I have a project in hand which involves a 500W 230V. ac heater coil bathroom floor installation that does not do the job it was intended. ie It barely reaches 21C. Apart from lifting the tiled floor and starting again I am going to use a "Buck - Boost" transformer setup to increase the heating output by using a 285V ac input to the circuit instead of the normal 230V. mains. From my calculations ( with the heater coil at approximately 105 Ohm)this should up the present current reading from 2.1A to around 2.75A giving a wattage of roughly 770W.I would be interested if anyone else has done this and how far can one go ? The transformer I am going to use , when it arrives, is a 230V ac to 55V. ac.
Kiwi2

Before you do anything, run the system up and measure the actual floor surface temperature. Once it's stabilised, if it's around 35 degrees C (95F), you are already at or near the limit. A higher floor temperature, created in an attempt to boost performance, might make walking on the floor in bare feet a distinctly unpleasant experience, if not dangerous to babies and pets. Depending on the cover material over the heater element, its temperature may already be close to the limit too, and it may even have a thermostat buried with it, which will negate all your plans. I'll let my betters here comment on the electrical implications, but my gut feeling is that this is a bad idea.

Alan
PS. Noticed you stated a 'tiled floor'. You can expect a maximum performance of around 10W per sq foot, (100W/M2) on ceramic/cement flooring, less on wood/plastic.

[This message has been edited by Alan Belson (edited 08-06-2005).]

I see a fire or injury in your future....your best bet is to consult the manufacturer of the heater coil. As Mr. Belson has suggested, the heater most likely has an imbedded thermostat which will simply cut power to the heater faster if you try to boost the voltage. That assumes of course that the heater doesn't instantly burn out from the excess voltage.

I would think that since it's tile above, the type of tile cement is crucial for proper heat transfer. You may end up having to lift the tile anyway.

What about the floor itself? Is it open on the underside? (like a second floor or built over a basement/crawlspace?) Could you insulate the bottom side to help avoid heat loss?

Please, consult the manufacturer, there are too many risks associated with your idea, and it would be unfortunate if someone got hurt!

Can you post the name of the manufacturer here as well?

Thanks for the replies. To answer, the installation is on a cement floor and the best it can do re the surface temperature is, as mentioned, only 21C.
Kiwi2

James,
Welcome.
What I'd be interested to know is how you're going to interface a 55V transformer in with the 230V mains?, to derive 285V.
What sort of a thermostat and controller are you using on this system?.
Besides, a 500W element is just that, it won't dissipate any more energy than what it is designed for.
This set-up sounds like it could easily end in tears to be honest.
I'm not aware of any underfloor heating elements that would enjoy being subjected to that sort of over-voltage and I'm pretty certain that that would instantly void any warranty in place.
Additionally, this type of heating system is required to have RCD protection.
I could be wrong, but 500W should be more than enough to heat the largest of Bathrooms.

[This message has been edited by Trumpy (edited 08-06-2005).]

Just looked up some data from my underfloor heating manual. For 100W/m2 emission, from a cement based floor, with ceramic tile over, the surface temperature should be around 28C, so you are low. The type of tile cement is critical for performance, as mxslick says, there are latex-rubber based flexible versions which will give poor heat transfer- a cement based product is best. An under-element insulation layer is vital, ( I bet this is your problem), or you will be warming the worms, [a few inches down, the earth's temperature will be 10C (50F), a massive heat sink].
My set up is hot water-pipe based, with a design heating of only 50W/m2, (wood floor, cork tiles over). The design floor temperature is only 23C so we are fitting supplimentary electric towel-rail heaters in each bathroom, using the floor as background only. If you want keep the aesthetics of a room uncluttered by radiators, rails or radiants, I'm afraid lifting the floor tiles may be the only option, since it seems you may need to put in a better insulation layer under. Also, for any underfloor heating to work properly, due to low emmission power per sq foot, the wall/ceiling insulation has to be really good too, R = 2.5 (metric) minimum = 100mm (4") of glass-wool minimum. Better is the new reflective foil multi-layer stuff, 7mm (0.28") thick is supposed to = 6" of rockwool but it's expensive. One thing all underfloor systems have in common is they don't work well in excessive drafts, ie. a ventilation fan tends to spoil the effect by dragging what little power you have outside to heat the sparrows! Since my guess is youre DIYing this project, get the floor tiles up, or improve the insulation and/or fit a heated towel rail and put this down to a learning experience, but leave the electrical stuff strictly conventional.
Stay safe,
Alan

Check the manufacturers specs. and stay within the voltage range given. If you go to the top of that range you may boost the output.

I suspect that the concrete floor is sucking all the heat away from the system. I'd consider adding electric baseboard heat before tearing out a tile floor.

Dave

Quoth Trumpy:

Most of the heating elements that I've come across are simple resistors, dissipating power at E^2/R; bump the input voltage up 10%, and the power dissipated goes up by 21%.

A 500W element is _designed_ to dissipate 500W. Increase the input voltage, and the power dissipated will increase, but the element may not be able to _safely_ deal with the increased power dissipation. Increased power dissipation generally means increased temperature, which means reduced component or insulation life.

-Jon

DO NOT DO WHAT YOU PROPOSE!!

I caution you for two reasons.

First, most products are intended to be used with a supply voltage no more than 10% over the nameplate voltage. You propose to exceed this.

Secondly, such systems are to be GFCI protected. Your GFCI would be the breaker supplying the tramsformer- and I fear the transformer will prevent it from protecting the circuit. Should you place it after the transformer, you will be exceeding the voltage it is designed for as well.

You'ld be better served by installing a recessed light in the ceiling, with a heat lamp bulb shining on the floor- assuming, of course, that this is outside the shower area!

Thanks for the all the responses so far. To explain to you all further ...The original installation was made during an extension to an existing house. The area in question is roughly 5M sq. or 57ft. sq so a time clock operated 500 W underfloor heating element should have been ample. The bathroom is properly insulated and there are no drafts or such. I personally watched the installation which was carried out according to the manufacturers specifications. Laid on a floor base of concrete and encased with the appropriate insulator/grout/sealer, then over laid with ordinary thickness ceramic tiles. This contractor has never had a failure like this before. I and my Electrician colleagues cannot understand why it will not heat up as compared to other identical installations which reach a nice 27C.+ within the hour and operate on a time clock. The current draw is 2.1A at 230V. ac. I have tried out a 30V. 'Boost' giving a 2.6A draw. Which resulted in only a 1C. rise over an hour, so as most of you have suggested, I think it looks like a base board heater to supplement unless anyone else can see a way around this I certainly do not want to tear it all up and replace, all though the contractor has agreed to do so. For the sake of a trial I am borrowing a 55V. transformer just to see what will happen although as several of you have said 500W = 500 W !
Trumpy , you can see how I do the voltage boost, see "Buck/Boost/choke test drawings" by 'Scott35' in the technical area.

James,
According to AS/NZS 3000 this sort of a transformer set-up would not comply.

Also bear in mind that connecting a device like this up, is Prescribed Electrical work (NZ Elec. Reg 17) and would need to be tested and certified before connection to a supply.

[This message has been edited by Trumpy (edited 08-06-2005).]

"DO NOT DO WHAT YOU PROPOSE!!"

____________________________________________

Good advice

Thanks Trumpy and others. Installing a heater ( to the code ) now rather than start again. We are still puzzled though why the installation would not perform ! Only thought is that the element run itself was spaced slightly too far apart ?
Kiwi2

Underfloor heating's generally not a great idea to be quite honest. I wouldn't use it at all if avoidable.

The heat output entirely depends on the make up of the floor and the floor covering. For example, you could be loosing most of the heat into the foundation of the house if the concrete isn't holding much heat.

Also, as suggested the tile adhesive or the tiles may be insluating the floor too making the heat transfer pretty poor. (particularly if the adhesive is put on very thin behind the tiles as you effectively create an air cavity insulation chamber!

Other reasons for not installing underfloor heating:

(1) Under wooden floors it creates a lot of problems as it tends to dry out and warp the wood. It can shrink, get brittle, change colour etc over time.

(2) In bathrooms, where it's usually used, it can create horrible smells. If the bathroom floor is in any way not 100% clean it can really create a whiff. Particularly where a toilet is present. i.e. any splashes will be evaporated.

(3) it's woefully ineffective in many cases and very unreliable.

(4) if it goes wrong you need to dig up the floor!

Personally, I think just heat your bathroom conventionally and don't rely exclusively on underfloor heating. It's a nice gimic to create warm tiles but, that's about it.

On a purely technical level, your idea of boosting the mains with a step-down transformer may not work too well anyway, due to the primary to secondary phase relationship which depends on load.

djk; Oooer! Well, I hope my bloody system works then, I'm £2500 ($4000) into the build for materials alone already! You are obviously somewhat disappointed with your system, but to answer some of your points:
Wood floors, absolutely granted, some shrinkage will occur, but I had my cedar boards kilned locally to 8%MC at a mill, after 4 years air drying, was able to do this as I bought the tree after a storm and had it milled. This should give them a c.2% MC region of non-movement, by some quirk of the kiln process. This also seems to have made the wood more brittle for some reason, a PITA on machining. I have deliberately down-rated the 2nd floor water temperature to 35c to ameliorate this problem. My back up position is the boards are t&ged, but screwed via the faces like a ships deck, and plugged, so it's possible to recramp, but not something I want to entertain!
Smells. Blimey! I hadn't considered this! I'll have to improve my aim, and cork tiles may have to be ceramic instead. Good point.

Ineffective/unreliable. Not the same thing. Any ineffectiveness must stem from poor design; ie. inadequate house insulation, poor design of the pipe runs, -(DO; run the first length of pipe from a feed manifold near the walls, tails to the middle of rooms; minimised spacing (8" diameter bend on 15mm pipe) maximises power; all loops in zones are 100m long, giving intrinsic balance from manifolds without excessive throttling, ball-jacks (valves) for full flow when open, automatic air-bleeds on all horizontal runs, a pump for each zone, a programmer for each zone, thermostat(s) for each zone ). Plus, inadequate design of the ventilation is another failure cause. All of which mean the power output of 100W/m2 max. can't cope. This problem is accentuated in bathrooms where you feel colder, there's a lot of vapor to dispel, and a small net floor area. I am addressing this with heated towel rails.
Unreliability comes in two areas; wrong sort of pipe, ( copper in cement will leak by corrosion within 10 years), and poor circuit design, components or materials. I don't see any difference to conventional systems here. I have a neighbor with a system built 8 years ago, (plastic pipe, oil-fired) and it performs beautifully- it was what sold me on the idea initially- his house beng so well heated in winter. I must ask his wife if her bathroom stinks!
Digging up the floor I have no fittings in the pipe runs in the concrete, and just 4 unavoidable in the wood-floor area to the manifolds. The pex-crosslinked plastic is incredibly tough. One fitting failed on air-test, & it turned out I damaged the 'O' ring on assembly by snagging it on the copper lining bush. Provided I don't drill any holes in daft places, I might be ok! Presently [this afternoon] it had held 1.5 bar (22psi) for the previous 36 hours on compressed air. I will be filling 2nd floor pipe run next week with 50% anti-freeze and corrosion inhibitor. I'm not looking forward to this, as the 1st floor pipe run was a sob to get all the air bubbles out!

I see drawbacks as; no open fireplace, (I rather fancied a big hearth with firedogs), the need, as with storage radiators, to foresee tomorrow's weather conditions,- you can't heat 12 tons of cement instantly on demand! - so once winter starts, it's on for 5 months! I have added a detail where in the night the 1st floor 'sets-back' to 10C on a thermostat. Wood floors heat up much faster. You also need really good insulation- 8" of glass wool or equivalent. Ventilation needs careful assessment to avoid excessive losses- I'm fitting centralised ducted venting which will run 24/24, using an 'S&P Akor' air-air heat exchanger and twin centrifugal fans with auto balancing, to recuperate about 50% air-heat or better, at the rate of 1/2 ACH ( = 250M3 per hour), run on a programmer for boost/run/trickle. Cooker hood will exhaust direct out, (grease problems!) Also going in is a 20,000 Btu reversible Fujitsu AC, which will allow 4kw of instant heat to be obtained in cold snaps if needed, and if I dun me sums right help keep it a bit cooler in summer. Plus, the plan is to fit 2 humidifiers (1 on each floor level) and H'stats to keep air levels between 30-50% RH, (still looking for suitable models), to improve the 'dry-throat' sydrome. All this is only feasible due to the insulated floors (2" of ext ps under the concrete), and 8" of Rockwool behind the sheetrock, and Pikington 'e' double-glazing. The theoretical heat demand, including venting, is just 3280Btu, less than 1Kw in a -1C frost and 21C (70F) inside. So, wish me luck chaps! I'll be posting a few pics shortly, I'm sure you could all do with a good laugh at my crap wiring!

Alan
PS. Air-spaces under ceramic tiles. These are easy to find. The tile cracks under the first person to stand on it. My French tiler has a good system. He lays tiles on 1.5" of mortar and beats them into submission with a rubber mallet. Thinset? Never heard of it!

[This message has been edited by Alan Belson (edited 08-07-2005).]

[This message has been edited by Alan Belson (edited 08-07-2005).]

I recently went through the "certification" for one manufacturer of under-floor heating.

Oddly enough, he focuses almost exclusively on the "bathroom" market.

His particular system uses a fabric-like mat, custom made for a particular floor- hole for the toilet and all. The material is also primarily intended for placing under a ceramic tile floor, though there are methods suggested for placement on other floor systems.

Of greatest interest were the extensive testing required at different times during the installation. The testing was centered upon megger tests, with documentation at each step. As you might guess, this is, in part, to find problems before you have to tear the whole thing up.

Alan : ... Oh Boy what a setup . I will be most interested with the results of your new heating system. From what you say I think you have more than ' dun your homework '. I look forward to the photos.Incidently, I think maybe my tiler was like your french man,I would imagine 1.5" of grout would certainly have negated a decent heat output from my bathroom installation ??
Kiwi2

I've installed a few Underfloor heating systems as well as Under-tile systems.
These have invariably been of the electric element type.
Some examples:
Warm Up NZ
And the one I've used a bit:
Devi-Flex

Some pictures of the Devi-flex system, before the concrete slab is poured:

The cold tails and sensor tube:



The element wires correctly fitted to the re-bar:



John (Reno),Is quite right about the testing required before, during and after installation of this sort of system.
It should also be stated that the main cause of failure of an electric underfloor heating system here in New Zealand, is by the failure of the installing electrician to fit Surge Diverters at the switchboard to protect the UF Heat system.
First lightning strike on the mains nearby, will cause the elements to irrepairably fail.

jamesnz,
Thickness of concrete/ mortar/ tiling over the heat source does affect output. Here is a table published by BSRIA. http://www.bsria.co.uk
The correction factor is applied to the design water temperature to achieve the surface temperature. eg. a 4% cf = increase water temperature from 40C to 42C.
Use of electric elements would have the same effect, ie reducing output because power is fixed.

concrete.....correction
over pipe.......factor
25mm, 1".......... 0.92
30mm, 1.25"........0.96
35mm, 1.4".........1.00
40mm, 1.5".........1.04
45mm, 1.75"........1.08
50mm, 2"...........1.12
55mm, 2.2".........1.16
60mm, 2.4".........1.20

So 1.5" of mortar, as opposed to thinset, will degrade output by 4%. It will also increase the inertia of the floor, increasing heat up time.
So I don't think thickness of screed was your problem.
Heat output from a floor depends on the finish material, for instance-
Up to 10mm ceramic tile- no cf needed
Stone, 20mm, cf = 106%
plastic tile, 5mm, cf = 115%
berber carpet, 8mm, cf = 142%
parquet, 10mm, cf = 130%
As you can see, a bathmat will have more effect than a few more tons of cement!

Mike, that looks a good way to support the elements/pipe, off the rebar. I didn't use rebar, just a floated 4" slab, but sectioned the floor off into 5 metre lengths with expansion joints. Pipe is fixed to the insulation boards with clips. Never thought about the lightning problem- worrying for the electric heating owner!

Alan

Thanks alan. Trumpy, Those photos reminded me of back in the mid seventies when I did a similar floor heating installation, one of the first of its type, and the element was copper sheathed,similar to Pyrotenax cabling.
I remember that it worked well but for how long I do not know as I went abroad not long after.

James,
Where are you in New Zealand?.
I hope you aren't from Canterbury, like Kiwi and PaulusGnome and myself.
Be nice to have a North Island perspective on things here.
As long as it isn't Auckland!. Grrr
Believe me mate, there is more than enough collective wisdom to solve the toughest of dilemma's here.
Keep coming back, we like that.

I can't say I've ever installed one of these heating systems, but as I work on new builds only rarely, that's not surprising.

Underfloor heating was popular in the U.K. for a time in the late 1960s/early 1970s, but fell out of favor.

When used with solid concrete floors, it also suffers from the same sort of problem as storage heaters -- Difficult to predict temperature and to control. That's a big point in the English climate where day-to-day weather can be very unpredictable.

Trumpy.... From the good old Bay of Plenty. born in Rotorua and now living in Tauranga, Semi Retired I like to call myself these days.I agree re Auckland but I must admit I got a really good apprenticeship with an all round Electrical Engineering firm in Parnell. Believe it or not Auckland still had DC power generated from Kings Wharf in those ancient days. ( You should have seen the arcing and carry on with the clapper typ contactors in the Lift rooms ! No heating needed !
Kiwi2

Thanks for that James,
So you're in Winston country?
BTW, that Pyro heating element wouldn't have lasted long being buried in concrete.
I'd be even more worried about lightning with a thing like that under the floor, especially considering that the power would have come in by Overhead reticulation back then.
The underfloor heating system would have just about made the ideal Earth mat.