1960s electric underfloor heating - broken?

Discussion in 'Electricians' Talk' started by Maikeru, Sep 28, 2014.

  1. Maikeru

    Maikeru New Member

    I have just moved into a 3 bed end-of-terrace built in 1969 that was supposed to have still-functional electric underfloor heating in the concrete.

    An electrician said it wasn't working and disconnected it (removed the thermostats and cut the cables running to the consumer unit, even though I did not specifically request this).

    In his report he based this judgement on the fact that the 3 underfloor heating circuits supplying the thermostats all gave 0 Mohms insulation resistance readings at 250v (between L-E, L-N and N-E. He went on to explain: "Upon further inspection, parts of 2 of the circuits (the first legs of the circuits running from the consumer unit to the stats) appear to have been rewired approximately 25 years ago with a newer pvc insulated cable, replacing the original mineral insulated cable that would have been installed when the house was built some 40+ years ago (likely a necessary repair due to corrosion over time of the original mineral insulated cables due to being buried in concrete). My conclusion is that the original mineral insulated cabling still running through all 3 circuits, to most parts of the underfloor heating, has corroded to the point that it can no longer correctly conduct electricity. The same insulation resistance readings were obtained from each of the stat locations on the original mineral insulated cables. No testing was carried out above 250v as individual disconnection of the heating mats was not possible."

    When I asked the electrician if there was a way of re-testing it now that he'd removed the thermostats, he said that it could be by knocking through the plaster where the thermostats were and testing the readings on the wires. Another electrician did this for me and got normal resistance readings. He said that the original electrician should have tested for resistance and not insulation resistance, as underfloor heating mats will always give a 0 Mohm insulation resistance reading.

    When I went back to the original electrician with this he said: "I carried out an insulation resistance test of three underfloor heating circuits at the consumer unit with the thermostats in the satisfied position (turned down/off) which breaks the line conductor. Finding a short circuit between Line/CPC, Line/Neutral and Neutral/CPC I concluded that the cabling running to the underfloor heating thermostats was defective. The only time you would expect a result of 0 Mohms is if you were to carry out an insulation resistance test on the underfloor mats themselves, which is not a mandatory part of the EICR. If you were to carry out an insulation resistance test on the underfloor mats themselves you would expect a reading of 0 Mohms between Line/Neutral ONLY, NOT between Line/CPC or Neutral/CPC. An EICR is only concerned with the installation wiring NOT the local accessory wiring. I believe the resistance readings you have informed me of were obtained locally by testing for resistance NOT insulation resistance of the underfloor mats themselves."

    The original electrician at first implied it wasn't worthwhile replacing the underfloor heating as the mats were in the concrete and be impossible to reach. However, he now seems to be saying that since we said we weren't going to use the underfloor heating anyway, rather than any rewiring to it disconnection would be the cheapest option.

    I am in the difficult situation of having heard two different things from two different electricians. I want to get the underfloor heating fixed if possible. My questions are:

    1) Can anyone hazard a guess on whether it's actually broken or not from the above?

    2) Is it possible to rewire it if so in order to make it functional? The second electrician quoted £720 to chase new cables to it. Would that fix it and is that even possible if the mats are in the concrete?

    The property has no gas supply and I will probably install fan-assisted storage heaters in any case, but I would like to salvage the underfloor heating since I've heard the in-concrete type installed in the 60s was very effective as a heat store.

    Thanks so much in advance.
     
  2. seneca

    seneca Screwfix Select

    If it's been in since the '60's and especially as there's MICC cables involved i'd scrap the system altogether!
     
  3. Rulland

    Rulland Screwfix Select

    I disagree Sen, MICC are bombproof imho, they'll last for ever and a day if undisturbed, no matter how old, If the mats are servicable why not use them.
    I find it hard to believe that all wiring to all the mats is defective.
     
  4. Maikeru

    Maikeru New Member

    Thanks for the quick responses. So do you reckon it's possible to replace the wiring to the mats (if that's what the problem is)?
     
  5. Rulland

    Rulland Screwfix Select

    The wiring should still be there tbh, if all the electrician did was remove the stats then cabling should still be fairly accessible to the mats from those locations, gonna be a whole lot easier than replacing the whole underfloor heating system.
     
  6. Maikeru

    Maikeru New Member

    He also cut the ends of the cables running to the consumer unit - they are locateable through his description though:

    "Two of the underfloor heating radial circuits were cut back to the services riser located in the garage area, the third is cut in the corner above the mains where the cables disappear into the wall."

    Do you know what a ballpark fair price would be for an electrician just to reattach these cables to the consumer unit (they'd need to be extended somehow due to being cut some distance away from the unit) as well as re-attach 4 thermostats if I supply them? Would any type of thermostat be compatible (the ones he removed were very basic ones installed when house was built in 1970)?

    I'm still trying to understand why he disconnected it all if it was still salvageable. At first he said the mats were the issue (and since they are in the concrete impossible to replace) but now seems to be implying in his reply above that the cables running from the consumer unit are the problem (at least that's what the second electrician can gather).
     
  7. seneca

    seneca Screwfix Select

    There could be moisture whithin the insulation of the MICC cables. I know Rulland described MICC as "bombproof" earlier but this certainly wasn't the case on a job I had recently. The copper sheath of the cables had corroded and split in an area of the house that had a damp problem and the moisture had permeated for quite a long distance along the cables.
     
  8. Maikeru

    Maikeru New Member

    OK so is it likely that if there was a problem with the underfloor heating it was with the mineral insulated cables rather than the mats themselves? This certainly seems to be what the original electrician is saying in his report. If this is the case, why wouldn't he recommend replacement cabling (the second electrician quoted me £720 for this which I would pay if I knew it would definitely solve the issue)! The original electrician definitely said on the phone that the underfloor heating mats were the problem but this doesn't seem to line up with what he says in the report above. Can electric underfloor heating mats give out like that or should they always be functional provided the cabling is good?

    I have absolutely know idea about any of this and it seems to baffle any electrician I ask.......
     
  9. seneca

    seneca Screwfix Select

    Well they are elements after all so I don't see any reason why they cannot fail just like any other element, eg, cooker, immersion, kettle etc. etc.
     
  10. Rulland

    Rulland Screwfix Select

    Sen I agree, mineral cables are prone to damp on occasion, just seems that the OP's post suggests that an electrician has condemned the whole underfloor heating for one reason or another, if the mats are not showing any leakage signs and are not OC, then try them, any standard stat will do to test them temporarily, or just put mains to them.
    Ohms will give you current draw.
     
  11. Rulland

    Rulland Screwfix Select

    How many mats n stats are involved?.
     
  12. Maikeru

    Maikeru New Member

    Sorry what does OC mean? There are 4 thermostats in total (I think each has two circuits each as the second electrician got resistance readings of 75.2, 22.2, 31.2 and 99.9 when he knocked through the plaster around two of the thermostats to test the wires). I wonder if that means there are 8 mats in total?

    Perhaps a silly question but is it actually possible to test the mats if they are in the concrete?
     
  13. Rulland

    Rulland Screwfix Select

    OC=open circuit, ie not a circuit, no current can flow, not working.
    If the mats are giving a resistance reading then theoretically they are a circuit, the differing readings would almost definately be from differing mat sizes.
    As for testing then yes, if there are no apparent insulation problems-ie leakage from live/neutral to earth-and they are in one piece, ie a resistance end to end, like you have, then they could well be servicable.
     
  14. Maikeru

    Maikeru New Member

    Thanks!

    Any idea how I should go about getting this done as any electrician I ask seems reluctant to touch this now and I don't want to the original electrician to come back now.

    I've submitted his work for investigation via complaint to the NICEIC (it wasn't just the UFH, he also didn't give an installation certificate after putting in a new fuseboard and did an EICR instead - which other electricians have said is just plain wrong; one or two of his other circuit readings which led him to recommend a partial rewire were also questionable).

    I'm guessing it would only be a few hours work to attach 4 new thermostats and extend the cut cables back to the consumer unit?
     
  15. Rulland

    Rulland Screwfix Select

    I'm NICEIC affiliated also, and can only give advice on what I 'gather' from a thread such as yours.
    I give such advise from years of experience, as do others, but our advise may differ, as you can see, it doesn't mean one or the other is wrong, just different takes on a particular scenario.

    I tend to favour the 'fault find and repair' touch, because I find it more rewarding, stimulates my thinking side, others prefer the opposite approach, 'replace' and you know it's going to be new/work-again I reiterate there isn't particulary any right or wrong as far as it goes, each electrician has their way.

    I would contact the initial electrician and just ask the questions you have here, again, give the guy a chance, maybe he's just a little out of his comfort zone ' box' no harm in asking tbh, if he is he'll say.
     
  16. Lectrician

    Lectrician Screwfix Select

    Skim reading, but, the original lecky said the supply cables were faulty, not the matts? However, it would be odd for them all to be faulty.

    MICC cable is a very durable and long lasting cable.......when terminated correctly. If it's not terminated correctly, you can quickly find your cable will absorb moisture and the IR will drop and cause issues. The only cure is to cut back and re terminate. Usually 300mm is enough.

    Your second electrician gave you end to end continuity readings for the matts, but did he also IR test them?

    The continuity readings you give equates to around 5kw. UFH tends to run for long periods before the stats are satisfied. That's quite some consumption.
     
  17. retiredsparks

    retiredsparks Super Member

    Ref'... MICC (pyro)..usually potting failure.
    Test continuity and IR........either failure can still be pot problem.
    If accessible remove pots....use blowlamp to drive out moisture...working towards open end...check continuity and IR.
    If ok..repot and sleeve.
    No real need to cut back.
     

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