How many underfloor heating spirals in a small room?

[mheden]

I'm having underfloor heating installed in a 2.75m x 2.5m bathroom extension. The plumber has laid out the tubing so that there are plenty of loops but, despite having a two-port manifold, he's laid it as a single spiral, leaving one set of ports on the manifold unused.

I've read somewhere that it's better to lay two spirals and arrange the flow and returns such that the adjacent pipes in the two circuits carry alternately hotter and cooler water in order to provide a more even heat over the whole floor surface.

Is this still the case in a small room, or does the uneven heating effect of a single spiral not apply when the floor area is less than 7 sq m? The total length of heating pipe used is probably about 45m.

Thanks,

Mike

[Ian Stirling]

For such a small floor, I'd say it doesn't matter so much. Unless you're heating from solar, or a heat pump, in which case it'll hurt efficiency a fair bit.

[The Natural Philosopher]

Yeah, its BETTER to do it that way, but anything up to a 100m run of pipe is OK, and the double helix arrangement is a BIT more necessary on

100m runs, but on a 25 meter run or so it makes sod all difference frankly.,

[The Natural Philosopher]

??? Not sure what you are saying there, but it sounds like bollocks anyway..

[Ian Stirling]

To clarify.

When heating with solar or a heat pump, the amount of heat you can get out of the system is more if the loop temperature is lower. Ideally, you want the floor temperature to be as even as possible, as this will mean that there are no uncomfortable hot spots, and the heat transfer to the room is efficient, meaning the output temperature is as near to the room temperature as possible.

[The Natural Philosopher]

Yup. Bollocks as I thought.

Or possibly some hazy concept that you haven't quite grasped and have misunderstood from someone elses dissertations.

[Ian Stirling]

Ok, I'm interested as to which bit you're disagreeing with.

Is it that heat pumps and solar panels produce more energy output for a given energy input the lower the working temperature.

[meow2222]

if your pipe corrodes, cracks etc, your ufh is history. With 2 spirals you can turn one off and continue.

Did you specify a twin spiral? If so, twin spiral is what you paid for. Whether its worth arguing over is another question.

NT

[meow2222]

if your pipe corrodes, cracks etc, your ufh is history. With 2 spirals you can turn one off and continue.

Also your required pumping power will now be around 4x what it was on the plan.

Did you specify a twin spiral? If so, twin spiral is what you paid for. Whether its worth arguing over is another question.

NT

[mheden]

"Did you specify a twin spiral? If so, twin spiral is what you paid for. Whether its worth arguing over is another question."

No, the job wasn't specified at that level of detail so I've no grounds on which to complain. I just assumed that it was accepted 'best practice' to lay two spirals and would be done that way as a matter of course - especially as there is no significant difference in the cost of materials or installation time. But it sounds like it's not a problem, so I'll just leave it as it is.

Mike

[The Natural Philosopher]

Well I am graplling wih my understanding of thermodynamics to find an example of any instance where the above might be true, but simply goving up.

For any heat device, its FAR easier to get MORE useful whatever out of it if the same amount of energy is in a high temperature working fluid, than it its in a low one.

Which is why tepid water engines are not used in powerstations, and cars don't run on them either. High inlet temperature is what ypu need, because the thermodynamics say that the efficiency is then related to inlet minus final exhaust temperature.

That goes for underfloor heating as well, if the inlet temperature is very low, its bloody inefficient as a heat exchanger.

Perhaps that is what you mean, that solar heating that can at best manage a 30C water temp or so, needs vast areas of pipes under the floor to have any heat output at all. And will have a woefully low efficiency anyay..

[The Natural Philosopher]

I don't think teh plastic pies 'corrode' and cracking is unlkiley. Riping or tearing possible..

[Guy King]

The message from [e-mail] contains these words:

That's all one spiral - it goes into the middle with twice the spacing you want, turns round and come back out in the gap.

[Ian Stirling]

Out of, yes. Not in to.

Oops - I see I've been a bit unclear. The 'working temperature' I'm referring to is the output temperature of the solar panel or heat pump.

The coefficient of performance of a heat pump is how much energy you get out for a given amount of energy in. At best, this is around 4-5 in practical devices. The maximum efficiency is set by the carnot limit, which is something like Tinput/Toutput-Tinput.

If the cold side temperature is 273K (0C), then if your output temperature is 68C, at best you get 4 times the energy out than you put in. If however, you drop the temperature to 45C, you can get 6 times the energy out.

So, the lower the input temperature of the underfloor heating, the less electricity it takes to run the heat pump for a given level of warmth. The other side of this is that if the heating of the floor is uneven, it may need a higher loop temperature to reach a given air temperature.

The same is true of (especially plate type) solar panels, where the output may fall to 0 at 85C, and be proportionately greater as the temperature drops.

[Ian Stirling]

Then there is Mr Nailgun.

[mheden]

"That's all one spiral - it goes into the middle with twice the spacing

you want, turns round and come back out in the gap."

Ah - I hadn't thought of that method. In my case though, the plumber has laid the pipe starting on one side of the room running backwards and forwards parallel to the wall where he started and progressing across the floor to the opposite side, so I suppose it isn't a spiral at all. Inevitably with that sort of layout the floor on one side of the room is going to be heated a bit more than the other.

Mike

[Guy King]

The message from [e-mail] contains these words:

The instructions that came with my Hepworth system were very clear - perhaps yours was different. It depends apparently on the sort of floor.

10 is about where you want.

[The Natural Philosopher]

Right. Finally it all makes sense.

Essentially cutting out the complexity what you are saying is that with either of these two you get lots of warm water rather than a little hot water and they work best that way, and therefore you want lots of pipe under the floor to extract that.

Fair enough.

Its an interesting pint, that perhaps one should run bath shower and basin waste under the floors to extract a bit of heat from them :-)

[The Natural Philosopher]

Under 3" of screed? unlikely.

Mr SDS is more likely when fitting a toilet.

I was ultra careful on that one.

[The Natural Philosopher]

Yes, but its no big deal really.

When I did my house, I put self leveling down, and you could see the double spirals as it dried out...i.e. the concrete away from the pipes was a lot colder than that in the vicinity of ANY part of the pipe.

A friend of mine has a small bathroom which apart from a radiator, has unlagged twin CH pipes running under the middle of the (wooden) floor. even with ply over and tiles, that is warm for MOST of the floor.