Heating only comes on if Hot Water switched on first

[thebladder]

Hi folks,

I have the above problem with my boiler and after reading through a lot of threads on here it sounded like the issue could be a faulty mid position valve actuator.
So I bought a new one, fitted it, but the original problem persists.

To give a bit more detail, if the HW is set to off and the CH on, the boiler does not fire.
If I then turn the HW on, the boiler fires up.
If I then turn the HW off, the boiler goes off, and then comes back on again. The heating can then be switched off and on with the programmer, or by turning down and up the thermostat (in the hall)

Does anyone have any ideas what the problem could be?

cheers.

 

[Echo the husky]

Does the valve move to the mid position when CH is first selected? It's almost as if the HW off signal isn't present at the valves grey wire at the start, the valve can't move past the mid position and over to CH only and fire the boiler without power on the grey wire, but it still doesn't add up.

Carry out these tests with a 2 probe voltage tester, one probe connected to neutral for all tests. You should measure roughly on the valve wires:

Heating on at programmer, room stat turned up, hot water off at programmer:
Grey wire 240V
White wire 240V
Orange wire 240V
Valve right over to heating only

Heating on at programmer, room stat turned up, hot water on at programmer, cylinder stat calling:
Grey wire 50-150V ish
White wire 240V
Orange wire 240V
Valve at mid position

Heating on at programmer, room stat turned up, hot water on at programmer, cylinder stat satisfied:
Grey wire 240V
White wire 240V
Orange wire 240V
Valve right over to heating only

Heating off at programmer, hot water on at programmer, cylinder stat calling:
Grey wire 0V
White wire 50-150V ish
Orange wire 240V
Valve spring returned to hot water only

Depending on what gets switched off last the valve may spring return to HW only or be held fully open to CH only when everything is off.

If the voltages are different that will pin point what is at fault.

 

[thebladder]

Does the valve move to the mid position when CH is first selected? It's almost as if the HW off signal isn't present at the valves grey wire at the start, the valve can't move past the mid position and over to CH only and fire the boiler without power on the grey wire, but it still doesn't add up.

Thanks Echo, yes, if the HW is on and the CH is then turned on, the valve moves to the mid-position.
Turn the HW off and the valve moves to the CH only position.
If the CH is then turned off, the valve remains in the CH position (should it return to HW only?)

I should have said, the system was working fine up till a month or so ago, then a few mornings the heating didn't come on, then after a few weeks of that, it started not coming on most days.
The workaround for now is just having water coming on a 5 minutes before the heating and then running them both together

Anyway, I'll get the multi-meter out tomorrow and report back.

 

[Echo the husky]

Yes do test, it almost sounds like a problem with there not being a HW off signal, perhaps due to a loose connection or a dodgy programmer, or a fault inside the valve with the switching. The valve should of course fully open when only heating is being called for, not stop at the mid position. When power is applied to only the white wire (CH on) the valve will move to the mid position, when it gets there a micro switch puts a diode in series with the motor connection which causes the motor to stall and stay in the mid position. If the HW is off/not calling there should be power to the grey wire (HW off) which bypasses the diode and allows the motor to move again to fully open the valve to CH only. When it is fully open a second micro switch is operated which sends power to fire the boiler and pump (via orange wire).

When only HW is on there is no power to the valve motor (so it spring closes to HW only) and the cylinder stat sends power directly to the pump and boiler.

 

[thebladder]

Test Results: Multi-meter readings in [], odd readings in [BOLD], additional notes in (italics)

TEST 1
Heating on at programmer, room stat turned up, hot water off at programmer:
Grey wire 240V [240V]
White wire 240V [0V]
Orange wire 240V [0V]
Valve right over to heating only [@HW]

TEST 2
Heating on at programmer, room stat turned up, hot water on at programmer, cylinder stat calling:
Grey wire 50-150V ish [95V]
White wire 240V [240V]
Orange wire 240V [240V]
Valve at mid position [@MID]

TEST 3
Heating on at programmer, room stat turned up, hot water on at programmer, cylinder stat satisfied:
Grey wire 240V [97V]
White wire 240V [240V]
Orange wire 240V [240V]
Valve right over to heating only [@MID]
(for the above set of readings, I think the hot water was satisfied, the boiler had gone off before I turned on the heating)

TEST 4
Heating off at programmer, hot water on at programmer, cylinder stat calling:
Grey wire 0V [0V]
White wire 50-150V ish [23V]
Orange wire 240V [240V]
Valve spring returned to hot water only [@HW]

(Additional Test. I repeated test 1, but turned the hot water on at programmer until the boiler kicked in, then turned off the water, the boiler went off, and then came back on and the readings were: )
Heating on at programmer, room stat turned up, hot water off at programmer:
Grey wire 240V [240V]
White wire 240V [240V]
Orange wire 240V [240V]
Valve right over to heating only [@CH]

Does the above point to a faulty programmer, or is it something else?

cheers.

 

[Echo the husky]

Either your room stat are programmer or its wiring is at fault, as 240V isn't being applied to the white wire when the heating should be on. Check that power is getting to the room stat when heating is on at the programmer. If it isn't highly likely programmer is faulty. A wiring diagram and an better explanation than I can do for the Y plan is available here. It certainly looks like your programmer is the issue. What is the Make/model of the programmer?

 

[thebladder]

Thanks again echo,
It's a Drayton lp 722.
I'll check the room thermostat tomorrow and report back.

 

[Echo the husky]

Easiest way to test:
power off, remove programmer from its base, make note of what wire is in terminal marked 4, with tape or whatever.
put that wire in the terminal marked L
refit programmer to base, power on.
set programmer to both HW and CH off
if valve moves to CH only and then fires boiler/pump when room stat is turned up, and boiler/pump stop when room stat is turned down. Programmer is faulty and needs to be replaced.
Power off, put wire back into terminal 4, and replace the programmer. (with a new one if it is faulty).

 

[J_N30]

Has the programmer recently been replaced? Do you have a hot water off wire going into the timer? Was your system originally a gravity HW system? Very hard to diagnose without getting a multimeter in there.

 

[Echo the husky]

Has the programmer recently been replaced? Do you have a hot water off wire going into the timer? Was your system originally a gravity HW system? Very hard to diagnose without getting a multimeter in there.

Looking at test 3 again, If the cylinder stat really was satisfied during test 3 there is no hot water off signal coming from the cylinder stat, yet there is a hw off signal from the programmer.

(for the above set of readings, I think the hot water was satisfied, the boiler had gone off before I turned on the heating)

Though the boiler going off could just be the circulating water was at a high enough temperature that the boilers internal thermostat had switched off the boiler, check that it is satisfied by turning cylinder stat down to ensure it is off. If the results of test 3 then match what they should be, my money is on some strange fault with the programmers electronics.

It would much easier to diagnose if I was there, as it's not a standard fault, but you are nearly there now!

 

[thebladder]

Echo, I'll try switching that wire and let you know what happens.

Robo, programmer was replaced a few years back, the old one wasn't faulty, I replaced it for more timer options. As far as I know it's always been a pumped system. I'll check the programmer wiring tomorrow.

 

[thebladder]

Easiest way to test:
power off, remove programmer from its base, make note of what wire is in terminal marked 4, with tape or whatever.
put that wire in the terminal marked L
refit programmer to base, power on.
set programmer to both HW and CH off
if valve moves to CH only and then fires boiler/pump when room stat is turned up, and boiler/pump stop when room stat is turned down. Programmer is faulty and needs to be replaced.
Power off, put wire back into terminal 4, and replace the programmer. (with a new one if it is faulty).

Progress, I moved the wire in 4 to L, and yes, when I turned back on the power, and turned up the room stat, valve moved to CH only and the boiler kicked in. Turning down the room stat switched off the boiler.

Does this prove it's the programmer? (I guess the re-wiring by-passed the prog?)
Do I still need to try test 3 again?

cheers.

 

[Echo the husky]

Good work! Programmer is faulty and will need to be replaced with a new one. You can do test 3 again if you wish, this will prove that the cylinder stat is wired/working correctly.

 

[thebladder]

Good work! Programmer is faulty and will need to be replaced with a new one.

Update, installed the new programmer last night and everything is working as it should.
Thanks for all the help, much appreciated.

 

[Echo the husky]

Excellent! Certainly one of the more unusual faults.