Electric underfloor heating

[Espain]

I originally posted this under Introductions (thanks for your comments AC_Hacker) but realise it should have been under this section, what I asked was:

I need advise on is underfloor heating. I have moved into a house which isn't quite finished one of the things being the underfloor heating, unfortunately it is electric rather than water and basically it is just 2 heavy duty cables coming out of the floor in each room at the moment. I have been told that the whole system is 12kw range is from 800w to 1500w per room so I certainly have no intention of connecting it to the grid, it does have a seperate circut board to everything else and each room has a termastat. The location is both sunny and windy, so I want to make use of both. Can I connect 24v staight to them or would I need to go through an inverter? If I feed 500w into a 800w room would it still heat but only be warm as opposed to hot? Any advise or ideas would be greatly appeciated.

[Xringer]

Since your power there is 220-240Vac, the cables coming into each room are likely
made to carry that voltage.

Are the resistance heating elements already installed into the flooring?

[Ryland]

You need to match the voltage as close as possible to what the resistance wire was designed for, this issue comes up in electric cars when people have a 120v resistance heater and want to run it off 72 or 96v and find that it does not put out nearly as much heat as it was designed for.
You could of course use it as a dump load if you have a wind generator that is the correct voltage, but for daily heating a heat pump is going to be the way to go.

[Espain]

Yes, unfortunately they are already installed, concreted in and tiled over and as you say we are 220-240v. The cables are marked "Cold cable PV 1.5mm2".

I agree a heat pump is the way to go but I way hoping to make some use of what is here as secondary heating. As a load dump can I get a 240v turbine or if it was 24v could I put it through an inverter?

Does anyone know what would happen if I ran 24v solar panels directly into the 240v cables, would it be the same as the electric cars and be less efficient or would it damage damage the panels? Is 10 x 24v panels in series possible or is that a recipe for disaster?

Thanks
Nick

[Ryland]

Most panels are good for being wired up in high voltage strings, it should say on the back of your panels how high you can go.
The voltage of your wind turbine is going to vary based off make and model.
From what I remember I believe that with resistance wire if you have half the voltage you get a quarter of the output, so running at 1/10th the voltage and you would never notice the wire get warm, but yes, you can use an inverter to run the floor, it's an expensive way to go but if you are going to have a good inverter anyway then why not? an inverter is a good idea because it allows you to use efficient modern appliances and the energy the inverter uses it made up for by reduced line loss in the rest of the house.

[Xringer]

The Cold Cable is the transfer cable. It's not effected by heat from the heating element cable.
At least that's what this spec says.
www.ohmewatt.be/files/Heating_cable.pdf

If you wanted to heat those wires in your floors, using DC voltage from PV, it should be possible.
But very very inefficient.

I've got a hot water heater running on my 800w PV array.
If it was working perfectly (it's not), it would only output 2729.7 BTUh.

The array is four 200w panels. On a non-tracking mount.
http://i46.photobucket.com/albums/f1...solarfarm2.jpg

Those panels are connected in series, so their output is high voltage DC (About 115vdc).
Since each panel can only put out about 7 amps, I can use a smaller gauge wire to bring the power into the house.
The heater load is about 10 ohms.
I use a 10A fuse, which will never be needed at 7 amps..
Even if I shorted the load to 0.01 ohms.

Since your heating element wires are buried in the floor, you will need to
check their resistance with a meter.
Once you know what their resistance is, you can use Ohm's Law to calculate
approximately how much (PV) voltage will be needed to heat them up.
(It's not exact, since the resistance changes as they warm up).

If the heater resistance in all the rooms is the same, that will make your work easier.
1500w / 220 volts = ~7A. That means eight CS6P-200 PVs (like mine), would
be a pretty good match to a 220v (31 ohm) heating element.

The problem is mismatching the panel voltage to the load.
If the load resistance is too low, (like mine) it will try to pull max amps (about 7A)
but, at a lower voltage. Volts x Amps = Watts, so a lower voltage means less watts/heat.
Of course, dropping the voltage into a load, means the load will be drawing
less amps..
So, I can't get my full 7 amps. The setup runs at 4 to 6 amps most of the time.


Two of your 31 ohm rooms in parallel would be equal to 15.5 ohms.
That would draw the max current of 7A, but the voltage out of the panels would drop.
If it dropped down around 100v, then you have 700w, instead of your 1500w.
The best way would be to buy another 8 panels for the second room..

If your had 4 rooms that were each 31 ohms, their loads would be arranged
in series-parallel to look like 31 ohms total.
The 1600watts of power (5459 BTUh) would be divided between them.
But 400w each is not much.. 1365 BTUh per room..

You might not have the idea load resistance, so matching it might be easier
if you had an impedance matcher.
I'm in the process if trying to build one for my hotwater heater now..
(I'm waiting for some new parts from China).

Anyways, read some solar panel specs and learn about maximum
power points (at what voltage the panel delivers it's rated watts),
and you will get a better idea of the challenge.

If you have the money & roof space for the PV, and don't need a lot of BTUhs,
PV heating might be one way to go..

Forget the heating wires??
Another way would be to add mini-split heat-cool units to the house
and use the grid or PV to run them. They are highly efficient..

PS:
The installed thermostats might be adaptable to work with DC, depending on how they
control the AC to the loads. It might be as simple as adding some relays or SSR relays.

[Ryland]

Even heating the smallest rooms 800 watts worth of resistance wire is going to take a lot of solar panels, of course you need to know how many sun hours you get per day (hours of full sunlight used to measure solar panel output) and you need to know how many hours per day you need the in floor heat to be on, or how much energy it takes to heat your house, but if it's your only heat source then I suspect it's a few hours per day at the very least, most likely it will end up being the same number if not more hours of demand for heat then hours of sun, so for that 800watt room figure $4,000 to $6,000 worth of solar panels, or more depending on their total installed cost and that is for less in floor heat then I have in my bath room floor.
At this point heat pumps are looking really really cheap, again the infloor heat might be a good dump load for a really windy day with a wind turbine, but not for every day use.

[Espain]

I'm convinced!! I am going to forget the underfloor heating. I was looking at a 1.4kw heat pump (Fan World) which returns 5.5 kw of heat for the equivilent of $450, seems like good value. I have a 2kw panel array which is grid tied, so will be dependant on the grid overnight. I'm waiting for a 1kw turbine which should help, unless I go for batteries & inverter etc.

Thanks for your advice guys.

[Xringer]

Good move IMHO. I hope the heat pump works out for you. But, I have to say,
a mini-split (air source heat pump) with inverter technology is a lot
more efficient that a standard heat pump. They cost more, but their
low power use makes them worth the price.

I wish you luck with the wind power. Most people live in homes that are located
in areas where wind power won't work very well. Mostly, it's a waste of money.
But, if you are on top of a good hill, no trees or buildings around,
(and a strong wind blows all the time), you might do good..

I have to admit, those turbines look neat, but they need to be spinning..

[Ryland]

Originally Posted by Xringer

But, if you are on top of a good hill, no trees or buildings around,
(and a strong wind blows all the time), you might do good..

I have to admit, those turbines look neat, but they need to be spinning..

Even being "on top of a hill" is not always good enough, that is why there are wind maps and people who are trained to site wind turbines and can estimate the yearly power output within 5% most of the time, allowing you to make sure it's an investment and not a gamble, there is to much in the news about tax money being spent on failed wind projects while we ignore that last year more renewable energy producing generation was installed then fossil fuel power generation and because most states now require that a site assessment is done before any money is spent, these are going to be adding to those wind farms that you never hear about.