No Heating at the Weekend (Evohome)

I passed that suggestion back to the Honeywell team some time ago. If Evohome calls for heat but the temps don't rise then it should cause an alert saying there's something wrong with the heating system.

P.

If you had to top up the pressure to get the system working again then yes, it is very likely that turning the heating off for an extended period of time triggered a boiler lockout due to the central heating loop pressure dropping below one bar. Most pressurised boilers lockout if the pressure drops to about 0.8 - 0.9 bars.

System pressure increases a lot when the system is hot due to the water expanding - typically the system is set to just over 1 bar when cold and the pressure may rise to around 2 bars when hot with all radiators open.

It sounds like your system pressure was a little low, however if the system remains on or does not go off for a long period of time for it to fully cool the pressure will remain high enough to prevent a lockout - but leave it off for a long time and the entire system will eventually cool and trigger the lockout. There's only really two possible causes for unexpected pressure loss - either you have a leak somewhere or the expansion vessel needs re-gassing.

If it's a leak it could be something like a tiny drip from somewhere like a TRV elbow joint or it could even be under the floor out of sight depending on the types of joints used there. It could be small enough that it wouldn't really be noticed, (as it would drip then evaporate later) or it may only leak when certain radiators are on or off as pressure changes.

It could also be the pressure relief/drain valve (usually found in the boiler casing) that has a slight leak from sticking slightly open - they are sometimes known to do this. In a flat we lived in the pressure relief valve had a slight leak - the valve is normally designed to vent pressure out through the overflow pipe to the outside if the system pressure exceeds 3 bars, and also doubles as a drain valve to drain down the system.

This particular valve had a slight leak that required us to top up the pressure every couple of weeks. After opening and closing the valve a few times to exercise it slightly and then re-topping up the pressure it stopped leaking.

The other main possibility is the expansion vessel needs re-gassing. It's just a big container with a rubber bladder filled with compressed air, sometimes located in the back of a combi-boiler or sometimes external to the boiler. It's connected to the central heating circuit to give the water somewhere to expand to when it heats up.

When the gas pressure gets too low the pressure rise in the system when it heats up becomes excessive until the pressure relief valve triggers, venting some water and pressure from the system. So for example you might top the system up to 1 bar when cold, when it gets up to maximum temperature with all radiators open it should only rise to about 2 bars, with a flat expansion vessel it would exceed 3 bars, the relief valve will drain some water out of the system, now that you have lost some water (harmlessly out the drain pipe) the system pressure when cold will be below 1 bar and you will get a lockout.

No matter how many times you top the pressure back up, it will keep happening again under the right conditions even though you don't have a leak as such. (The pressure relief valve is doing its job by preventing excessive pressure build up)

If the gassing is marginal you might find that if you run at a lower flow temperature or don't have all radiators on at once (both of which reduce pressure rise) that you won't see a problem, however worst case scenario is all radiators full on and maximum flow temperature, followed by the system being off for long enough to completely cool.

You can test the expansion vessel fairly easily - turn all radiators fully on (schedule all zones to 30 degrees on evohome) but disable the boiler itself by turning its power off so the boiler can't heat anything up. Let everything cool down ideally for 24 hours, but 12 hours is probably ok. Check that the pressure is correct, eg slightly above 1 bar or whatever the manufacturer recommends.

Then re-enable the boiler, turn your boilers flow temperature up to your winter maximum, whatever that is, schedule all zones to a high temperature of say 24 for a couple of hours and then check the pressure reading on the system.

If its below 2.5 bars the expansion vessel is OK, if its above 2.5 bars it probably needs re-gassing and if it is showing 3 bars or more it definitely needs re-gassing as that is usually the opening pressure for the relief valve.

I would also go around the house with the system fully on and check all visible pipe joints and around radiators for any sign of weeping - touch the underside of the joints with a finger and see if it feels wet. Even a single drip is a sign of a leak - all joints should be perfectly dry. If you can't find anything and you have to keep topping up the pressure you probably need a plumber to check the expansion vessel and pressure relief valve and also check for hard to find leaks.



Good idea in theory, but I'd like to see the heuristic that could do that reliably without false alarms. For example if I schedule my hallway (only) for 20 degrees and it only increases from 16 degrees to 17 degrees and won't go any further, is that cause for alarm ? What if I left all the doors in the hallway open and all the other rooms are cold so that one radiator can't make any real dent on the temperature ? (which is certainly the case here if I tried to do that)

Quite often the temperature differentials between rooms asked of the system are not possible if doors have been left open, so it would have to be fairly clever to ignore such scenarios otherwise it would be going off all the time. (Another example is our bathroom - scheduled for 22 in the morning but if we forget to close the door at night it has a fat chance of achieving that with a much colder hallway attached)

What I would like to see though is notification of the fault log book by email or in the app though - it seems silly that the system can detect low batteries in actuators or loss of communication with the boiler relay and log it in the fault log book, but not email me to let me know if I was away from home...

Thank you for this comprehensive answer (it is much appreciated as I like to know why things go wrong - but in layman's terms!) - I will digest it thoroughly tonight when I get in from work.

My heuristic is simple, @DBMandrake - Evohome calls for heat and there are to temperature rises within, say, 30 minutes. In any zones.

I agree that it wouldn't be perfect - solar again on a hot day, for example, might cause temps to rise. But I think we'd only see false negatives, not false positives, and eventually any such fault WOULD show.

But it's all about temperature rises, like I said in my original, not reaching setpoints which is what you seem to have assumed I meant.