Is a combi boiler supposed to come on if the thermostat is on 5 degrees in the summer?

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iamabigtree@reddit

For a combi boiler the hot water should not be on a timer, they work on demand so won't heat any water you are not using unless it has a hot start system and even then just leave that alone.

Usually the central heating should not be on a time either as that is controlled by the thermostat. Encountered many an issue with boilers in built timers doing weird stuff, they should be turned off in most cases.

The radiators and hot water are seperate systems controlled by a diverter valve, sometimes this can fail and heat the radiators. Either way I can have the radiators off for months and still have hot water.

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_Cridders_@reddit

Who's downvoted you for that? This is why I hate Reddit sometimes 😂

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Teh_yak@reddit

The systems to heat the radiators and heat the water are entirely separate. The radiators are a closed loop, the hot water you use is cold water from the mains and heated up.

The radiators being on sounds wrong. Are you sure it's paying attention to the thermostat? Are they coming on with the timer?

Also, if the hot water is on demand, it doesn't need a timer. You just leave it on.

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_Cridders_@reddit

This isn't strictly true tbh, but might lead to the answer, OP. The main thing separating the heating and hot water is the diverter valve, if that's letting by (leaking internally), it'll be sending heat to the radiators every time you run hot water

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Ok_Shirt983@reddit

Seems like the timer you think is for hot water is actually just another timer you've set for heating, combi boilers don't have a timer for hot water as the boiler automatically fires up whenever you turn on the hot tap, and then switches off when you close the hot tap.

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star-bynight@reddit (OP)

The settings have timers for both the hot water and heating which can be turned off, timed, once or on for each individually but if I just press for hot water the heating comes on too.

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miklovesrum@reddit

Are you 100% sure it's actually a combi boiler?

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Spanner1993@reddit

I think youre 'water' timer is actually a 2nd heating timer. Hot water is on demand.

I could programme my last system to come on and off multiple times a day, if I wanted. You've probably programmed it to give another blast of heating, in the belief its the water.

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BoiledEggOnToast@reddit

The heating shouldn’t have come on if the thermostat is set to 5 degrees. Run a hot tap and see if you can hear the boiler fire up for the hot water on demand. Does the boiler have a small display to indicate if the radiator heating loop is active?

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terryjuicelawson@reddit

Do the radiators get hot when you run a hot tap? As this can be an issue with a particular valve. Landlord will need to advise on the setup or get it fixed either way, that is what they are there for.

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Slow-Kale-8629@reddit

A combi boiler comes on to heat water on demand - so only when you're running a tap or shower. It wouldn't have a hot water schedule. Hot water schedules are for people using a hot water tank, so they wouldn't have a combi boiler.

Whatever kind of boiler you have, the water for your taps and showers is completely unrelated to the water in your radiators. You're not drinking or showering in radiator water! If your heating is turned all the way down then your radiators should not come on. Something is wrong with your setup if it's doing what you describe.

The boiler probably has a setting (directly on the boiler itself) where you can turn off the heating altogether (and not the hot water) while you get someone to help troubleshoot.

Maybe someone replaced the boiler recently and left the old thermostat in place but disconnected, because they couldn't be bothered to redecorate. If that's the case, maybe when you're setting this "hot water schedule" and changing your thermostat to 5° you're not actually changing anything at all. Have a look for another thermostat.

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uncertain_expert@reddit

Asking a neighbour politely for help might be the easiest- chances are they have exactly the same boiler/thermostat if it is a moderately new property.

Find the user manual!

Combi boilers will have a frost-protection mode that will activate if the sensors indicate it is <5C or thereabouts, but that is the air or return water temperature being measured, not the temperature you have requested on the thermostat.

Some combi boilers have a diverter-valve. This valve allows either water from the central heating system OR water from the mains to pass through the heat-exchanger. The valve should change position when you turn on a hot-water tap. If the valve doesn’t move freely you may be heating water in the central heating loop whenever you have demand for hot water from the tap.

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JWK3@reddit

Some combi-boilers have a small internal hot water tank that it'll periodically reheat, hence why you'll hear it firing up randomly in summer, but if the radiators are hot, then something is amiss.

Have you tried turning your thermostat off/batteries out instead of down, just to see if the signal is accidentally been sent to the boiler?

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Lonely-Job484@reddit

regular 'old school' wired thermostat? Surprised it goes as low as 5 degrees.

The obvious answer is to just set it to 'off' rather than 'timer' (or 'auto', or whatever the switch/dial/menu calls it)

as u/BanzaiMercBoy says though, a combi boiler won't generally have a timer for hot water as the whole point is it's on demand. Some modern/higher end boilers have an 'eco' and 'non-eco' mode with a small internal reservoir that can be kept up to temperature to allow 'faster' hot water supply, but I don't think they have timer functions for it either.

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