Hot Water but No Central Heating

[keithmdw]

For reference the boiler is a condensing Potterton Promax HE (with Potterton thermostat and programmer). System is about 3-4 years old and was put in by the house builders I assume.

I also know VERY little about how central heating/boilers etc. work.

The last week or so we have been using the central heating as it has got colder - didn't really use it all summer except on the odd occasion.

We have had it back on timer to come on in the morning and again at tea time.

Any way, it's been working fine, until this morning.

Quite cool in the house this morning when I woke up, and the radiators were cold.

Checked in the tank cupboard and the little electronic box (I guess something to do with the pump) which has two lights on (one for water one for heating) had a dim light on for heating - I have never seen it dim before.

Went downstairs and checked and the thermostat (dial type) was at around 20C. Turned it up and it clicked around 25C (certainly didn't seem that warm).

Still nothing though. Boiler didn't fire and pump stayed silent. The light on the box in the cupboard did come on bright though.

Tried the hot water (by turning the slider to on - as it's usually in the off position) and that fired instantly, so not a boiler problem.

Tried several things this morning, including leaving the front door open for a while to cool it down (thermostat is at the bottom of the staris inside the front door). And by then the thermostat was clicking around 20C but still no boiler or pump.

Gave up and came to work!

Is this likely to be a problem? And if so what could it be?

We had a new programmer back in January as the old one broke (broken slider meant we couldn't turn heating on), so I doubt it's that at fault.

Is there a way to reset the entire system? Will simply turning the power off to the programmer reset it all?

We are renting, so I will call the landlord later, but it would be good to know what it could be (and whether it's a job for a plumber or an electrician so I don't waste time off work), or if indeed it is nothing.

It may work fine tonight when I get home, but what with the cold weather closing in I don't want to take any chances.

 

[ChrisR]

1) was the "little box" one of these?

2) if the heating came on when you turned the stat up, it would appear to be working, other than the calibration of the stat ?

 

[keithmdw]

Yes, the little box was exactly like that.

And no, turning the stat up made no difference at all, other than it went click when it passed the 'current' temp.

All we got was the light on the 'little box' and the light on the programmer indicating we had switched it on.

Turning the stat past the click would normally have kicked off the pump and fired the boiler at that point (or within a minute of doing so).

Does this sound like a dodgy stat or something else?

If we switch the hot water slider on the programmer on the boiler fires up and pump starts running so suspect the boiler and pump are not to blame, but I am a plumbing idiot so what do I know

How would a non-plumbing person like me test what aspect of the system is likely to be at fault?

 

[keithmdw]

OK, landlord called, and plumber booked. Not until next week though (at least it's not too cold at the moment).

Still would appreciate any advice on the above.

 

[MANDATE]

That little box is the actuator head on a 3 port valve. The valve can be in any of three positions. ie HW or HW & CH shared or CH. Within the head is a small motor and a spring. The motor turns the valve against the spring or the spring turns the valve and motor back to the start position.
Problems are usually related to either a 'stiff valve spindle' or 'knackered motor' so valve can't move away from HW position. Also there are microswitches which cause problems and even if valve moves to CH position the boiler would not function if microswitch faulty.
Do a search on 3 port valves, you'll find loads of info.

 

[keithmdw]

Thank you for the reply.

We noticed back in January when the programmer physically broke, that if we turned on CH (nothing happened because the slide switch was broken, but we could 'wiggle' the slider to make enough of a connection to get the lights on this valve to turn on), and then also turned on DHW (which did work fine) both the hot water and radiators got hot. I guess then this was because the valve was open for both CH and DHW and the boiler had been fired by the (working) DHW slider. I seem to recall this only worked until the DHW tank reached max temperature (but it was a while ago and I could be wrong on that fact).

Yesterday we tried the same. Switched the non-functioning CH on (which did nothing other than turn the light on this valve on). Switched DHW on and got hot water AND hot radiators. Seemed to work for longer than I would have expected if it was going to be tripped out by the DHW tank thermostat.

Now one thing I did notice, that might be important, was that a few minutes after we turned it all of last night, there was a groaning, creaking kind of noise from within this valve. I have NEVER heard this noise before.

Also, something which we thought nothing of. Pretty much ever since the programmer was replaced (when it wasn't broke of course), when we turned the CH on there would sometimes (not every time) be a delay of a good 30 seconds or more before we heard this valve or the pump actually do anything, and before the boiler fired.

So from what you have said, there is a switch in this valve, and if that switch was broken it would stop the boiler firing and pump starting?

If that is the case, could we get the above happening?

I guess (with my VERY limited plumbing knowledge and from what has been said) that the CH valve is opening (as if both CH and DHW are on we get hot water), or is stuck somehow open, but that the CH micro-switche in the valve could be faulty as CH alone won't fire the boiler but DHW will.

Does that make sense?

I want to call the plumber back with as much info as possible so he can try and be prepared and limit the chance of him having to go away and get parts once he has diagnosed. At least if he knows what it could be he can be prepared and save me having to take a second day off work.

 

[MANDATE]

I think you have a fair understanding of it, however the boiler/pump gets its power direct from the cylinder stat if HW or HW/CH is demanded but if the demand is CH only then the power comes through the 3 port valve.
This is only achievable if the valve is moved beyond the mid position where it can activate the relevent microswitch and of course the microswitch is not faulty. The problem seems to be fairly common. I believe in most cases the motor needs replacing, but in my case the head and valve moved to the correct positions leaving the microswitch as the culprit.
A new head (£38 ) fitted in 15 minutes cured the problem. (2 days later dismantled and repaired faulty microswitch)
It's important to make sure the valve spindle is not stiff as this makes it harder for the motor and is what ruins them.

 

[keithmdw]

Thank you.

I have relayed these musings to the plumber on the phone and he is in agreement that the valve/microswitch sounds like the problem, so has taken the model number and is getting hold of one befoe he pays me a visit so if that is the problem once he has had a look he will at least have the part in his van.

You say A new head (£38 ) fitted in 15 minutes cured the problem. (2 days later dismantled and repaired faulty microswitch) so is the head and microswitches seperate, or did you mean you repaired the faulty switch in the old unit later? I thought it was a single unit with everything inside it (valves, switches and all).

I wish I knew more about these things as I think it's always good to know.

One question (unrelated to this problem) I have wondered. The 3 port valve takes a feed from the boiler and splits it into CH and CHW. How does the 'dirty' water from the CH get heated without contaminating the DHW?

 

[2face]

the water never meets it goes round a coil in the cylinder and the heat from the coper pipe warms the water in the cylinder through heat transfer .
does this make sense ?

 

[keithmdw]

That makes perfect sense. I kind of suspected something of that nature but really had no clue

Thank you

 

[MANDATE]

The valve is in the valve body in the pipe work with just the spindle protruding. The actuator head comprises a toothed quadrant driven by the motor through about 90 degrees. There is also a strong spring, so that when the power is removed the spring pushes the quadrant back.
Because there is a middle position, it requires a printed circuit board on which the two microswitches are mounted. How the valve is held in the mid position is quite clever, it uses a reduced and modified voltage which is not enough to drive the motor forward but strong enough to hold the quadrant against the return spring.
Your other quiry! the DHW is in the cylinder fed from your loft tank, this is heated by a coil of pipes within the cylinder. This coil uses the water in the boiler and CH so the two waters never mix.

 

[keithmdw]

I know the diagnosis is pretty certain, and my plumber agrees and has ordered the replacement part, but here's something else I noticed.

Did some more checking today, and when you turn heating on and create demand (turn the stat up) you can hear the valve open, but as expected, the boiler never fires. I have never listenend to this before and took any notice of it, but it sounded quite laboured, so maybe the valve is knackered not the switch and it's just not opening enough to press the switch.

Also, when using the 'bodge' of turning hot water and heating on together (to make the boiler fire for water and in turn give us heating), the upstairs radiators get to full heat in the usual (very) quick time, but the downstairs ones (which usually take a little bit longer anyway) get only luke warm (if that) after a couple of hours, in the same order they usually get warm, and the last one in line is stone cold.

Are all these things related? I guess so as the radiator heat and speed were fine before this valve started playing up, but how would the valve cause this anomoly? Or could there be something else too?

 

[MANDATE]

I doubt if the valve itself is the problem, but it's wise to ensure there is no stiffness there.
When you can't get CH only and you rely on HW/CH shared, it presents another oddity. The HW side soon becomes satisfied unless you constantly run off hot water and the heat is contained much better with insulation on the cylinder. Now as soon as HW is satisfied it automatically moves to CH only, it is not as if you have set it and it stays put. In your case it either aint moving to CH or if it is the boiler aint lighting. So I would say it is not on the mid position long enough for you get the radiators up to full heat.
knackered motors appear to be the cause of most problems but my motor was ok. my problem was the microswitch which I proved after changing the actuator head. I have since repaired it and now have a spare head.
I was surprised at the strengh of the return spring, but there was a large ratio between the gears.
I reckon the motor does about 6 complete turns to move the quadrant from HW to CH.
Another point that is misleading, is that in a correctly operating head when you switch the system off it does not automatically return to HW as one might think. It stays put till you next use it and it moves accordingly depending on what has been selected and which aspect is or is not satified.

 

[keithmdw]

Tried the CH this morning WITHOUT DHW and the system works just fine. Just as it did before the fault!

Guess whatever is broken has started working again!!

 

[MANDATE]

That will be dirty contacts on the microswitch me thinks