Central heating only comes on when hot water is active

Discussion in 'Plumbers' Talk' started by Dcwt2010, Mar 25, 2021.

  1. Dcwt2010

    Dcwt2010 New Member

    Hi all,

    As the title says, my central heating only comes on when hot water is active. I'm an amateur so apologies for the sparse info or incorrect terms. We have a boiler and hot water cylinder that is controlled by a Hive system. One admission is that the Hive was installed by a relative and he didn't have time to check all the wiring (lockdown happened and changed a lot of plans). One of the outstanding issues was that the original wall mounted thermostat needed to be wired for the heating to work at all.

    Anyway, it's been functioning exactly as expected for the last 5 months but earlier this week I noticed the heating wasn't coming on no matter what I did with the hive settings. So I tried the basics of checking the boiler, the hive itself and then looked in the airing cupboard but could see no obvious fault except that the motor for the 3 port valve was on constantly with a buzzing noise. Long story short, a plumber did come by to diagnose and did a next day job to replace the fault 3 port valve and motor.
    However, the same problem was still persisting; the heating will not come on until the hot water was set to go. Furthermore, the hot water needs to be actively required (level low or temp not reached) otherwise nothing happens for the heating. I can see clearly the heat pump only activates when the hot water is engaged.
    We do have an electrician coming out hopefully soon but my google research is only giving me limited answers, any ideas what's happened given most people solved their issue with replacement of the 3 port valve?

    Thanks for any advice in advance.
  2. PlumbyGas

    PlumbyGas Member

    Did he change the Whole valve and not just the Head (silver box on the brass body) this would have taken at least 1 1/2 hour as he would have needed to drain the CH and Hot Water down and refill, bleed etc.
  3. Dcwt2010

    Dcwt2010 New Member

    That's right, he was a pretty decent guy and explained it as he went along so it did take about an hour and half total.

    An electrician came along for a quick look but the heating box for the cables is just a jumble with no labels so nothing obvious for their first glance. I've read that it is possible for the hot water cylinder's thermostat to be faulty and that could interfere with signals going to the boiler?
  4. Dcwt2010

    Dcwt2010 New Member

    So the radiator pump comes on when hot water is activated but not if only central heating is turned on. Is the pump supposed to come on just for the hot water?
  5. terrymac

    terrymac Screwfix Select

    The pump should run when there is a demand for central heating or domestic hot water.
  6. Dave does Gas

    Dave does Gas Screwfix Select

    Sounds very much like a wiring issue
  7. Dcwt2010

    Dcwt2010 New Member

    Thanks for your input. Any ideas why this would happen now after 5-6 months of functioning normally? Initially, the motor for the three way valve was definitely broken but the plumber has replaced the whole thing.
  8. sam spade

    sam spade Active Member

    Can you post a close up pic of the heating box (aka wiring centre) with the lid off? (Take care: 240Vac present.) Another pic showing the boiler and pipework etc would also help.
    Dcwt2010 likes this.
  9. Dcwt2010

    Dcwt2010 New Member

    Sure thing. The boiler is in the garage so I'll try and take a picture tomorrow. Thanks for taking an interest!

    PXL_20210324_133139425.NIGHT.jpg PXL_20210324_133128092.NIGHT.jpg 00100dPORTRAIT_00100_BURST20180717162302716_COVER.jpg

    Attached Files:

  10. sam spade

    sam spade Active Member

    Thanks for the pics. The wiring centre is a bit of a muddle!

    Do you have a multimeter and know how to use it?
  11. Dcwt2010

    Dcwt2010 New Member

    Nope, the last time I did use one was back in A level electronics but I am always keen to learn.
    Yes, the wiring is a cluster F and that's why we've had 4 electricians/plumb engineers who came and ghosted us afterwards.
  12. sam spade

    sam spade Active Member

    These four diagrams show the status of a mid-position valve when:
    1. Calling for hot water (DHW)
    2. Calling for CH and DHW
    3. Calling for CH only
    4. When CH only has been satisfied.
    When calling for DHW only, the valve is not operational; the boiler is controlled directly by the cylinder thermostat.
    CH and HW together uses the cylinder stat to control the boiler; but also required voltage on the white wire to hold the valve in mid position.
    CH only requires voltage on the white wire and on the grey wire (HW Off) to make the valve move from mid-position to the CH only position.
    When CH only has been satisfied the valve still stays in the CH only position - it's a quirk of the mid-position valve. However, as the valve only consumes about 6W, it's not normally considered a problem.

    Mid pos -1.jpg Mid pos -2.jpg
  13. Dcwt2010

    Dcwt2010 New Member

    Thanks, that's a very clear and comprehensive explanation.
    Today's been a warm day so heating has not been on and I've just noticed that the casing for the motorised valve is still warm when all the pipes are just lukewarm. So for some reason, it is always active...?
  14. terrymac

    terrymac Screwfix Select

    It's not unusual for a mid position valve to be held in its last position ,and the motor remaining energised, thus staying warm. As Sam spade pointed out in his post.
  15. Dcwt2010

    Dcwt2010 New Member

    Thanks for the clarification, I just reread that bit.

    Is it possible for the plumber to have accidentally mis-wired the newly installed motorised valve so as to cause the signal to move the valve to CH to not function but mid and HW to work?
  16. sam spade

    sam spade Active Member

    I thought of the same thing, particularly as there is a connector between the new valve and the junction box. Separate the connector and check that the colours match in each half of the connector.

    The valve's grey wire has to be fed from two places: (1) Programmer HW OFF terminal, (2) cylinder stat "satisfied" terminal. All three terminals of the cylinder stat are connected on a mid-position valve (aka Y Plan) system.
    Dcwt2010 likes this.
  17. Dcwt2010

    Dcwt2010 New Member

    Right, looks like your sneaking suspicion was correct. As you can see, the orange cable was not correctly seated and thus did not complete the circuit (this is on the motor side so I didn't even have to turn off power to the system to fix this). I can now switch on the central heating without the hot water also being active. Many thanks!

    PXL_20210330_112957097.jpg PXL_20210330_113004372.jpg
    sam spade likes this.
  18. Harry123456

    Harry123456 New Member

    i have a hive for hot water and heating on a Y plan. what do i need to change to stop the priority HW when i turn the central heating on? cheers
  19. sam spade

    sam spade Active Member

    Simple. Start a new topic.

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