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Downstairs underfloor heating is dead due to electrical problem

Discussion in 'Plumbing and Central Heating' started by foxo, 7 Mar 2021.

  1. foxo

    foxo

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    Hello all, I'm hoping someone can advice me on my heating problem - I seem to have lost all downstairs UFH, but the upstairs heating and hot water are fine.

    About our system:
    We have a gas boiler feeding an 5 zone UFH system downstairs, some radiators upstairs and the hot water cylinder. This is all controlled by an EPH timer which controls the water and a single heating zone (i.e. all the heating). The timer seems to be wired back to two different wiring centres - one (EPH model) seems to handle the upstairs heating and the hot water, and a second one (mystery manufacturer) which seems to cover the downstairs UFH. Into this wiring centre seems to be the cable to the timer, all the downstairs thermostat connections, connections to the actuators on the UFH manifold, and the UFH pump and valve.

    What happened:

    Today I went to replace a thermostat in a downstairs zone and I noticed that once I did this, it seemed to kill the power to all the downstairs thermostats (the little lights went off). The thermostat I was installing is a basic EPH CM2 model that had perviously been installed upstairs before I replaced it with an electronic 7 day timer model (which has worked perfectly for a few years now).

    What have I tried/measured:

    I immediately disconnected the new thermostat and checked the EPH timer which seems fine. It still seems to control the hot water and upstairs heating. I took it off the wall and checked the voltages on the back plate L & N are fine. I even briefly directly connected the L to terminal 4 'CH ON' - no difference (light on thermostat still dead).

    Returning the timer into position, and ensuring it was set to 'heating on', I measured across the L & N on 2 of the downstairs thermostats - both are dead (previously it was 230v).

    Possible conclusion:
    I can't help but think that something has somehow blown in the second wiring centre. It's a mystery model - the only thing that looks like a model number is 'SAM 8.1/1' and some of the text looks to be German. It's more than a basic set of connector blocks, with 2 relays, some capacitors, a small traffo, a push-in-twisty knob (potentiometer?) in addition to all the connectors; but it has nothing that looks like a fuse.

    If I knew who made it I could get try to some data on it and try to understand more about what it's doing. I'll try to post some pictures of this mystery wiring centre

    So I'd be very grateful if anyone could advise, or even any tips on how to get more info on the wiring centre. Finally, is it a safe assumption that the EPH timer is not the problem given that the upstairs heating is ok, running from the timer's single heating zone?

    And finally - if I do end up needing to call in a pro (likely) - is this a plumber or electrician?

    IMG_0541.jpeg IMG_0538.jpeg IMG_0537.jpeg IMG_0535.jpeg
     
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  3. Johnmdc

    Johnmdc

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    The fuse is circled in yellow. Use a screwdriver to turn it. You have probably connected the thermostat wrong and blown it. IMG_0538.jpeg~2.jpg
     
  4. foxo

    foxo

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    Thank you so much @Johnmdc - i actually just found this myself just now and came to post about it - it needs a push and turn and the fuse pops out. Testing continuity on it shows that it is indeed blown. I've never seen these small 20mm fuses before but I'm hoping the local hardware shop stocks them in the morning. My old man eyes prevent me from seeing the engraving on the terminals, but it seems like it's 15A (surely not!) - I assume a 3A replacement is in order.

    Again, thanks for taking the time to read and respond.
     
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  5. dilalio

    dilalio

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    Probably 1.5amp they're known as fast-blows. Also found in boilers and TVs.
    You'd be better going to an electrical counter.
     
  6. Madrab

    Madrab

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    You do need to understand why it blew. Otherwise all you'll be doing is eating your way through fuses. Only concern with that is that do you know how the original worked, was it a switched live or a volt free?

    Typically when these things happen and the fuses start to go it's because it's getting current it shouldn't be.

    What is marked on that white label on the board?
     
  7. ianmcd

    ianmcd

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    If you do indeed have a CM2 as you say, then it doesnt have a Neutral that you refer to, the CM3 does, the CM 2 switches between 1&2 do not connect anything to 4, does your thermostat have a little neon to indicate heating on ? if it does then the Neutral is required (CM3)
     
  8. DIYnot Local

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