Central heating coming on by itself

Discussion in 'Electricians' Talk' started by nck, Jan 9, 2021.

  1. nck

    nck New Member

    I have an Ideal H18 boiler which is about 18
    months old.
    It’s controlled by a Hive thermostat.
    Recently the central heating has been coming on with no demand from the thermostat. After a while, it will turn itself off.
    im pretty sure it’s not the thermostat as you can hear the micro switches when they turn on and that doesn’t happen.
    If I power down the whole system for 20 mins or so it seems ok for a while, sometimes days at a time, but eventually turns itself on again.
    The boiler has an on/off knob and if I turn it to off(so the boiler still has power) I’ve noticed that the central heating pump continues to run.
    The boiler is under warranty still but nothing else is - the pump is about 20 years old.
    Is my problem likely to be with the boiler itself or another part of my system?
     
  2. Bazza-spark

    Bazza-spark Screwfix Select

    Could be the built in frost protection kicking in, maybe faulty.
     
  3. Mike83

    Mike83 Screwfix Select

    Could be a faulty zone valve or slow closing valve.
     
  4. The Happy Builder

    The Happy Builder Screwfix Select

    Frost stat without a pipe stat to turn it back off when the heated water has circulated?
     
    nigel willson likes this.
  5. nigel willson

    nigel willson Screwfix Select

    Where is your boiler located?
     
  6. nck

    nck New Member

    In the garage.
    I know it has frost protection and it is cold at the moment but surely if this were the case the boiler would be circulating water and not the whole central heating system
     
  7. Mike83

    Mike83 Screwfix Select

    Heat only ideal will have external frost protection.
    It will more than likely circulate around the heating circuit when the frost stat kicks in.
     
  8. The Happy Builder

    The Happy Builder Screwfix Select

    It would be useless frost protection if it didn't circulate the water around the full system, the house could be soaked through from burst pipes.
     
  9. ElecCEng

    ElecCEng Screwfix Select

    Do you have TRVs fitted on your radiators? (You should ) There needs to be one that doesn’t have a TRV, could be a towel rail, which ensures that the hot water circulates round the whole system but only through one radiator in this situation.
     
  10. spinlondon

    spinlondon Screwfix Select

    Could be a sticking diverted valve.
    When you turn on a tap, the water flow causes a micro switch to operate, which then tells the boiler to fire up.
    If the valve or the micro switch is sticking on, the boiler will fire up.
     
  11. fred812

    fred812 Active Member

  12. nigel willson

    nigel willson Screwfix Select

    Have a low limit stat fitted on the return pipe, this will turn off the heating again when it’s warmed up
     
    The Happy Builder likes this.
  13. The Happy Builder

    The Happy Builder Screwfix Select

    The boiler may well have built in frost protection with or without a return stat.

    My parents lived in a bungalow with a old gravity heating system with a gas boiler and tanks in the loft. my dad had me fit a frost stat in the roof by the tanks and the heating ran most of the winter, but the tanks didn't freeze and they would have had the heating on anyway apart from when they had a month abroad in Jan/Feb, whilst they were away he just knocked the room stat back to 10 degrees.
     
    nigel willson likes this.
  14. ScottyP

    ScottyP New Member

    Check your zone valves if you have them (standard boilers only with separate water & heating - not combis) as when the heating controller opens the zone valve, the end switch in the valve calls the boiler for heat, which in turn fires up the pump and the burner. When the valve closes (assuming the valve isn't sticking open) then the end-switch can remain on if faulty or corroded, so the boiler keeps firing with the pump dead-headed against the closed valve, wasted gas and red hot pipes ensue (bad zone valve actuator switch). If the pump sticks on and the heating is actually flowing in the rads when not supposed to be on, you will know. Hot radiators with no heating selected, (bad valve body / zone valve motor).

    To test this, when the boiler is firing on its own with no heating or hot water called for, unplug the zone valves one at a time (if you can - mine are connected on 4 pin plugs - Danfoss HPA2s) and the bad zone valve will shut off the boiler and pump when unplugged. They can fail at less than 10 years old. My heating ZV failed at 8 years old due to a tiny leak from the valve stem corroding the innards.
     
  15. MGW

    MGW Screwfix Select

    Ideal and Hive, I thought Ideal boilers were opentherm enabled, hive is not opentherm, so wrong thermostat for boiler. But Hive has a trick up its sleeve, demand for heat, if the Hive TRV senses room under temperature it can tell the central thermostat to turn on even it that room is warm enough.

    The main point with modern boiler is not to switch off too often.
     
  16. ScottyP

    ScottyP New Member

    That's true, if the OP has TRV's that are Hive smart ones (agree they can call for heat by themselves).

    Also opentherm boilers don't necessarily need to have that input connected to anything, so it just wouldn't use it, if it's not wired to an opentherm aware controller / receiver. Plus it's used to modulate the boiler output when running, not to decide when it fires? I don't think all Ideal models are OT enabled either.
     
  17. MGW

    MGW Screwfix Select

    I have never used OpenTherm as here using oil, and last house had Worcester Bosch which don't seem to have embraced OpenTherm.

    But most gas boilers are now modulating type, it is mainly to allow them to gain latent heat, which needs the return water to be cool enough, if we turn the boiler off with a on/off device be it programmer, thermostat, or simple switch, any heat in the boiler is lost out of the flue, so we want the boiler as cool as possible before turning off with an external on/off device.

    Also when it is switched back on again it does so flat out, where if switched on internally or with a thermostat connected to ebus, it turns back on at minimum output, which reduced the hysteresis.

    So either we use the TRV to control room temperature which in turn controls boiler due to return water temperature, or we use some form of ebus control, be it OpenTherm or other.

    So where there is no ebus control there is a problem as weather improves, boiler can never switch fully off, although the anti-cycle software can reduce how often the boiler fires up, it can never switch fully off as it needs to circulate the water to assess if required.

    So my method with silly Worcester Bosch with no ebus in my case was to carefully set the hall TRV and wall thermostat to compliment each other, the wall thermostat set higher than TRV so in winter never activated, but on a warm day it would turn off boiler, it is a balancing act to set the TRV, lock shield, and wall thermostat to work together, far easier to have the wall thermostat RF linked to the TRV, which is what Hive does.

    Setting the lock shield valve before having digital TRV heads was a problem. *123456 on the TRV is about as good as a chocolate fire guard. OK 20 degs C is some where between 2 and 3 on most TRV heads, but it is gradual so fully open at 18 degs C and fully closed at 22 degs C so you have two controls neither with any usable scale to balance together.

    Now this Flat Battery TRV.jpg makes it far easier, you know setting of TRV, so if after some time to settle target is under current close lock shield a little, or if over open lock shield a little. Once set you can put old mechanical TRV back, they do work well, but just impossible to set the two variables.

    But once all TRV's set, the wall thermostat does very little, the TRV can be set on a schedule 61dmtMm13BL.jpg it also auto turns off if window opened, well in my case back door to unload food, so if the wall thermostat did go wrong, it would make very little difference, as the TRV's are controlling the room temperatures.

    It is clearly not what we want for the boiler to fire up, the by-pass valve open, and it to shut down again every couple of hours as it tests if required, but not any big problem if it does.
     

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