Boiler sometimes doesn't heat water when it should

Hennymore

Our new house has a system boiler that pre-heats water to go into a hot water cylinder.

It seems that for some reason, the boiler sometimes doesn't actually heat water for the cylinder, even though it should. Grateful for any suggestions for what to try.

The boiler is a Baxo Megaflo on an S-plan arrangement, with two separate valves for hot water and heating.

I have myself replaced the previous Salus RT500RF wireless room thermostat with a Nest thermostat. I only connected the same wires as the previous thermostat used, so the Nest currently only controls room heating, not hot water. Or that's my assumption, anyway…

There is a programmer (Invensys "Lifestyle" LP112) that is set to "Always on" for heating and on a schedule (several hours a day) for hot water.

The cylinder has two "therm-o-disc" thermostats, both set to 60 degrees. The cylinder doesn't appear very well insulated – we can tell by touch if the water in it is hot or not.

We've learned that as a workaround, if there's no hot water even though the programmer allows it, we can toggle the heating (not the hot water) on the room thermostat to "Off" and then back to "On". If we do that, the boiler kicks into action, and I can feel the pipe under the hot water valve get hot.

I've also tried restarting the boiler (turning it off and on again), but that didn't help. Turning off the isolator to all those devices (boiler, programmer, room thermostat) did work on a previous occasion.

Any thoughts on what it could be, or on things I could try to pinpoint it?

I'm thinking it could be an issue with the programmer, or that I messed up somehow with the Nest? It seems to work fine for the room heating, though.

bris

Was it working ok before you installed the Nest? If yes you have done something wrong. Although the room stat is wired separately from the cylinder stats


If not, turn the heating off and the hot water on does the boiler fire up? If not you may have a faulty cylinder stat or actuator head on the zone valve.


The sequence is as follows on an S plan. Turn on the hot water and it sends a signal to the cylinder stat, the cylinder stat then sends a signal to the actuator on the valve, the actuators motor then turns the valve on and makes a connection to a switch which then fires the boiler.
If the boiler doesn't fire then you could try and open the valve using the manual lever, slide it along feeling the resistance as you go locking it in place, this should fire the boiler and you will feel heat either side of the valve. It's usually the actuator head that goes, or the motor inside it, if you are able you could just try changing the synchronous motor.

Hennymore

Thank you very much, bris.

We installed the Nest fairly early on, and the problem seems intermittent, so I'm not entirely sure. But as I remember it (and my partner has the same impression), it worked fine both before and after installing the Nest – we installed it almost a month ago, but starting seeing this issue maybe two weeks ago. I guess that means it's less likely that it alone is the culprit.

Yes, if I turn the heating off and the water on from the thermostat, the boiler still fires, and the pipe below the hot water valve gets hot. The boiler fires now, anyway – but the issue I've been seeing is that sometimes it does not.

There's another data point, and I'm not sure if this is a clue or if it is unrelated and just confuses the matter:

There is a towel rail next to the cylinder cupboard on the second floor. I noticed the other day that the towel rail no longer got hot. Nor the pipes outside its valves. But I do remember it being hot when we moved in. Other radiators on second floor have been working fine.

I bled the towel rail the other day and there were several seconds of gas escaping before I got water. Even then the rail didn't get warm. But today, after doing the above workaround to get the hot water flowing to the cylinder, I also noticed the towel rail being warm again. So I'm thinking that this towel rail might feed off the hot water rather than the heating. If so, maybe that is a factor somehow?

EDIT: Another theory. I just had a closer look at the programmer, and I saw that it consists of a wall-mounted backend with six electrical cables, and then the main unit has six metal prongs that connect to those cables. But the main programmer unit seemed really loosely attached to the backend and came off without me having to even unscrew it. So perhaps it's as simple as those connections glitching, and when we've worked around it by toggling heating off and on, perhaps we simply happened to correct that glitch temporarily by exerting force on the buttons… I've screwed it on tighter and I guess we'll see if that helps.

bris

The towel rail gets hot because it's a bypass, heat dump, its how the excess heats gets dispersed, when there is nowhere else for it to go.


You might want to get someone in to look at the s plan and controls.


Problem is, it can be a bit tricky finding someone that can understand S and Y plan systems. Electrician and installers tend to struggle with them.

getmore4less

But the main programmer unit seemed really loosely attached to the backend and came off without me having to even unscrew it. So perhaps it's as simple as those connections glitching, and when we've worked around it by toggling heating off and on

when you said heating on/off I thought you meant on the nest , now you clarify it was on the loose unit....

might be worth doing a full wiring diagram of the actual installation while you are on the case(check it look right and the connectors are all tight ) and keeping it somewhere handy for future reference, if something ever goes wrong or need changing.