Problem with Central Heating Three Port Valve

[DavidA]

Hi

My mother-in-law's central heating system has a problem. When HW only is selected, the hot water is heated and the central heating is off. However, when we select CH only, the CH heats up but so does the HW. By the hot water cylinder and pump is what I assume to be a Mid- Position Three Port Valve. The motor unit (Honeywell 40003916-003) was recently replaced by a plumber (to fix the opposite problem whereby CH came on when HW was selected).

I believe the plumber to be competent but he appears not to know what is wrong. Please can anyone suggest what the problem is?

BR

David

[Roger Mills]

The 3-port mid-position valve has 3 positions - HW-only (in the powered-off, rest state), CH-only (maximum travel) and HW+CH (mid position). If you are getting CH+HW when you should have CH-only, it indicates that the valve is not moving through its full travel. The most likely cause of this is that the 'wet' part of the valve (the part which wasn't changed recently) is partially seized, and requires more torque to move it than the motor in the actuator can deliver. You can check this by removing the actuator from the valve and trying to turn the spindle of the valve. It should move freely through at least 90 degrees. If it is stiff, you may be able to free it by turning it back and forth with a pair of pliers. If it cannot be freed it will need to be replaced, requiring a partial drain-down.

The other possibility is that it's wired up wrongly. Is your plumber competent at electrics as well as plumbing? I assume from the part number you quoted that he changed the complete actuator head, not just its motor? The correct wiring for the valve is shown in the Y-Plan section of

[DavidA]

How do I remove the actuator?

David

[Roger Mills]

There'll be a couple of screws somewhere which hold the actuator (motor head in Honeywell parlance) onto the valve body. You can't see then in the picture shown at

so I imagine they probably go upwards through the base-plate into the actuator case.

Hopefully others will know for sure. [I have a Danfoss valve, and the screws on that go downwards - but the Honeywell valve is not quite the same in detail even though it's functionally the same].

[DavidA]

Roger

at

so I imagine they probably go upwards through the base-plate into the

Ok, I took the actuator off (by the way, I believe the entire actuator was replaced). I would say that the spindle moves only +/-30 degrees around the midpoint i.e. 60 degrees total. I couldn't move it beyond that range. Watching the motor, it seems to be moving in the right directions as I switch between the 3 combinations of CH and HW. Therefore, would you think that the fault is pretty conclusively with the wet part of the valve?

David

[geoff]

In message , Roger Mills writes

It depends on age

The later ones have two screws and two locating pins, BUT the older ones (few and far between now) are one piece and not removable without the rest of the valve

Just hope that yours is the later version

[geoff]

In message , DavidA writes

It should normally be 90 degrees in total (i.e. +/- 45)

I would try to free it off and hope that it lasts until the summer when it is easier to drain down

You can open it up and try and free it off, but it's not a good idea in situ

[Roger Mills]

I would expect it to move more than that. How much does the actuator move when not connected to the valve? [You should be able to find out by manually putting the wet part in the mid position and then selecting all the options on the programmer/stats (HW-only, both together, CH-only)]. If the spindle of the actuator moves more than the valve will let it, there's definitely something wrong with the valve.

[Ed Sirett]

I think the head can move 90 degrees but the valve constrains the head to only moves about 60.

[Ed Sirett]

If you have one of the older ones it has FOUR screws that hole the head to the body. If you undo a couple you will begin to get wet.

[geoff]

I don't think so

otherwise the cam wouldn't progress far enough to operate the microswitches

It should be possible for the OP to check, as there will be a hard stop as the paddle or ball hits the end stop in the valve

this will be absolutely obvious to the OP