DIY Doctor

Main navigation

Honeywell 3 Port V4073A Motorised Valve and Issue With no Heating

Postby S1mon » Mon Jul 09, 2018 1:45 pm

Hi Everyone, I have an issue with my central heating system where I can turn on the water and the boiler hearts the water, if I turn on the heating the Honeywell 3 port V4073A motorised valve does not move the valve to heating even if the thermostat is set to max. This sounds like a simple job of replacing the synchronous motor. I though it may be the motor as when system is turned to heating I can move the lever over to the right and hey presto when I hear the relay click the heating comes on and the radiators get hot.

Now this is the unusual bit. Normally I read when people have this issue they move the valve lever over to the right and over the hook to hold the lever in the open position so they can get heating. For my situation, I don’t have to do this I move the lever to the right and when it clicks the micro switch the lever stays there on its own (No Need to hook it).
Also noted, if the heating is switched off the lever will stay where it is, if then the water is turned on the lever will come back on its own and water starts to heat. It appears there may be a voltage present to hold the switch in position but not move the motor. The valve seems to move ok (freely enough) as the spring obviously brings if back to enable the water to heat. If I turn all power off and the valve is set to heating the lever will fall back to the left, again making me think there is a voltage holding the switch.

I have replaced the motor with a new one and it made no difference.
The system has worked without issue for many years, and suddenly I had an intermittent period were occasionally the heating would not come on and over the last winter I have had to manually move the switch to bring the heating on, although recently while trying it before changing the asynchronous motor, one time the heating turned on correctly.

All wires in the wiring box and also the timer appear to be tightly screwed in and are never touched usually. The stat is an old square box Honeywell stat , I cleaned it out and checked it makes a good connection when on.

I am now thinking of changing the Honeywell powered V4073A as I see there is a small circuit board in the box under the lid. I can replace the head only as its the two screw version to the valve. Do you guys know why the valve switch can stay over to the right when manually moved and not being hooked, although if moved and stat is off (Temp already high) the switch will automatically come back on its own.
Many thanks for looking.

Rank: Labourer
Progress to next rank:
Posts: 2
Joined: Mon Jul 09, 2018 8:14 am


Simply Build It

Postby S1mon » Tue Jul 10, 2018 7:36 pm

Hi. Thanks to all that looked.

I decided to have a look around internet for how the head actually works, voltages etc.
I found the necessary info. The reason my synchronous motor was holding its position inside the valve is due to a diode and couple of resistors supplying some ac voltage, just enough to hold it in position but not enough to move the motor, this is what should happen so that part was working correctly. The reason when selecting heating the motor wasn't automatically running and moving the valve (remember I had to move the switch on the actuator by hand to the on position) is one of the two micro switches was not working on the internal pcb of the Honeywell actuator head.

I took the head off the valve and undid the screw that holds the small PCB inside the head. Pulled the pcb out leaving the wires connected and checked the switches with an Avo meter to prove I had a faulty switch. I then de-soldered the faulty Micro switch and carefully too it apart.

The reason the Micro switch was not working is the contacts were a little black. I cleaned the contacts and put the switch back together, tested it to check it was working. Soldered it back in the pcb, put the pcb back into the Honeywell head unit. Screwed it all back together and tested the heating. Hey presto it’s all working as it should.
I will get a new head in time, but at the moment really happy it all works as it should. And also proved it was the head unit as well.

I have added a couple of pictures for your amusement. Electronics is a passion of mine that’s why I went this far to repair the Honeywell head. You guys may want to do a similar exercise, just remember to turn off any power to the parts you may be touching. You don’t want an electric shock.

One final thing. You can actually buy all the parts that go on the internal PCB of the Honeywell Head from Farnell etc. But If the head is diagnosed as faulty, its a lot quicker and easier to replace the head.

Best regards
Switch back together
switch taken apart
Pcb-after-switch desoldered.jpg
Pcb after switch desoldered
Head with pcb taken out
Rank: Labourer
Progress to next rank:
Posts: 2
Joined: Mon Jul 09, 2018 8:14 am

Postby ericmark » Tue Jul 31, 2018 6:07 pm

I had same with daughters central heating, seems if you latch the manual lever which is really there to allow bleeding then it seems that the system is working, however put your hand on radiator and you will find they are warm.

Some one had cheated by latching lever before my daughter bought the house, and my son-in-law dislodged the lever when cleaning out airing cupboard, took me ages to find fault, as in fact three micro switches, by plan says one, so my tests were giving odd results.
Rank: Project Manager
Posts: 2656
Joined: Fri Oct 02, 2009 8:49 pm
Location: Llanfair Caereinion, Mid Wales.

Display posts from previous
Sort by
Order by

  • DIY How to Project Guides
  • DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!

  • Related Topics