Please explain how my electric underfloor heating works!

[fungit]

I moved in recently and can't make sense of how it actually works. I'm trying to work out why my electricity bills are huge (on my own in a 2-bed flat and I'm using 48 kWh per day), so I need to know how everything in the flat works.
It has no timer and no on/off switch (that I can find). It has thermostats in each room. It seems (although I may be wrong) that I am not actually in control of the level of heat (despite the fact that the controls are dials with temperature marks on them) but if I turn the thermostat fully off, the heating will not come on in that room.
The heating comes on overnight but I don't know when. I also don't understand how it knows to come on overnight! There is no timer, so how does it know??? And how can I be sure it comes on when electricity is cheap for me? (It seems to be impossible to find out when the overnight cheap time actually is anyway.)
I've taken some photos (sorry they're sideways) but there is no name on anything except the thermostats! Everything was probably built in when the flats were built in 1970.
Please help!

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• fungit
• 25 Apr 2013
• 1

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• fungit
• 25 Apr 2013
• 1

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• fungit
• 25 Apr 2013
• 1

 

[SimonH2]

This is probably better asked in the Electrics UK section.
Can you post a photo of your meter ? My guess is that you have an "off peak" tariff - some variation of what used to be known as "Economy 7". With these, there will be either a time clock or radio switch that will turn on the off-peak loads, and at the same time switch the meter over to the off-peak register. On your bills, you'll have two quantities of electricity shown - off peak which shows all the lecky used during the off-peak period (heating plus everything else), and peak which is everything used outside the off-peak period. Off-peak units are normally a fraction of the cost of peak units.

The fuse box (the one with 4 blue spotted rewirable fuse holders) will supply the heating, the consumer unit (the one with the circuit breakers) will supply everything else.

 

[fungit]

This is probably better asked in the Electrics UK section.
Can you post a photo of your meter ? My guess is that you have an "off peak" tariff - some variation of what used to be known as "Economy 7". With these, there will be either a time clock or radio switch that will turn on the off-peak loads, and at the same time switch the meter over to the off-peak register. On your bills, you'll have two quantities of electricity shown - off peak which shows all the lecky used during the off-peak period (heating plus everything else), and peak which is everything used outside the off-peak period. Off-peak units are normally a fraction of the cost of peak units.

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• fungit
• 25 Apr 2013
• 1

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• fungit
• 25 Apr 2013
• 1

Thanks SimonH2. You're right, there are 2 tariffs (1 and 2) but I don't know how the underfloor heating knows to come on only during tariff 2. If there's no timer... I mean, the lights come on whenever I switch them on; why doesn't the heating work like that?

 

[SimonH2]

You see how there are two cables (one red, one back) going from the supply to the meter, but 4 cables going through the wall ?
Well the two grey cables (one red, one black inside the grey sheath) will go to one board (I'd say the consumer unit), the other two will go to the other board (I'd say the off-peak fuseboard). The meter has two outputs, the second one (the thinner red one) is switched on/off by the meter which I guess has an integral time clock or remote switch*.

What I do think may be wrong is that the cables for the off-peak board are a) unsheathed, and b) don't look very large compared with everything else. Several earths coming out of the service head probably isn't right either.
But those are things better commented on by someone who know's what they're talking about.

* The are radio switching systems that allow the supplier to remotely turn on/off the off-peak supply. This allows them to use people's off-peak heating as a form of load management - controlling when it turns on and off to reduce the impact of the normal daily variation in demand. You'll get 7 (or whatever your contracted hours are) of off-peak lecky, but not at a fixed time, and not necessarily in one chunk.

 

[fungit]

Thanks a lot, SimonH2. That's very helpful. Thanks for your time.

I'm taking your advice to post elsewhere too. No, I'm not - it appears to have moved already...

 

[flameport]

I'm trying to work out why my electricity bills are huge (on my own in a 2-bed flat and I'm using 48 kWh per day)

They are huge because the electric underfloor heating in your flat is designed to use vast amounts of electricity with no regard given to cost whatsoever.
It's a left over from the 1960s when some people believed that nuclear power would provide virtually free and unlimited electricity for ever. They were wrong.

The main problem is that there is no way to control how much heat is stored in the concrete floor slab. By the time the room thermostats switch off, the heating elements have already been on continuously for hours.

The easy and free way to save money is to switch it off and not use it ever again.

The other option is to have storage heaters installed instead, which will at least be controllable as to the amount of heat stored.

 

[fungit]

I'm trying to work out why my electricity bills are huge (on my own in a 2-bed flat and I'm using 48 kWh per day)

They are huge because the electric underfloor heating in your flat is designed to use vast amounts of electricity with no regard given to cost whatsoever.
It's a left over from the 1960s when some people believed that nuclear power would provide virtually free and unlimited electricity for ever. They were wrong.

The main problem is that there is no way to control how much heat is stored in the concrete floor slab. By the time the room thermostats switch off, the heating elements have already been on continuously for hours.

The easy and free way to save money is to switch it off and not use it ever again.

The other option is to have storage heaters installed instead, which will at least be controllable as to the amount of heat stored.

Oh. That's a bit worrying. But my understanding is that a washing machine uses about 1kWh. Surely my underfloor heating isn't using the equivalent of about 40 washing machine cycles overnight???

 

[ban-all-sheds]

I think that the typical consumption of UFH when it's used as the primary heating with concrete floors is 200w/m².

So if you have it on for 8 hours, and your total floor area is 30m² then you will indeed be using 48kWh.

It is a seriously bad way to heat a property.

 

[SimonH2]

Well the idea was sound, but in practice it works less well that storage heaters - and off-peak lecky never got as cheap as it could have been.

Theory: You use cheap rate lecky to heat up a big lump of concrete. The heat stored in that lump of concrete is released during the day to keep you warm and toasty - especially as it keeps your feet warm. The amount of heat stored is controlled by the room stat - with the floor needing to be warmer when the outside weather is cooler as the floor-room and room-outside heat transfers act as a simple potential divider.
It'll be cheap to run as they need to get people using lecky overnight to avoid cycling some large plant (nuclear in particular, but the big coal plants are best run at constant power as well). It'll be cheaper than gas as our gas is all expensive and imported (or derived from coal in coke-making plants).

Practice: You use not so cheap rate lecky to heat a big lump of concrete during the night. The room stat gives only coarse control as the night temperature isn't always a good indication of what is needed the next day. So the user needs to look at the weather forecast and adjust each evening - it can be freezing at night but the next day is bright and sunny so lots of solar insolation (not a lot of heating required), or it can be mild overnight but cold and grey the next day (more heating required).
During the day, most heat is given out during the morning, tapering off during the day, and reaches it's lowest point during the evening when you most want it. Bear in mind that the majority of people only occupy the living space for a short time at breakfast and then in the evening.
Because you can't control the heat output from the floor, you are heating the property almost 24 hours a day when you only need heat for perhaps 6 to 9 hours total.
It'll cost a fortune as you're heating the place when you don't need to, and the lecky never got as cheap (relative to gas) as expected - what with North Sea gas driving the price down and us not building nuclear plants in the quantities required to get economies of scale.

We have storage heaters in the office, and even though they (in theory) have some output control - in practice they are absolutely sh*te as keeping the office at a comfortable temperature. So bad in fact that we often have to open windows to let the heat out.

 

[D_Hailsham]

... our gas is ... imported (or derived from coal in coke-making plants).

Are you saying that some of our power stations still use Coal Gas (aka Town Gas), not Natural Gas?

I would be interested to know which ones.

 

[fungit]

Practice: You use not so cheap rate lecky to heat a big lump of concrete during the night. The room stat gives only coarse control as the night temperature isn't always a good indication of what is needed the next day. So the user needs to look at the weather forecast and adjust each evening - it can be freezing at night but the next day is bright and sunny so lots of solar insolation (not a lot of heating required), or it can be mild overnight but cold and grey the next day (more heating required).
During the day, most heat is given out during the morning, tapering off during the day, and reaches it's lowest point during the evening when you most want it. Bear in mind that the majority of people only occupy the living space for a short time at breakfast and then in the evening.
Because you can't control the heat output from the floor, you are heating the property almost 24 hours a day when you only need heat for perhaps 6 to 9 hours total.

This is a pretty accurate description of my situation Looks like I might have to re-think my heating setup...

 

[fungit]

I think that the typical consumption of UFH when it's used as the primary heating with concrete floors is 200w/m².

So if you have it on for 8 hours, and your total floor area is 30m² then you will indeed be using 48kWh.

It is a seriously bad way to heat a property.

Thanks for your help and advice everyone. I'll need to do some thinking before next winter.

 

[dhutch]

The main problem is that there is no way to control how much heat is stored in the concrete floor slab. By the time the room thermostats switch off, the heating elements have already been on continuously for hours.

The easy and free way to save money is to switch it off and not use it ever again.

The other option is to have storage heaters installed instead, which will at least be controllable as to the amount of heat stored.

I dont know the details of your ufh, but underfloor heating, installed into a concrete floor, run at night from off peak meters on a dual tariff (as aposed to wet ufh run from a boiler, or 'tile warming mats' ) is a form of storage heater.
- They work in exactly the same way, there just in a thin layer over the floor, not a tin box on the wall.

It is however true that due to large amount of hysteresis in the system (ie, takes time to heat up the floor, and once hot it stays hot for a day) it is hard to control it effectively using just a room thermostat or timer.

These days if it where me I would get a controler designed specifically for the job, which typically electronic and have a sensor for room temperature and outside temperature, as well as a 7day timer, operating a large relay (switch) to turn the heating on and off, some can switch each room induvidually.

Alternatively a cheaper option (in terms of installtion cost atleast) is to use an imersion heater timer (bog standard timer) to drive the relay(s) and use your own knowlage of the system, weather, and desired room temp, to adjust the amount of time its on during the 7hours, or whatever time you get with the supply your on.

If you have a room thermostat(s) there must already be a relay(s) although as I cannot see anthing obvious around the fusebox/consumerunit they may be in the rooms themselves.

In the mean time, as its almost summer, you could do a lot worse than turn the big switch on the silver fuse box to off most of the time!



Daniel

 

[SimonH2]

... our gas is ... imported (or derived from coal in coke-making plants).

Are you saying that some of our power stations still use Coal Gas (aka Town Gas), not Natural Gas?

I would be interested to know which ones.

No, it was a reference to back in the 60's 70's a lot (nearly all ?) of the gas used in homes came from coke ovens. I suspect (it was before I had any interest in fuel & heating bills !) the argument went along the lines of :
Gas is not cheap, and isn't likely to get significantly cheaper. Lecky is cheap (off peak), and is expected to get cheaper*. Therefore it makes sense to use off-peak lecky for heating.
Having made that decision, it's then a case of whether you have a lump of concrete spread a few inches thick across the floor, or stacked up in a tin box under the window. Having a heating system that takes effectively zero space must have looked quite attractive.

Of course, since then North Sea gas came along and altered things, and nuclear hasn't become as large as I suspect it was planned.*

* In hindsight they really f***ed up with nuclear (IMO). The argument was that by getting several different groups to build power stations, they'd compete. Instead what we got was several (nine is a number that comes to mind) different designs with all the costs that go with it. Had they built all the same design then I suspect it would have been a lot cheaper - and we'd probably have been able to build more. Also, if North Sea gas hadn't come along and dropped gas prices/improved availability, then there's have been more of an incentive to build more nuclear.

If you have a room thermostat(s) there must already be a relay(s) although as I cannot see anthing obvious around the fusebox/consumerunit they may be in the rooms themselves.

No, the thermostats will be rated to switch the heaters directly. The fuses are blue dots, which from memory means 15A (or is it 20 ?) each - no problem (at least back when this went in) getting stats rated for 15A or 20A resistive (and it will be a nice easy to switch resistive load).

 

[dhutch]

No, the thermostats will be rated to switch the heaters directly. The fuses are blue dots, which from memory means 15A (or is it 20 ?) each - no problem (at least back when this went in) getting stats rated for 15A or 20A resistive (and it will be a nice easy to switch resistive load).

Fair enough, blue is 15amp afair, so if you can get stats rated at that I expect your right, it also makes sense as to why they is one per room.

My knowlage of UFH is purely from the fact its what my parents have in there house, built in the later 80's when I was a kid. They have around 13kW of elec UFH, split over 10 circuits (most rooms have a small and a large circuit) , with a single relay running of a manual timer which they adjust based on demand. They did have a 'weatherwatcher' controler but it wasnt that great and the ended up ditching it in favour of just switching it themselves and have not got around doing anything else since.

Its not the cheapest option, but given they are not on mains gas, its not hediously much more then oil, and means they dont have to have a boiler or have the heating replaced.

Daniel