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Ask Tim Cooke Your Own Question
Tim Cooke
Tim Cooke,
Category: HVAC
Satisfied Customers: 1167
Experience:  Owner at Self-employed
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I have an open vented gas central heating/domestic hot water

Customer Question

I have an open vented gas central heating/domestic hot water system. Why does the boiler fail to start when CH and DHW are both selected? The boiler works normally when one or other is selected. Your advice would be greatly appreciated,
JA: What's the brand and model of your central heating? How old is it?
Customer: The boiler brand is Potterton and is around 15 years old. The system is controlled by a Honeywell ST400C programmer and a Honeywell mid-point diverter valve\actuator.
JA: How long has this been going on with your central heating? What have you tried so far?
Customer: The problem's been on-going for several months. The complete diverter valve\actuator assembly was replaced last summer. I'm not sure if the system worked correctly after the new valve was installed.
JA: Anything else we should know to help you best?
Customer: No, except that both the central heating and hot water work fine when selected individually on the controller/programmer.
Submitted: 6 months ago.
Category: HVAC
Expert:  Tim Cooke replied 6 months ago.

Hello, I can help you with this.

Normally, I would say that the MZV actuator is faulty, but that's less likely since yours is fairly new. Did the problem start as soon as you had the new actuator, or before?

Customer: replied 6 months ago.
Unfortunately I can't be sure. The mid-position distributor valve was replaced during the late summer when the CH/DHW system had not been in use for several months. The valve was replaced because the hot water system had stopped working.
Customer: replied 6 months ago.
I'm not at the premises at the moment so couldn't carry out your instructions. Can we continue with the on-line chat?
Customer: replied 6 months ago.
Hello, Tim. Are you still on line?
Expert:  Tim Cooke replied 6 months ago.

I'm online, but this isn't quite like an interactive chat, so there will be natural gaps in our exchanges. But rest assured I will see your replies and will respond.

Customer: replied 6 months ago.
OK. I'll stay logged in and look out for your responses. Thanks
Expert:  Tim Cooke replied 6 months ago.

Assuming for now that the new actuator has no fault, then the fault is likely to be in the wiring to it, or of the system. It might have predated the change in valve/actuator, but of course the installer should have fully tested it. I guess you're finding out much later that there's a fault, and can't be sure that they tested it?

Customer: replied 6 months ago.
Thanks, Tim. Do you mean that the new valve\actuator may not be compatible with old Honeywell ST6400C controller/programmer?
Expert:  Tim Cooke replied 6 months ago.

I don't mean that it might be incompatible, no. There are no compatibility issues of that nature.

When the actuator is replaced it involves re-making connections at the wiring centre. If the previous actuator was a different make, then it increases the chances of making a mistake in those connections. Or, a mistake could have been made long ago, and the wiring could always have been wrong, and could still be wrong. Or the new actuator could have been faulty out of the box. Or could have become faulty.

Before we go too far in any direction, please could you tell me the exact model of the Honeywell actuator? There should be a label on the casing.

Customer: replied 6 months ago.
It's a Honeywell V4073A mid-position 3-port diverter valve with 22 mm compression fittings to connect to the plumbing pipework. Is this enough information?
Customer: replied 6 months ago.
Perhaps I should have added that the new valve/actuator is identical to the one that it replaced.
Expert:  Tim Cooke replied 6 months ago.
Ok, that’s useful, thank you.Do you have a wiring centre in the airing cupboard?
Customer: replied 6 months ago.
Tim, apologies for the long delay in replying. Yes, there is a wiring centre in the airing cupboard.
Expert:  Tim Cooke replied 6 months ago.
That’s no problem - take as long as you need.Have you ever used a multimeter?
Customer: replied 6 months ago.
Expert:  Tim Cooke replied 6 months ago.

OK. If you have the multimeter with you and have time today, I can post here a list of the initial tests you need to do. The first steps will be to turn off the power, remove the wiring centre cover, take a photo (or photos) of all the wiring connections, and then turn on the power and get ready to use the multimeter. You will be testing live connections, so you need to be confident in doing that safely, and sure that there's nothing live that you can accidentally touch, like stray conductor strands.

I presume that you're a contractor and you have to visit the property?

Customer: replied 6 months ago.
Tim, thanks. I shan't get to the premises until the end of the day so I'd appreciate your posting the list of tests.. I'll get back to you with the results if not tomorrow then on Thurs. Thanks again.
Expert:  Tim Cooke replied 6 months ago.

OK. Bear with me - I'll post them at lunchtime so you should have them in plenty of time.

Depending on the outcome of the tests, we might need some more interaction here, but I suggest taking it one stage at a time.

Customer: replied 6 months ago.
Sounds good. I'll look out for the test list. Thanks again.
Expert:  Tim Cooke replied 6 months ago.

First of all, some sanity checks.
Locate the orange, grey, and white wires from the MZV.

1. Power up with no demand on DHW and CH.
Verify that there is 240V on grey, and 0V on the other two.

2. Create a DHW demand.
Verify that there is 240V on the orange, and 0V at the other two.

3. Remove the DHW and create a CH demand.
Verify that there is 240V at all three of the above.

4. Create a DHW demand, and leave the CH demand on.
Verify that there is 240V on the orange and white, with 0V on grey.


If any of the above reveals a problem, let me know, and meanwhile identify the cable from the cylinder thermostat - note the terminal numbers and let me know what those numbers are.

I'm expecting that tests 1-3 will pass, and test 4 will fail. If this is the case, then you'll need to look for a problem with the connections to the cylinder thermostat.

Customer: replied 6 months ago.
Thanks again, Tim. I'll update you before the end of the week.
Customer: replied 6 months ago.
Good morning, Tim. Results of the tests are:
Tests 1, 2 and 3 all checked out OK as expected
Test 4 gave a steady 240v on grey only. The voltage on orange and white fluctuated erratically, and there were audible clicks from the controller/programmer as it appeared to switch itself on and off at random.
The cable from the cylinder thermostat contains 4 wires: blue, brown, black and yellow/green (earth). Blue goes to terminal 9, brown to terminal 6, black to 5, yellow/green to 1.
Your further advice would be greatly appreciated.
Expert:  Tim Cooke replied 6 months ago.
Your programmer seems to have a fault. That will certainly cause a problem.I suggest you isolate the power to the system and remake the live and neutral at the programmer terminals. Also the other end of that cable at the terminals in the wiring centre.
Customer: replied 6 months ago.
Thanks again, Tim. I'll let you know how it goes.
Expert:  Tim Cooke replied 6 months ago.

I meant to ask you to attach a photo of the inside of the wiring centre. That might not be needed, but just in case it is, could you arrange one?