Underfloor heating power

[Goku]

Hi everyone, we're thinking of getting some works done in our loft and have a chance to get underfloor heating installed. I have been reading around on this forum and the web and would like some advice before deciding on next steps.

We want to get 3 x underfloor heating mats (UFH) across 3 different areas on the same floor: bedroom, dressing room, and bathroom.

There used to be a jaccuzzi bathtub in the loft which had a dedicated MCB in the consumer unit (CU) and was fed by a 2.5mm cable. Could this be repurposed to power all 3 UFH in a radial-type circuit (from consumer unit to an accessible junction box and then split to each UFH from there)? Would the power draw from each in isolation and all together be sufficient for this circuit type and cabling?

Would this configuration require DP switches for each UFH?

The path to the Consumer unit would be open so we would have a chance to upgrade anything there and inbetween. There is a spare slot in the CU, but I am earmarking that for an additional RCD that would support the loft. However, if the advice is to upgrade and make use of the spare slot in the CU, then we can think about changing the loft MCBs to RCBOs.

Pretty sure I've missed important information, so please let me know if there's any additional information required.

TIA

 

[JohnW2]

There is a spare slot in the CU, but I am earmarking that for an additional RCD that would support the loft.

Most, if not all, RCDs occupy two 'slots', so you might end up with ....

However, if the advice is to upgrade and make use of the spare slot in the CU, then we can think about changing the loft MCBs to RCBOs.

However, even if you decided to 'upgrade' the circuit that used to suppl the jacuzzi, you would presumably be able to use the slot that was previously occupied by the 'jacuzzi MCB', but that would still leave you with one spare slot - not enough for a standard RCD.

As for your actual question, everything depends upon the power requirements of thee UFH. Depending upon the routing of the cable 2.5mm cable protected by a 20A (or 25A if one is available for your CU) would b good for a total of up to about 4.6 kW (or about 5.75 kW with a 25A MCB).

Kind Regards, John

 

[Goku]

Hi John,

Thank you for the helpful reply. Thanks for the reminder about the RCD size. The space I have is actually 2 slots rather than 1, but hopefully I can still keep that free for a RCD based on your reply.

Apologies that I didn't mention the power requirements of the UFH. If it helps, I've listed these below along with an attachment which resembles the routing and approximate lengths of the cabling:

Bedroom: 3080W
Dressing room: 490W
Bathroom: 1000W

Would you also be able to advise if each stat should have a dedicated DP isolator switch?

 

[ericmark]

I fitted UFH in mothers wet room, idea was to speed drying of the floor, as she was an amputee. However in real terms it was a failure, it would not heat the room enough, with floor max temp at 27ºC the heat lost through the extract fan was more than the UFH could replace.

However the big problem was heat up time, today to reduce energy losses we have two methods, one more insulation and use of heat recovery units for ventilation, and two only heat just enough and when required.

The latter has a problem, fans produce some noise, the fan assisted radiator is likely the best answer, and with 5 speed auto select fans noise is reduced to a minimum, but in real terms installation cost and noise means they are not used much, but other end of the scale is the off peak storage heater, these can't stop releasing some heat when not required, but the UFH is getting close to the storage heater with the delay between heating being switched on, and room getting warm.

It may work with a bedsit where same room used 24/7, and at the bottom of the house, as heat raises, but from switch on to floor getting warm looking at over an hour, so switch on until room getting warm over 3 hours, so for a bedroom used for 7 hours a day, not the right selection.

 

[EFLImpudence]

Most, if not all, RCDs occupy two 'slots',

Is this one of those occasions when we are supposed to know what is meant even though that is not what is written?

 

[Murdochcat]

Heating rooms by electric underfloor heating only?

Could prove to be a very expensive mistake

 

[ericmark]

Heating rooms by underfloor heating only?

Could prove to be a very expensive mistake

I was lucky, also fitted a towel rail. But my sister talked about her friend and said how good it was, so got taken in.

It seems the whole home heating system is being turned upside down, and full of false claims. We have boilers designed to modulate, (turn up and down) then we are told to fit motorised valves which turn on/off, rather than just use TRV's which do turn up/down like the boiler.

Then we are told turn off heating when not required, which means we need fast recovery times, then fit heat pumps which are so small if you don't leave them on 24/7 they are no good, as only big enough to maintain the home, not big enough to re-heat the home once allowed to cool.

Then told heat pumps are three times more efficient to boilers, but they forget to say electric three times more expensive than gas. More when compared to oil heating, as to solid fuel pellets not sure.

And there is more natural globule warming from Iceland than man can do in around 100 years.

 

[JohnW2]

Most, if not all, RCDs occupy two 'slots',

Is this one of those occasions when we are supposed to know what is meant even though that is not what is written?

 

[JohnW2]

Hi John, Thank you for the helpful reply. Thanks for the reminder about the RCD size. The space I have is actually 2 slots rather than 1, but hopefully I can still keep that free for a RCD based on your reply.

Fair enough.

Apologies that I didn't mention the power requirements of the UFH. If it helps, I've listed these below along with an attachment which resembles the routing and approximate lengths of the cabling: .... Bedroom: 3080W ... Dressing room: 490W ... Bathroom: 1000W

Well, that total (4,590 W) is just about within the capabilities of a 20A circuit (and well within the capabilities of a 25A one), which could probably be fed with 2.5mm² cable.

Would you also be able to advise if each stat should have a dedicated DP isolator switch?

Not 'required' by anything, as far as I am aware, but would make very good sense, in case a fault were to develop in one of mats.

More generally,I do agree with those who are questioning the wisdom (both in terms of effectiveness and running cost) of trying to heat a loft (or anything, come to that!) with UFH. How is the rest of the house heated?

Kind Regards, John

 

[EFLImpudence]

Most, if not all, RCDs occupy two 'slots',

No they don't, so what did you mean when you wrote that.

 

[JohnW2]

No they don't, so what did you mean when you wrote that.

I meant what I wrote, because I believed it.

Do you not at least agree that 'most' RCDs (RCCBs, not RCBOs) occupy 'two slots'?

 

[EFLImpudence]

I meant what I wrote, because I believed it.

It is not true.

Do you not at least agree that 'most' RCDs (RCCBs, not RCBOs) occupy 'two slots'?

Really?

 

[JohnW2]

It is not true.

I see. I clearly need to be educated.

Really?

In recent decades I have not seen an RCD in/for a domestic CU that did not occupy '2 slots'. In the very distant past I had some which occupied 3 or 4 slots, and I've personally never seen one (as opposed to RCBOs) that only occupy one slot. What have I missed?

Has anyone reading this got a (21st century) CU which contains an RCD which occupies other than two slots?

 

[Goku]

I fitted UFH in mothers wet room, idea was to speed drying of the floor, as she was an amputee. However in real terms it was a failure, it would not heat the room enough, with floor max temp at 27ºC the heat lost through the extract fan was more than the UFH could replace.

However the big problem was heat up time, today to reduce energy losses we have two methods, one more insulation and use of heat recovery units for ventilation, and two only heat just enough and when required.

The latter has a problem, fans produce some noise, the fan assisted radiator is likely the best answer, and with 5 speed auto select fans noise is reduced to a minimum, but in real terms installation cost and noise means they are not used much, but other end of the scale is the off peak storage heater, these can't stop releasing some heat when not required, but the UFH is getting close to the storage heater with the delay between heating being switched on, and room getting warm.

It may work with a bedsit where same room used 24/7, and at the bottom of the house, as heat raises, but from switch on to floor getting warm looking at over an hour, so switch on until room getting warm over 3 hours, so for a bedroom used for 7 hours a day, not the right selection.

I was lucky, also fitted a towel rail. But my sister talked about her friend and said how good it was, so got taken in.

It seems the whole home heating system is being turned upside down, and full of false claims. We have boilers designed to modulate, (turn up and down) then we are told to fit motorised valves which turn on/off, rather than just use TRV's which do turn up/down like the boiler.

Then we are told turn off heating when not required, which means we need fast recovery times, then fit heat pumps which are so small if you don't leave them on 24/7 they are no good, as only big enough to maintain the home, not big enough to re-heat the home once allowed to cool.

Then told heat pumps are three times more efficient to boilers, but they forget to say electric three times more expensive than gas. More when compared to oil heating, as to solid fuel pellets not sure.

And there is more natural globule warming from Iceland than man can do in around 100 years.

Thanks for your reply and useful insights. Your reply and others here have made me rethink if I should drop the idea of the bedroom and dressing room having UFH under the carpet

In terms of heat loss and insulation, we've ensured that we've taken this part seriously. All the rooms have been insulated with foil-lined celotex and expanding foam around any imperfections with the cuts to each board. We've even gone as far as putting insulation foil over the holes in the celotex that had been cut out for spotlight downlight cans to prevent any airflow above the celotex creating cold spots or draughts!

Do you recall what Wattage the wetroom UFH was? I have opted to go for the maximum Wattage that the supplier provides (200W per m2) to overcome the heat up time. I also have a dual-fuel towel rail which will mainly be heated by the system boiler, and will have an electric element when we want towels dried or the radiator to be warm outside of the heating system schedule.

 

[Goku]

Heating rooms by electric underfloor heating only?

Could prove to be a very expensive mistake

This is a good point. The bedroom is quite large and I only had one radiator planned for it due to some stying features around the room making it less than ideal to have a second radiator. Your reply has made me plan in a second radiator for the room in a way that doesn't ruin the aesthetics.

Thank you