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General: Low voltage electric UFH??

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    Hi everyone, please could you give me your opinion on whether a 24v UFH system is worth installing / will be cheaper to run than a wet UFH system run on a gas condensing boiler particularly as I am hoping to install solar pv panels in the next few years. I have a victorian terrace house in sw London which we are renovating currently.

    http://www.stepwarmfloor.co.uk/index.htm
  2.  
    A wet system is the most flexible - you could heat the water electrically. If you install a low voltage system you'll need much thicker wire due to the higher current and you won't easily be able to use any other source of energy. Electrical UFH systems have a maximum coefficient of performance (COP) of 1.0; if you use a wet system you could heat the water with devices that give a COP of > 1.

    Paul in Montreal.
    • CommentAuthorjoe.e
    • CommentTimeApr 21st 2008
     
    Better to install a wet system then fit solar hot water panels rather than (or as well as) PV's. It would be much better in terms of saving / first cost ratio, and probably more efficient in terms of the amount of available solar energy that actually ends as heat energy in your house (although I could be wrong about that - anyone?).
    • CommentAuthorCWatters
    • CommentTimeApr 21st 2008 edited
     
    Electric UFH is very unlikely to be cheap to run I would say. See middle col for the price per KWH after "boiler" efficiency is taken into account...

    http://www.nottenergy.com/energy-costs-comparison2

    I don't believe low vs high voltage makes any significant difference to running costs. At the end of the day your house needs a certain amount of power to heat it to compensate for losses through the wall's floor and roof. Power = I x V so if V goes down then I goes up to keep the power the same. Your electricity meter measures power not current so I feel the low vs high voltage issue is a red herring.

    Electric may well be 100% efficient (COP = 1) but that's only true if you ignore the 40-60% losses at the power station. My wet UFH system is 100% efficient if I measure it at the right place :-)

    If capital cost is no object then an Electric powered GSHP seems to be the thing to go for. The COP of 3 for the heat pump more than compensates for the losses at the powerstation. But do calculate the payback period for all options if that's important to you. Anyone know if gas powered GSHP exist?
    • CommentAuthorPete1951
    • CommentTimeApr 21st 2008
     
    I am building a CSH level 6 house and I have chosen Step Warmfloor to provide heat for the wet rooms of the house. I chose it because it’s low voltage DC and as I understand it, it is self regulating in that as the temperature of the film element rises the resistance increases and the output is reduced. It is good at providing low level heat which is fine in a very well insulated property but not so good in an old draughty house. If you are generating from PVs then you can use the output directly via batteries which is more efficient than converting to AC and then back to DC. I hope that is of some help.
    •  
      CommentAuthorMartian
    • CommentTimeApr 22nd 2008
     
    Electrical underfloor heating of any sort is going to drag down your energy efficiency rating badly. I just signed off a block of flats where the developer has insisted on using it, and even with pretty well specified insulation levels, the SAP ratings were very poor.
    This is due to the "fuel factor" weighting of 1.47 in the SAP calculation, which means that when you select electricity as your fuel it is rated as approx 2/3 as efficient as gas. The poor energy assessor was pulling his hair out by the end, from trying to limbo dance under the regulation bar by aggregating the flats into a single figure for the whole block, and even then he only just made it.
    My advice is to avoid until we get nuclear power again.
    • CommentAuthorBluemoon
    • CommentTimeApr 22nd 2008
     
    I rather fancy the stepwarmfloor system and will check it out pricewise. The bathroom amounts to around 5% of the total house area, remainder is oil/rads.
    • CommentAuthorPete1951
    • CommentTimeApr 22nd 2008
     
    Bluemoon, I got estimates for my house and for the bathroom which is about 6.3m2 the coverage would be 5m2. That was for 62.2Watts/m2 and the total cost including transformers etc was £475 + VAT. It would obviously vary from house to house depending upon insulation levels etc.
    • CommentAuthorCWatters
    • CommentTimeApr 23rd 2008
     
    > it is self regulating in that as the temperature of the film element rises the resistance increases and the output is reduced

    Why is that seen as an advantage? Doesn't it mean that the floor temperature is more or less fixed? In which case the output power is fixed? Fine if that matches what the room requires but what hapens if it doesn't? IHow do you turn it up/down if it self regulates? Just curious as I've no direct experience with electric UFH.
    • CommentAuthorPete1951
    • CommentTimeApr 23rd 2008
     
    It is self regulating for safety. The output can be controlled by a thermostat. They carry out heat loss calculations for each room and therefore the maximum wattage required.
    • CommentAuthortony
    • CommentTimeApr 23rd 2008
     
    Electric underfloor resistance heating is the least flexible form of heating, impossible to repair or change without ripping up the floor, so to avoided.
    • CommentAuthorPete1951
    • CommentTimeApr 23rd 2008
     
    Tony, that is true but it could be even worse if problems were encountered with wet UFH with pipes buried in screed. The advantage of low level electric UFH is that in a highly insulated house it provides just enough heat. Wet UFH would produce to much heat and the cost of installing GSHP or boiler unjustifiable.
    • CommentAuthortony
    • CommentTimeApr 23rd 2008
     
    That sounds good -- no boiler I love that idea. Still if it were mine I would run in pipes and steal some heat from the solar hot water to heat with most of the time = free underfloor heating most of the year? Then the rest of the time it will cost the same as the hot water does - hopefully less than direct electric resistance heating. Should gain you some level 6 brownie points too.
    • CommentAuthorPete1951
    • CommentTimeApr 23rd 2008
     
    Tony, I'm hoping the 4kWp PV array will produce enough electricity to cover all my needs. I will be installing a solar hot water system but I hadn't thought of using it for UFH. Something for me to think about.
  15.  
    As Tony said, a wet UFH system is far more flexible than an electric system - you can use anything you like to heat the circulating fluid - doesn't have to be a heatpump. With electric, you're stuck with only one source of energy with a maximum COP of 1.0 - with a wet system you could still use electricity but, if you use a heatpump (doesn't have to be ground source) you could get a COP of 2, 3 or more.

    As for self regulating, that's just marketing hype. Any electric underfloor heating system is self regulating in that it can't provide more heat than being run continuously - the temperature will come into an equilibrium (i.e. self limit) based on the heat loss. All electric systems can be fitted with a thermostat to maintain a certain temperature and the duty cycle can be surprisingly low. I have a very small (3 feet by 3 feet) electric UFH system in my bathroom - "full on" power is only 180W but with the programmable thermostat and timer, it's only on for a small percentage of the day (1st thing at morning, late evening) and once it's at temperature, the duty cycle is very low. The area being served is so small that a wet system makes no sense. For a primary source of heat covering the entire ground floor, say, a wet system is by far the best way to go. A properly zoned wet system will also be thermostatically controlled - we have a system in the new house using Wirsbo equipment which has temperature sensors embedded in the slab in each zone controlling motorized valves.

    Paul in Montreal.
    • CommentAuthorPete1951
    • CommentTimeApr 23rd 2008
     
    I can't justify the use of a heatpump on cost grounds. The only areas of the house to have UFH are the wet rooms and as you say for small areas it makes no sense to use a wet system. I would look at their website for an explanation of their unique system. I have been to see them and it is a very good system for what I want.
    • CommentAuthorBluemoon
    • CommentTimeApr 23rd 2008
     
    My little bathroom is 2.7 x 2.9 metres, taking away the sanitaryware leaves about 4M2. Bearing in mind that the UFH only has to make the ceramic floor comfortable to walk on, as I've made provision for a rad as well; using resistance heating is probably the best option
    • CommentAuthortony
    • CommentTimeApr 23rd 2008
     
    I dont think that resistance heating is ever the best option nor will it ever be.
  19.  
    Posted By: tonyI dont think that resistance heating is ever the best option nor will it ever be.


    Depends on the area - for a small area like a bathroom, it probably has the lowest life cycle cost. The equipment is simple and cheap to install and consumes very little power - even if a wet system is more efficient (using whatever measure you want) because it is more complex the saving may never be realized. The small resistance setup I have in my bathroom uses such a small amount of power that I can't detect it in my religious meter reading (I take readings at least once a week and have done over the past three years to correlate climatic conditions with energy usage). I agree with you, though, for the primary heating source for a large area.

    Paul in Montreal.
    • CommentAuthorCWatters
    • CommentTimeApr 24th 2008
     
    I think I'd agree that for a bathroom something like electric UFH in conjunction with a towel rad is probably a good option. I wouldn't use it for for a whole house though.
  21.  
    Thanks for all your opinions. The company that supply the flooring claim that it is more efficient and cheaper to run than a wet system see quote below. Do you think that their data can be acurate?

    'STEP Warmfloor™ only uses on average 50 Watts/m² whereas Water underfloor heating products use between 70 Watts/m² and 85 Watts/m² and Electric Cable products can use between 150 Watts/m² and 300 Watts/m². We therefore use less energy to achieve the British and European Standard for floor temperatures of 28ºC.'
  22.  
    Methinks their claim is nonsense. Floor temperature is determined by how much power is supplied versus how much is lost as heat and there's no difference whatsoever in the source of the heat, everything else being equal. It's the "on average" bit that's the clue since this implies it's the thermostatic control that gives this power usage. The same would also be true for a high voltage system or a wet system "on average". Sounds like another case for the ASA to look at!

    Paul in Montreal.
    • CommentAuthorPete1951
    • CommentTimeApr 24th 2008
     
    It has an inbuilt thermostatic control as I described before. The temperature of the floor cannot go above 28C because the resistance increases with temperature and cuts off at around 28C. It is possible to control the room temperature in the normal way with a room thermostat. It is not recommended to have a high temperature wet UFH system and you are relying on the water temperature thermostat to control that.
    • CommentAuthorCWatters
    • CommentTimeApr 25th 2008 edited
     
    Anyone here do O Level Physics? Anyone remember the law of Conservation of Energy?

    Basically this means that if the air in your house is at a constant temperature the energy coming in and going out will be equal. In other words the power supplied by your heating will exactly match that being lost through the walls, floor and windows etc.

    In _theory_ this means that it doesnt matter what heating system you use...if the house is leaking 10KW it needs 10KW of heating or it will get colder.

    In practice there are some differences, for example UFH is claimed to heat the bit of the room that people are in (the bottom 6 foot!) allowing the average air temperature overall to be cooler for the same level of comfort. I'm not particularly convinced on that one.

    If the floor is well insulated very little will escape downwards and most will go into the room. So I'm with Paul on this. The source of the heat should make very little difference.

    > It is not recommended to have a high temperature wet UFH system and you are relying on the water
    > temperature thermostat to control that.

    In my wet system I reckon three things would probably have to fail.

    The stat on the thermal store.
    The mixer on the manifold.
    The room stat.

    Perhaps you could obtain government statistics on the number of people a year who burn their feet due to faulty UFH?
    • CommentAuthortony
    • CommentTimeApr 25th 2008
     
    You wont find any of them there aren't any cases of that yet.
    • CommentAuthorPete1951
    • CommentTimeApr 25th 2008
     
    CWatters, I'm sorry your house leaks so much heat that you need a wet UFH system. As I explained above I am using an electric UFH system because it is only going to be used in the wet rooms and it isn't justifiable on cost grounds to fit a wet system. What you say in simple terms is exactly correct. In order to maintain a constant temperature then heat needs to be put in to balance what is lost. It is how the UFH heat is supplied which varies. If you use electric cables which have a small cross sectional area then those cables have to be very hot in order for the heat to be dissipated. It is dependant on the how efficiently the heat is conducted away from the cable and what the cable is bedded in. It would be possible for the floor to have a lot of hot and cold spots. The same applies to wet systems which is why some use aluminium plates to spread the heat. Obviously if the whole area is covered with a thin film then it doesn't have the inherent problem that the other two cases have. The system reacts more quickly, heats more evenly and is therefore more comfortable. It is easier to insulate directly under thin film electric enabling a more responsive system and preventing less heat from going where you don't want it to. How that is possible with electric cables I don't know. The only way I know with water pipes is to use aluminium spreader plates. I have nothing to do with StepWarmfloor except that I have compared their system with all the other types and have come to the conclusion that it is the best system for my particular needs.

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