1960s electric underfloor heating - broken?

[Maikeru]

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.

 

[Octopus]

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.

Nobody could really "hazzard a guess" from what you've written to be honest.... maybe get a 3rd opinion, to check what the 2nd chap said?

EDIT: Have you any idea how expensive underfloor electric heating can cost?

 

[PEG]

Hi dude,my folks,and several pals,live in semi-detached properties,built in 1963,which all have underfloor electric heating in concrete floors,one for front section,one for the back. It actually works VERY well,and,save from fitting new thermostats three years back,have not given a problem. Another property on the same road,one my missus nan owned,had the heating condemned by a spark 20 years ago,but it was due to the stat,and imminent fitting of gas central heating...i tested the feeds to the floor grid,and gave similar readings to my folks,which i know well,and when thermostat was bypassed,it functioned ok. It is not unheard of,for failures,but this would give open circuit on L-N,with possible low IR readings,but trying to de-cypher your results,an IR between L-N would yield that,but the others raise the question of some other issue. Did he check the installation wiring (as per EICR),or was there "mission creep" to test further? If there has been any movement/displacement of the floor structure,coupled with any moisture ingress,O/C on element/grid,and low IR readings MAY be the case,but,as ever with these things,viewing it directly is required. The fella quoting £720,can NOT be anticipating replacement of the existing system,merely the section feeding it,which will go downwards from the thermostat position,to the floor,1.5 M,sound about right? Breaking out existing,prepping to regs,and refitting new,is considerably more expensive,i know,i have done it! The best advice,not knowing all your details,would be to get the feed to your floor grid/element,exposed,and tested thoroughly,maybe connecting up temporarily,to verify function,load,etc. Get a competent spark to do this,as over the years,many deviations occur,and i have seen spurs taken off these "off-peak" supplies,and even feeds to them,from both meters,to enable use at any time. Old is good. Do not dismiss what could be a viable system,without knowing for sure,i mean,we ALL love the Queen,don't we?:stooge_curly:

 

[ayjay]

if he removed the stats and cut the cu cables without ur permission he sabotaged your electrics
sue him

 

[shanky887614]

if he removed the stats and cut the cu cables without ur permission he sabotaged your electrics
sue him

if he thoughy it was faulty the only thing he should have done was put the cables in connector strip or equivalent till you decided if you wanted it ripping out or repairing

 

[ayjay]

we agree then shanky
the other post that concerns me is replacing the heatind circuits for £ 760-
its inpossible to replace heat wiring thats cast in a slab
the concrete slab forms part of the buildings structure
it would be totally impossible

 

[Deleted member 72666]

Hi dude,my folks,and several pals,live in semi-detached properties,built in 1963,which all have underfloor electric heating in concrete floors,one for front section,one for the back. It actually works VERY well,and,save from fitting new thermostats three years back,have not given a problem. Another property on the same road,one my missus nan owned,had the heating condemned by a spark 20 years ago,but it was due to the stat,and imminent fitting of gas central heating...i tested the feeds to the floor grid,and gave similar readings to my folks,which i know well,and when thermostat was bypassed,it functioned ok. It is not unheard of,for failures,but this would give open circuit on L-N,with possible low IR readings,but trying to de-cypher your results,an IR between L-N would yield that,but the others raise the question of some other issue. Did he check the installation wiring (as per EICR),or was there "mission creep" to test further? If there has been any movement/displacement of the floor structure,coupled with any moisture ingress,O/C on element/grid,and low IR readings MAY be the case,but,as ever with these things,viewing it directly is required. The fella quoting £720,can NOT be anticipating replacement of the existing system,merely the section feeding it,which will go downwards from the thermostat position,to the floor,1.5 M,sound about right? Breaking out existing,prepping to regs,and refitting new,is considerably more expensive,i know,i have done it! The best advice,not knowing all your details,would be to get the feed to your floor grid/element,exposed,and tested thoroughly,maybe connecting up temporarily,to verify function,load,etc. Get a competent spark to do this,as over the years,many deviations occur,and i have seen spurs taken off these "off-peak" supplies,and even feeds to them,from both meters,to enable use at any time. Old is good. Do not dismiss what could be a viable system,without knowing for sure,i mean,we ALL love the Queen,don't we?:stooge_curly:

Not the same without Freddie I'm afraid!

 

[PEG]

Not the same without Freddie I'm afraid!

...Sometimes i think you lot just arn't taking circuit vandalism,seriously enough...:bow:

 

[Deleted member 72666]

...Sometimes i think you lot just arn't taking circuit vandalism,seriously enough...:bow:

Ha ha not true! Do you remember......."no disassemble Jonny 5", Well I was really gutted when they smashed him to pieces!

 

[Maikeru]

It is not unheard of,for failures,but this would give open circuit on L-N,with possible low IR readings,but trying to de-cypher your results,an IR between L-N would yield that,but the others raise the question of some other issue. Did he check the installation wiring (as per EICR),or was there "mission creep" to test further? If there has been any movement/displacement of the floor structure,coupled with any moisture ingress,O/C on element/grid,and low IR readings MAY be the case,but,as ever with these things,viewing it directly is required. The fella quoting £720,can NOT be anticipating replacement of the existing system,merely the section feeding it,which will go downwards from the thermostat position,to the floor,1.5 M,sound about right? Breaking out existing,prepping to regs,and refitting new,is considerably more expensive,i know,i have done it! The best advice,not knowing all your details,would be to get the feed to your floor grid/element,exposed,and tested thoroughly,maybe connecting up temporarily,to verify function,load,etc. Get a competent spark to do this,as over the years,many deviations occur,and i have seen spurs taken off these "off-peak" supplies,and even feeds to them,from both meters,to enable use at any time.

This is the problem - I can't seem to find any electrician who's familiar enough with the system to test it definitively. I know you're in Manchester but any chance you'll be heading down south (London) for anything in the near future? I'm serious.

I've heard mixed reports from neighbours about the quality of the heating - one said it was great and still working well; another said it was no longer functional on their ground floor or kitchen. Another installed under-laminate heating for better control, though I'm not sure this would provide anywhere near the amount of warmth the old system does (whereby I understand the whole concrete floor essentially becomes a heat store).

You said you've replaced this type of UFH before - how expensive was it and how did you manage to do it if the mats etc where in the concrete? Did you take up the screed?

Cheers in advance!

 

[Richard Burns]

It sounds like your first electrician was only doing an EICR (i.e. fixed wiring not equipment) he got low insulation resistance results testing the entire circuit with the mat line not connected (thermostat off).
He then tested the cables at the thermostats (presumably with the thermostat still off) and got low results.

So long as he did not disconnect any connections:
The first test would indicate damage to the supplying cables, or the thermostat allowing a low reading.
The second test would be testing via the mats back to the supplying cables and so would show the same result (the resistance of the mats would be immaterial)

He then performed criminal damage to your property, as it was without your permission.

The second electrician only performed low resistance readings (presumably directly on the mats), this would only tell you if the mats would be able to heat i.e a sensible reading from line to neutral shows the mat heating element is continuous.
The resistance readings from line to earth and neutral to earth would not normally show any problem on a low resistance test, unless something was badly wrong (dead short circuit to earth).

The mat without any connections to anything else should also be tested for insulation resistance from line and neutral to earth, this would indicate if there were a problem with the cable from the test position to the mat or with the heating element within the concrete connecting to earth (Which should not happen).
Since the second electrician's results showed the mat heating element to be continuous (presumably) If the insulation resistance results came up OK then you would be OK to connect a working power supply and thermostat to the mats and run them successfully.

The type of thermostat would depend on how the mat temperature were being measured i..e. an air temperature thermostat seeing if the room warms up or a buried sensor in the floor giving a signal to a thermostat switch if the concrete warms up.

The cost of replacing the supply and thermostats would have to be determined on site.

 

[PEG]

This is the problem - I can't seem to find any electrician who's familiar enough with the system to test it definitively. I know you're in Manchester but any chance you'll be heading down south (London) for anything in the near future? I'm serious.

I've heard mixed reports from neighbours about the quality of the heating - one said it was great and still working well; another said it was no longer functional on their ground floor or kitchen. Another installed under-laminate heating for better control, though I'm not sure this would provide anywhere near the amount of warmth the old system does (whereby I understand the whole concrete floor essentially becomes a heat store).

You said you've replaced this type of UFH before - how expensive was it and how did you manage to do it if the mats etc where in the concrete? Did you take up the screed?

Cheers in advance!

Hi dude,Richard has added,as usual,a good,definitive explanation of your current situation. Sadly,a week in Oxford has used up my southern travel patience quota! I can,however,give a little more information which may assist. IF your system is similar to the ones i have played with,and age wise,it is,then it is not a "mat" or "grid" as such,but a serpentine of resistive cable,set in the screed or floor slab. Remember,this was a newish concept in the 50's and 60's,and thought for set-back and temperature monitoring were not catered for. There will probably be no floor temp. therms,and the original wall thermostats were wax-bulb or similar,mechanical affairs. The feeds to your floor circuit,may ONLY be Line and Neutral,but a decent spark would see this was apparent,and could run his tests accordingly. I am hoping the previous "executioner" has ONLY severed these connections at the site of the wall thermostat,and,with a little forethought,a repair would suffice. If any settlement/movement of the slab has occurred,you WILL get some weird results,as building techniques then,precluded sheet DPC's,and the concrete could have had soil,spoil or anything else,as a blinding. Under a section of the concrete floor at my parents house,i found,mixed in with all the rest,a FULL pint bottle of milk from 1963,so raspberries to the regs...If you get a decent spark (recommendation by someone helps),furnish him with what you know,and maybe he could power it up to prove,then,all you need is a load appropriate air temp only thermostat,and some decorating. If you intend replacement of system with the same,you will need a week,three lads,a skip,clear barrow runs,tape for all the doors......in other words,it is a fair job! :stooge_curly: Keep us informed,for interest sake,or if we could assist further. Good luck.