Heating control question with an oddity

[CountryFan]

I’ve not done many heating controls and wanted to know, on a standard 2 zone heating system with a combi, existing programmer is rated as 6 and 2.5A (resistive and conductive I think) can I swap to a lower rated 3A one? Since, surely the fcu fuse will only be rated at 3A anyway.

Also, the oddity, but me (gas engineer) and the electrician think it’s coincidence: The customer was complaining that when she put her heating on it works fine, but once the temperature drops and the thermostat kicks in again, the smoke alarms go off. I’ve been and checked the wiring, which seems ok, couldn’t see any shorts and the download from the heat/CO in the kitchen (where the boiler is) doesn’t show any CO. Nothing is penetrating the wiring, eg a nail in the wiring. No gas cooker and she claims it does this every time. I’ve swapped the alarms - previously the optical alarm with 9v battery backup to multi sensor fire alarms. Anyone else had something similar? It worked fine when I was there.

 

[Harry Bloomfield]

I’ve not done many heating controls and wanted to know, on a standard 2 zone heating system with a combi, existing programmer is rated as 6 and 2.5A (resistive and conductive I think) can I swap to a lower rated 3A one? Since, surely the fcu fuse will only be rated at 3A anyway.

More likely 6amp resistive load, 2.5amp inductive! An inductive load, is higher when switched both on, and off. Such that it can weld contacts up due to the current surge, and spark. A resistive load is much gentler on contacts. Your 3amp fuse, will not care so much whether the load is inductive or resistive, because it has some inertia - it takes it some time to respond to an overload.

 

[CountryFan]

More likely 6amp resistive load, 2.5amp inductive! An inductive load, is higher when switched both on, and off. Such that it can weld contacts up due to the current surge, and spark. A resistive load is much gentler on contacts. Your 3amp fuse, will not care so much whether the load is inductive or resistive, because it has some inertia - it takes it some time to respond to an overload.

Sorry, still don’t understand what that means. I understand I’ve got the 2 the wrong way around. There is some brown thermal damage on the front of the existing programmer/timer.

 

[Harry Bloomfield]

The customer was complaining that when she put her heating on it works fine, but once the temperature drops and the thermostat kicks in again, the smoke alarms go off.

They are responding to the inductive pulse spark, across the contacts. You can fit suppressors across the contacts, which might help.

There was a certain pump, and boiler combination, where the pump was switched by the boiler. The switching was so inductive, that it would weld up the relay contacts in the boiler, needing the pcb to be replaced. The boiler manufacturer, sold a suppressor to deal with it.

 

[CountryFan]

They are responding to the inductive pulse spark, across the contacts. You can fit suppressors across the contacts, which might help.

Since there is already some thermal damage, and it makes a grinding noise apparently, can I change the programmer for a digital 2 zone one, rated as 3[1]?

I think the boiler/pump you’re referring to is a Worcester?

 

[Harry Bloomfield]

There is some brown thermal damage on the front of the existing programmer/timer.

That could be due to many things.

Since there is already some thermal damage, and it makes a grinding noise apparently, can I change the programmer for a digital 2 zone one, rated as 3[1]?

Grinding noise suggests it is a mechanical unit? A digital 2 zone ought to be designed to cope with the expected load type, so why not, especially, if the boiler is switching the pump on? The only load on the digital programmer, will only be carrying the current for the valve, and the signal current for the boiler.

 

[CountryFan]

Grinding noise suggests it is a mechanical unit?

Yes, it is.

if the boiler is switching the pump on?

Yep, it’s a combi boiler. 2 zone setup

 

[Harry Bloomfield]

Yes, it is.

Yep, it’s a combi boiler. 2 zone setup

I would put the brown discolouration down to the motor over-heating, due to age, wear and tear.

 

[CountryFan]

I would put the brown discolouration down to the motor over-heating, due to age, wear and tear.

I definitely thought overheating, as it was warm today when she had the heating on and not near another heat source. I’ve already put it down to be replaced, so it’ll either be a gas engineer or electrician. Thanks Harry.