualified electrical technician and I fitted the programmer myself which has been fine for a number of years. When this problem first started the first thing I did was to remove the programmer but the boiler still fired up over night.
Customer: replied 19 days ago.
I'm quite deaf, so phone calls aren't appropriate
Customer: replied 19 days ago.
Are you still there?
Expert:  Tim JA replied 19 days ago.

I am still here.  I was having a lunch break.  I will just read your reply now. Please bear with me....

Expert:  Tim JA replied 19 days ago.

Since you have no room thermostat, and the programmer is disconnected, there should be no heating demand coming for central heating.  That indicates three possibilities:

1. There is a wiring fault resulting in a heating demand from outside the boiler.
2. The boiler is reacting to a phantom DHW demand, possibly from a faulty sensor.
3. The boiler has a fault control module.

The first can be easily checked by you, by disconnecting the switched live connection to the boiler, inside the boiler.   The other two can be progressed by a Gas Safe registered engineer.  Are you still in touch with the person who last serviced the boiler?

Customer: replied 19 days ago.
I can call them
The boiler shuts down when I turn the switch on the wall off, so it must be an internal problem
Expert:  Tim JA replied 19 days ago.
I don't know which switch that is, but that alone doesn't indicate an internal problem.
Customer: replied 19 days ago.
The boiler is supplied via a switch fused spur on the wall, turning it off isolates the boiler. With the switch off, there is no power to the boiler and it shuts down
Customer: replied 19 days ago.
There are no other external controls or supplies
Expert:  Tim JA replied 19 days ago.

OK.  But if you remove power from the boiler it simply cannot run under any circumstances.  That doesn't mean the fault is not with the switched live that originates outside the boiler.

Where is the programmer normally mounted?

Customer: replied 19 days ago.
I don't understand exactly what you mean. All the isolator switch does is supply the boiler with power 24/7 to enable it to control itself. I can't fathom how it can cause the boiler to activate randomly at any time. If you know, I'd be happy if you would tell me?
Expert:  Tim JA replied 19 days ago.

I'm not withholding anything, or being cryptic - I've listed the possibilities.  If you aren't logged on to the Just Answer web site then please do that because then you can see the exchange of posts more clearly.

If you want to eliminate one of the causes that I've listed then that's a matter for you, but from the information you've given I would not eliminate a wiring fault.  If the programmer is mounted on the boiler then that's a different matter; that's why I asked you about that.

Customer: replied 19 days ago.
Tim, I'm think we've exhausted all possibilities so I'd best contact the service engineer. Thank you for your suggestions.
Expert:  Tim JA replied 19 days ago.

You're very welcome.  I will be interested to hear the outcome.