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Tim Cooke
Tim Cooke,
Category: HVAC
Satisfied Customers: 1068
Experience:  Owner at Self-employed
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Find this forum whilst trying to figure out why our heating

Customer Question

Hi James. Find this forum whilst trying to figure out why our heating is being temperamental. We have a Vaillant boiler, Honeywell command module and wireless HCW80 thermostat. W also have underfloor heating separately. These modules are based downstairs and our issue is getting the upstairs rads to come on when we need them. Sometimes they do - sometimes they don't! The thermostat and command module appear to be connected when testing. Any advice?
Submitted: 7 months ago.
Category: HVAC
Expert:  Tim Cooke replied 7 months ago.

Hello, do have a conventional installation for the non-underfloor zones, i.e. a circulating pump and motorised valves?

Also, please give the model of the Vaillant boiler and the Honeywell command module.

Customer: replied 7 months ago.
Hi Tim. I'm not much of an expert. We have a vaillant ecotec plus 937 but there's no markings on the command module other than the brand
Customer: replied 7 months ago.
Yesterday the rads came on and the underfloor heating was on but they they go off without warning and we seem to have little control
Customer: replied 7 months ago.
I put the main Honeywell heating control onto constant rather than on schedule but that hasn't made an difference
Expert:  Tim Cooke replied 7 months ago.

That's ok - I'm not expecting you to be an expert! :-)

1. Could you post a photo of the Honeywell command module?
2. Do you have an airing cupboard with heating components in?
3. Could you post the make and model of your room thermostat, or a photo of it?

Customer: replied 7 months ago.
Cant see an option to attach photos
Expert:  Tim Cooke replied 7 months ago.
There should be a paper clip icon somewhere.
Customer: replied 7 months ago.
Not that i can see. We have no airing cupboard and the thermostat is a Honeywell wireless HCW80
Expert:  Tim Cooke replied 7 months ago.

Apologies - I missed the fact that you'd already given the model of the thermostat!

The first thing to check is the batteries in the thermostat. Ensure that they are fresh, just to eliminate that possibility.

Next, check that the thermostat is calling for heat when you believe it should be, and that the LED on the thermostat behaves according to the instructions.

Pay attention to the binding behaviour - again, see the instructions.

https://ensupport.getconnected.honeywellhome.com/s/article/How-to-bind-the-HC60NG-receiver-to-the-HCW80-thermostat

Customer: replied 7 months ago.
Changed batteries twice. Turn to low and command module turns off - increase and it turns back on like it should
Expert:  Tim Cooke replied 7 months ago.

Without meaning to sound picky, how are you determining that the command module (which I take to be the thermostat wireless receiver) is turning on?

Customer: replied 7 months ago.
The green light is on. Goes off when i drop the temp on the stat and back on again when i increase the temp
Expert:  Tim Cooke replied 7 months ago.

OK that sounds like it's behaving predictably and correctly.

It will be interesting to know the state of the green light at the moment that the radiators are going cold, i.e. when you're expecting them to be hot.

Turning to the programmer, do you have one somewhere? If so, what make and model is it?

Customer: replied 7 months ago.
It stays green - always. Not sure what you mean about the programmer? Do you mean the main Honeywell heating control unit?
Expert:  Tim Cooke replied 7 months ago.

The programmer allows you to define on & off periods for the heating and hot water functions - the periods during which those functions are active and subject to demands from the respective thermostats.

Customer: replied 7 months ago.
we're talking about the same thing. Two days ago I moved the function onto continuous
Expert:  Tim Cooke replied 7 months ago.

Two questions : which make & model of programmer do you have?
Did the current problem start before you switched the heating to continuous?

Also, I owe you an apology - I neglected to pay attention to the fact that your Ecotec is a combi boiler. If cylinder charging was activated by the commissioning engineer, then during those minutes when its frost protection is in effect, the boiler will stop heating the radiators. I wouldn't expect it to last long enough for you to notice and feel cold, but it's possible. The fact that the boiler is outside does mean that it could easily fire up as per its frost protection function.

I suggest that next time you notice the problem, you nip out and look at the boiler display and see what it says.

In the meantime, are you saying that an old part was fitted to the new boiler? I suspect you aren't, in which case are you saying that the old room thermostat was used? Or if neither of those, what old part was fitted and to what?

Customer: replied 7 months ago.
Not sure what you mean - I haven't said anything about old parts or the boiler being outside?
Expert:  Tim Cooke replied 7 months ago.

Er, yes, hang on - I mixed up replies on two different topics. I'll correct that...

Expert:  Tim Cooke replied 7 months ago.

Hm - it seems that I can't edit an older reply. Please ignore that one and I'll ask my question again...

Two questions :

1. Which make & model of programmer do you have?
2. Did the current problem start before you switched the heating to continuous?

Customer: replied 7 months ago.
St9400c. I'm not sure that change made a difference - the rads will come on or off at will
Expert:  Tim Cooke replied 7 months ago.

Hm. Curiouser and curiouser.

When did this (temperamental heating) problem start?
How old is the underfloor heating portion of the system? Has any work been done on that recently?

Expert:  Tim Cooke replied 7 months ago.

Just a thought - when you turn up the HCW80 thermostat, and the green light comes on, does the boiler go into its ignition sequence immediately?

Customer: replied 7 months ago.
The whole system was installed about 4 years ago when we built the house. Just checked and if i make the receiver go off then on again - the boiler doesn't kick into action
Customer: replied 7 months ago.
the green is on constantly but the boiler doesn't do anything
Expert:  Tim Cooke replied 7 months ago.

OK. That's a step in the right direction - clearly the boiler *should* ignite.

When did this temperamental problem start?

Customer: replied 7 months ago.
Not sure. It seems like it could have started yesterday but to be honest I've only just started paying attention as the weather turned
Customer: replied 7 months ago.
It was cold on Friday evening and still the following morning so that was when i turned out to constant. Everything came on but then dropped off without me making any changes. Now it won't come on again regardless of what i try
Expert:  Tim Cooke replied 7 months ago.

One possibility, which is straight out of left field, is that the radiators only heat up when the underfloor controller is calling for heat. It isn't supposed to be configured that way, but if the installer got it wrong, or did it on purpose for an as yet unknown reason, then it would explain things.

If you leave the HCW80 on maximum, with the boiler not running, and then turn up one of the underfloor heating zone thermostats (assuming that you have at least one), does that have any effect on the boiler?

Another question - do you have a "proper" underfloor heating manifold, with its own circulating pump?

Customer: replied 7 months ago.
It's possible but i don't think the two are linked. The underfloor heating manifold has its own control and thermostat and doesn't seem to link with the other.
Expert:  Tim Cooke replied 7 months ago.

Something odd about your installation is that you appear not to have a motorised valve for the radiator zone.

The two (radiator zone and underfloor zones) are not supposed to be linked, but each is supposed to be capable of calling for heat from the boiler. Since things aren't behaving how they're supposed to, I'm guiding you in looking for a fault, all the while keeping in mind that you don't know if it ever worked as it should.

Did you do the test with the HCW80 on maximum, and turning up a thermostat in a room heated by the underfloor heating?

Expert:  Tim Cooke replied 7 months ago.

I still need to understand whether or not you have a "proper" underfloor heating manifold, with its own circulating pump.

Customer: replied 7 months ago.
there is a full manifold with pump. Thanks for your help thus far but I need to close off the chat now - early start. Perhaps there's no choice but to get someone to look at the system close up
Expert:  Tim Cooke replied 7 months ago.

I'm available tomorrow, to continue.

I'm mean this in a kind way, but if you do the tests that I'm suggesting, it will lead to the solution.

When you have time, please do the test with the HCW80 on maximum, turning up a thermostat in a room heated by the underfloor heating, and see what happens.

Expert:  Tim Cooke replied 7 months ago.

By the way, if it turns out that there is a motorised valve somewhere, that you're not aware of, or perhaps don't realise what it is, then that could be faulty. These are a common source of problems, and a faulty one could entirely explain your symptoms.

Customer: replied 7 months ago.
Thanks understood. Am wondering if it's worth changing to a nest set up
Expert:  Tim Cooke replied 7 months ago.

A Nest system is good, but in your case it will add cost and complexity without solving any problems, because it replaces only the room thermostat (and its receiver) that I suspect is working.

If there's a wiring centre that someone has connected up wrongly, then that problem isn't fixed by installing Nest. If a motorised valve is faulty, then that won't be fixed either. The first rules of fault-finding are to change only one setting at a time until the problem is found, and not to add extra complexity because that just introduces more suspects.

Customer: replied 7 months ago.
Morning. I reset the boiler and controls from the spur this morning and it seems to have worked. Always frustrating for a problem to resolve without really knowing why for future reference!
Expert:  Tim Cooke replied 7 months ago.

It would have been good to get the result of the test with the underfloor heating thermostat first, but I'm glad you have heat now.

Clearly you'll be monitoring this. I have a hunch that the boiler, if it got cold and now heats up, might decline to ignite again. Temperamental problems like this can be down to a dry solder joint on the boiler PCB - with the heating and cooling cycles you'll then see the problem recur and magically disappear.

The next time it happens, I urge you to do the thermostat text *before* resetting the boiler.