Underfloor heating question

We've just had an extension built and we specified wet underfloor heating for it (by Osma/Wavin). The plumber's installed it, the manifold gets warm, as do the flow and return pipes, but they're not nearly as hot as the rads, and I can barely detect any heat emerging from the floor at all even if I turn the wall thermostat up to 25C. I've never seen the ambient temp in the room go higher than 17C even with the boiler temp full blast at about 80C.

On top of the heating boards is 17mm plywood, some thin underlay (recommended by the manufacturers for UFH) and then B&Q click-to-fit wooden boards (also said on the pack that they would be OK for UFH).

We've got a beefy combi boiler (15 lit/m) and the rest of the heating in the house is fine. I balanced the rads a couple of weeks ago to see if that might help, but it didn't.

The plumber says it's working, and that UFH is just "very subtle." I agree there is some heat going into the floor, and I realise the floor won't be as hot as a radiator or anything, but I was expecting at least *discernible* heat from the floor when walking on it in bare feet. Is that not the case?

If anyone has any tips, I'd be grateful. I know next to nothing about plumbing though.

Reply to
BlueJohn
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The water from the boiler is cooled before it goes into your manifold and on to heat the floor. On ours, there is a thermostat near the manifold (looks a bit like the head of a thermostatic valve) which sets the temperature of the water for the UFH. Ours is currently set on the lowest setting (about 35 degrees I think). If you put you hand on the manifold, it doesn't feel that warm - it is just below body temp! For our room, Polypipe suggested a temp of 52 degrees. Maybe yours is set too low?

In terms of heat output, when the pipes were laid, do you know how close together they were? Obviously the closer they are the more heat you will get out from them.

Also, how long have you had the heating on for? I have the pipes laid in a concrete floor and it takes a good day (or longer) for it to heat up with the UFH on continuously.

It is a bit deceiving as the heat is very subtle it is nothing like a scorching radiator but if your room doesn't get above 17 degrees, something doesn't seem right.

The only other thing I can think of is that the manifold flow rates can be adjusted for each circuit. Might be worth checking with the manufacturer to see what they should be set on. Polypipe do a free technical spec for your room and give you these figures, pipe layout, temp etc.

Reply to
Lee Nowell

BlueJohn wibbled on Saturday 09 January 2010 15:46

In the UFH installations I've seen, the floor was comfortably warm.

There is a significant thermal lag though. How long have you had it running?

The other thing - has the mixer valve been adjusted to the correct setting?

Reply to
Tim W

Just but there is a very long thermal lag in UFH. Lag measured in days rather than hours and you have lots of "insulation" over the floor.

Sister has under floor heating covered by stone flags, they aren't cold and you don't get cold feet if you stand still for any length of time in bare feet but "warm" they are not.

The feed to the under floor pipe work will be thermostatically controlled somewhere to around 40C if that, so winding the boiler output up to 80C won't have any real affect on the UFH.

Reply to
Dave Liquorice

Ours is set to maximum. I've no idea what temp it's running at, but he manifold feels almost at medium-ish radiator heat.

They were the standard width from the manufacturers as they were set in to insulated boards - about 6 inches apart.

It turns on in the morning at about 7:30, it goes off at about

11:00pm. Never have I seen the thermostat on the wall (the one that controls the UFH) rise above 17C, and it's set to it's maximum of 25C.

If "subtle" means "running it for 12 hours at apparently maximum output and we feel next to no warmth" then yes, it's subtle alrighty. About the most subtle use of =A31,800 I've ever spent in fact.

BTW this is wet (not electric) UFH and it's in bats suspended between wooden joists (not in concrete/stone).

I think I'm going to have to give them a call.

Thanks for your help though.

Reply to
BlueJohn

Are there no markings on whatever control sets it? Ours is clearly marked, from 35C to 60C, and we run it in the 40-45C range.

I've not measured the maximum rate-of-change of room temperature that our system can achieve but I'd guess it's in the region of 0.5C per hour even in the current weather conditions. Ours is a modern, well- insulated house.

So you have little or no thermal storage in the floor itself? That's very different from our system (wet UFH in concrete screed) so I'm not sure how it ought to behave.

Richard.

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Reply to
Richard Russell

BlueJohn wibbled on Saturday 09 January 2010 17:57

Normally you wouldn't want it too hot in a solid floor as you risk cracking the screed - but as yours is in a suspended floor, there's less risk.

Are they set into heat spreader plates?

The solid floor UFH is normally ready reckoned to give about 100W/m2 but I have no idea what a suspended floor system is like.

What have you got between the pipes and your feet? Wooden boards and carpet for example?

Reply to
Tim W

Probably a daft question but does your UFH manifold have a pump on it? How many circuits do you have?

Reply to
...

Hello,

I recently installed wet UFH in a 10m2 conservatory, using a kit bought online. I had way too much pipe so laid it at 100mm centres, with 2 circuits, on top of 4" foil-backed foam insulation, and then laid 75mm screed over the top, with porcelain tiles over that. It's on it's own zone with it's own programmable stat, fed from a 10 year old back boiler which wasn't sized with this in mind! The mixer valve is set to max, and the floor takes a few hours to get warm after switch-on, but once it is stays around 35ºC all day (according to my IR thermometer), maintaining the conservatory at 20ºC. The boiler seems to be coping fine, once the conservatory is up to temperature it gets a few short top-ups every hour. It's become the dog's favourite room to lay in...!

In your system it may be that the underlay and the wood floor are acting as insulation to the hot pipes.

Alan.

Reply to
AlanD

they shouldn't - indeed must not - be: should be about 43C flow temp, obviously lower on the return.

, and I can barely detect any heat emerging

that's the hot water output to taps which is quite irrelevant to heating radiators or UFH

and the rest of the heating in

Yes, it should be comfortably warm, and so should the room if the system has been properly designed and installed.

The keys are how hot the water going into the UFH is, how hot the water coming out of it is, how fast the water is flowing through the UFH, and how much heat is being lost downwards from the floor. You can measure the first two (flow and return temps). The flow rate may be indicated by a little gauge on the manifold but otherwise is hard to measure. How much heat is being lost downwards depends on the construction of the floor.

Did you see (and better still, photograph) the floor being constructed?

Reply to
YAPH

Something wrong there then. Should be no more than an hour or two even with a thickish concrete slab.

Reply to
YAPH

Oh the floor may be warn in an hour, but the actual rate at which it transfers to the rooms is pretty low.

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Reply to
The Natural Philosopher

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember BlueJohn saying something like:

Probably too much insulation on top of the pipes, in the form of a floor, and I suspect there were no heat spreader plates in the system. If the pipes were fitted straight into a moulded foam carrier, the heat has to rise just above the pipes and can't spread out very much. This is also assuming the foam pipe trays were well insulated underneath, as heat robbing undersides might be contributing to the problem. How thick are these foam trays, and were they fitted straight on top of concrete, or in a joist gap and if a joist gap, was the gap filled with rockwool? How much foam is under the pipe when it is fitted in the tray?

Reply to
Grimly Curmudgeon

We have underfloor in concrete downstairs and aluminium heat spreaders between joists upstairs. Downstairs you feel the warm floor more than upstairs. If you put a cushion on the floor for about 30 mins how warm does it feel underneath it?

You can use the cushion and thermometer to map the heat distribution quite accurately by the way. Simply place the cushion on the floor for a fixed time (about 10 - 45 minutes, the shorter the better) and note the temperature on a thermometer placed between the floor and the cushion. Repeat in a grid pattern across the whole room.

Reply to
Peter Parry

Fair enough: depends on relative output of floor and heatloss of room. But the OP was complaining that the floor didn't even get warm.

Reply to
John Stumbles

it may well not if the heating input is marginal and the room is cold.

My open floor spaces are not hugely above ambient at full chat on the heating. It gets very warm under the sofa though. Everywhere else the heat goes straight into the air by and large.

I am runing at 100W/sq meter max. That is NOT a lot of heat output.

Th e pipe runs through the corridors at up to 6 times that, do get perceptibly warm. Feline resting area, that.

Reply to
The Natural Philosopher

It is what it is. The $64K Q is what's the heatloss of the room?

Reply to
YAPH

"BlueJohn" wrote in message news: snipped-for-privacy@o28g2000yqh.googlegroups.com...

Well for sure something's wrong as the room isn't up to the design temperature which will likely be 21C with an outside air temp of -1C, ie. a

22C difference. So either the design is wrong, or the installation is wrong, or the setup or the usage.

Presumably your system designer did heat loss sums for the extension. You don't tell us much about it but let's assume it's a fairly normal room built to today's insulation standards, not stupidly long and thin or uninsulated etc. UFCH product from the major players claims about 100W/m2 which for most normal rooms works well. UFCH is getting a bit like lego now, buy the standard bits, plug them together and it simply works, so it's hard to see how the designer could have gone very wrong.

Your builder had more latitude to mess up, often they are not experienced in exotic things like UFCH. The spacing between the pipes is important as too wide a spacing limits heat output. Screed depth on the solid floor is also critical as too thick a screed makes the system very slow to respond, although it should still get there in the end. That said, provided he used the standard bits and bothered to read the instructions it should simply work.

Setup is something which is often ignored as it takes time and is a bit fiddly, something that builders are often not good at. There will be a thermostat on the manifold which controls the water flow temperature, it does this by mixing colder return water from the UFCH pipes with hotter water from the boiler. Obviously this has a major effect on the temperature of the floors. On my Polypipe system the thermostat is a red knob which clicks 1 notch per degree, on the OSMA I think (but I'm not sure) it looks like a horizontally mounted radiator valve, a big white thing with numbers and a pointer on it. Try turning yours to a hotter setting, the flow pipe should get hot. It should get to a temperature you don't really want to hold on to for very long (say 65C-ish). Each individual heating circuit (one per room usually) will have a way of restricting its flow in order to balance them. Its quite possible they've been turned down too much but I don't know how its done on the OSMA. From memory that system lacks any flow indicators so you may end up having to measure the temperature of the flow and return of each circuit with a thermometer (or hand), they should be roughly 11C different. Mind you for your purposes simply opening them up would at least prove it is capable of heating the rooms in the short term. The UFCH is pumped separately form the main heating so I suppose the pump could be faulty or indeed it could be adjustable and set too low. There will also be a couple of isolating valves which could restrict the flow if they weren't open properly. Wow, I didn't realise there was so much potential to mess up.

Usage of UFCH is a little different than radiator circuits in that it needs a little while to heat up. People have suggested it needs all day, not on a properly installed system it shouldn't. Mine gets the screed noticeably warm in about 30 minutes and fully up to temperature in about 60. The wooden floors are quicker.

Hope those ramblings help.

Reply to
Calvin Sambrook

Reply to
Doctor Drivel

What's perhaps not so readily appreciated is that the length of the loops of pipe making up the UFH circuits is important too. For example in a recent installation one area was about 16m^2, and I needed 83 Watts/m^2 to heat it. With a 5C drop between flow and return and I'd have had needed almost 1 bar to pump the required amount of water around a single pipe loop - far higher than a typical central heating circulator ("pump") can manage. However by splitting it into two loops it only requires about 0.16 bar.

Reply to
YAPH

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