Underfloor heating & engineered flooring thickness

Yeah, well, for wet UFH it works very well. As long as you have a large floor area devoted to heating, and much better insulation under the pipes than above, then of course the heat has to come upwards. I have three rooms working this way, and no complaints at all. It's just a case of correct design.

DetlefSchmitz said:

Yeah, well, for wet UFH it works very well. As long as you have a large floor area devoted to heating, and much better insulation under the pipes than above, then of course the heat has to come upwards. I have three rooms working this way, and no complaints at all. It's just a case of correct design.

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Yes, in the situation you describe, the rate of transfer of heat would be much greater upwards, but the rate of that transfer, even upwards, would obviously be dependent on the thickness and nature of the material above the pipes.

In the fullness of time, thermal equilibrium would obviously be achieved, but that could be a long period of time if there were appreciable amount of relatively thermally-insulating material above the pipe - and I would have thought that 20mm of wood would qualify as that ... and all that before one considers how long it takes for a warm floor (presumably not a lot hotter than the desired air temp in the room) to heat up the room above it. Hence my suspicion (which, of course, may be wrong) that, with a lot of 'insulating' material (like wood) above the pipes, the heat-up times could well be such that it might well need 24/7 operation to be a practical form of room heating

However, if it works for you, then that's great. I wonder what is above your pipes?

Kind Regards, John

As I said, wood. About 20mm. I think there are too many gut feelings on here. People need to do the maths. (I also have two UFH-heated rooms with tiles, which are of course also excellent in performance.)

DetlefSchmitz said:

As I said, wood. About 20mm.

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Fair enough. I guess the post in which you told us that must have got deleted!

DetlefSchmitz said:

I think there are too many gut feelings on here. People need to do the maths. (I also have two UFH-heated rooms with tiles, which are of course also excellent in performance.)

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It's not entirely gut feelings - I've certainly known a good few people who have been disappointed with wet (as well as, more understandably, electric) UFH. However, if/when I have a little time, I'll try to rise to your challenge!

Kind Regards, John

JohnW2 said:

Fair enough. I guess the post in which you told us that must have got deleted!

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I don't think so...

JohnW2 said:

... I would be considering the effect of putting 20mm of wood/wood products between the heat source (whatever) and the room I was hoping to heat.

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DetlefSchmitz (replying to JohnW2) said:

Yeah, well, for wet UFH it works very well.

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Can't speak for electric, I imagine its only really good for a warm feeling on tiles. But a friend of mine has whole house underfloor (wet) heating with the heat supplied from a geothermal system and the place is like an oven! Engineered wooden floors as well. But its on pretty much 24/7 because it takes a couple of days to actually warm up.

The floor doesn't feel particularly warm - it doesn't seem to emit a lot of heat per sq/m - but there is a lot of sq/m of it. The house is very new with a lot of insulation which helps of course.

DetlefSchmitz said:

I don't think so...

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OK, fair enough. I didn't realise that the first sentence of that post of yours referred to your own installation.

Kind Regards, John

ub7rm said:

But a friend of mine has whole house underfloor (wet) heating with the heat supplied from a geothermal system and the place is like an oven! Engineered wooden floors as well. But its on pretty much 24/7 because it takes a couple of days to actually warm up.

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Indeed, much as I said.

One of my cousins had a large house extensively refurbed (virtually rebuilt!) two or three years ago, and it is now primarily heated with wet UFH, which is also extremely good at keeping the house hot. However, he has to keep it on more-or-less 24/7 throughout the winter since, just as you say, he says that it can take a couple of days to heat up the house from cold. He also has blown hot air available to heat the place up quickly at times when the UFH has not been on (at all, or for long).

His refurbed place is pretty well insulated, but I'm not sure of the economics of having to run the UFH continuously during winter.

Kind Regards, John

DetlefSchmitz said:

I think there are too many gut feelings on here. People need to do the maths.

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ub7rm said:

The floor doesn't feel particularly warm - it doesn't seem to emit a lot of heat per sq/m - but there is a lot of sq/m of it.

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OK, some quick preliminary maths ....

Assume a 'typical' thermal conductivity of dry wood of 0.07 W /m /°C, wood 20 mm (0.02m) thick, floor surface temp 25 °C and heating pipe temp 70 °C. Also assume that the entire lower surface of the wood is in perfect thermal contact with the heating pipes (probably far from the truth in practice). Then:

Heat transfer = [0.07 * (70-25)] /0.02 W/m² = 157.5 W/m²

So, for a 20 m² room, a rate of heat transfer of 3,150W, which I think (but may be wrong) is a around half of what would be regarded as the radiator output requirement for a 20m² (x 2.5m high) room in an 'averagely'-well insulated house. If the entire under-surface of the wood were not in good thermal contact with the pipes, the heat transfer could be considerably lower than these figures.

Is that roughly right? Are my assumptions reasonable?

Kind Regards, John

My first house had hot air central heating, to anyone visiting it would have seemed great, but to us living there moving the air around resulted in higher losses, and a very dry house, we would put trays of water in the air intake.

Point is visiting a house toasty warm with underfloor electric central heating does not mean the system works well. There is a chemical version which does have some self regulation and combined with heat recovery units and very good insulation in the house I am sure it can work well in a property kept warm 24/7. It is however as I found out not easy to install. The digging up of the floor and replacing it with polystyrene blocks then wood then the heating mat then a surface means the floor is now very well insulated. So even with the underfloor heating off the floor never feels cold.

The big question is if you fitted all that insulation in the floor without the heater what would the rooms feel like then? My houses the central heating has always done a reasonable job. Mothers house however was very different, the design was poor from the day it was built, we all know open fires use the air in the room for combustion, so that air needs replacing in a way where it will not cause a draft, some where it needs a vent near to the fire, otherwise it will suck air from under the doors, hence the use of high backed chairs.

So job one when central heating is fitted is to block up many of the vents. However we still need some fresh air, but walk into a house which is near hermetically sealed and you will not notice the lack of fresh air, it is only when you live in the house you notice the problems. I notice it with father-in-laws house compared with mine, next door but one so same design to start with, but he has triple glazing, sealed front door, two sealed doors to rear, never opens a window, and until this year ran a open flue gas fire, and still cooks on gas. His humidity is around 80% compared with ours at around 65% we have got into a habit of leaving his doors open to vent house.

His house reminds me of living in Hong Kong. And I have noticed homes with only underfloor heating are the same, they can't allow air changes as the floors are not up to reheating air after the air change, step one is always fit a heat recovery unit, only then can you get the air changes required and keep the house warm.

Note I say only underfloor heating, add a few fan heaters be it water or electric in the plinth in kitchen, and add a towel rail, or a inferred heater in the bathroom and you can supplement the heating enough. For a retired person at home all day, or young family again at home all day great. But in the main we want to save money by turning off the heating while not at home, this is just not an option with underfloor heating.

If the UFH has half the output of radiators, then it makes sense that it would take twice as long to warm up. Where the argument falls down is to say that leaving the heating on all day is somehow going to be hugely costly. Afterall, it's on all day, but at half the power input. Therefore the only additional cost is the losses during the part of the day when the heating would have been off, but is on. But of course during that time when the rads are given up heat and gone cold, the higher thermal mass of the concrete floor (with insulation below it...) is still giving up heat and upon your return hasn't cooled down anything like as much as the house WOULD had done if the rads were in service. So needs less time to warm it up to "useful heat emitting temperature" again. With the invention of night fall back, there temperature can be set back by 2 degs and the UFH is then giving up heat long in to the cold winter night.

Nozzle

Nozzle said:

If the UFH has half the output of radiators, then it makes sense that it would take twice as long to warm up.

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It's really far more complicated than you suggest, and my quick calculations only scratched the surface.

The 'heating up time' is only a small part of the story - the fact that the floor surface eventually comes to a constant temperature of 25° (my assumption) in no way necessarily means that the room will be adequately heated.

When, after the 'heating up', thermal equilibrium is eventually achieved, the equilibrium temperature of (the air in) the room will depend upon the rate at which heat is being put into the room and the rate at which heat is being lost from the room (through walls, doors, windows ceilings etc.) - regardless of how that heat is supplied.

Hence, if a radiator had to have a certain heat output (rate of heat transfer) in order to heat the room adequately in the 'equilibrium state', then UFH which supplied only half of rate of heat transfer would not heat the room adequately, even when in that equilibrium state. That's simple physics.

Nozzle said:

Where the argument falls down is to say that leaving the heating on all day is somehow going to be hugely costly. Afterall, it's on all day, but at half the power input. Therefore the only additional cost is the losses during the part of the day when the heating would have been off, but is on.

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As above, if, in the equilibrium ('heated up') state, the UFH cannot provide heat at the rate that a radiator would have to in order to heat the room adequately, then the room would not be heated adequately by UFH, even in that equilibrium state. The fact that UFH takes a long time to get to that equilibrium state merely adds insult to that injury.

The Laws of Physics being what they are, and assuming 'losses' are much the same in all cases, maintaining a certain room temp for 24/7 will require the same amount of energy (hence cost) over any period of time, regardless of how that heat is supplied.

Nozzle said:

But of course during that time when the rads are given up heat and gone cold, the higher thermal mass of the concrete floor (with insulation below it...) is still giving up heat and upon your return hasn't cooled down anything like as much as the house WOULD had done if the rads were in service.

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Yes, if, after a period during which the UFH has been off, the surface temp of the floor remains a bit above 'ambient' temp by virtue of stored heat released by the concrete, that would to some extent speed up the 'heating up' process, but that would not have any effect on the adequacy/inadequate of the UFH to heat the room. I haven't a clue as to how much heat the concrete might store (less the inevitable 'losses'), but I suspect this would not be a major consideration.

Kind Regards, John

It can be a big enough consideration when the weather is unpredictable, or above-average acute temperature changes occur, and you can't suddenly turn the heating on or off.

It was actually the allegedly positive (as proposed by Nozzle), rather than negative, aspects of having heat stored in a concrete floor that I doubted would be a major consideration, and I suspect that is equally true of the negative aspects. The need for a meteorological crystal ball obviously applies to any sort of 'storage heating', but if not a lot of heat is being stored, and if it can only get slowly from the 'store' into the room, I continue to doubt that it's likely to be a major consideration.

I get various feedback from customers that have underfloor heating.

Retro fitted systems on ground floor, are oftrn expensive to run. Not too bad as under tile mats but primary source heat no good. I would say primary source heat is only suitable when on 100mm of celetex and in screed.

I had a customer with underfloor heating in a conservatory and he kept telling me he suffered condensation on the roof. Eventuakky we discovered the UFH didnt heat the air enough, so the central heating was extebded and a couple of rads installed. Problem solved.

Wet underfloor heating seems very effective in orangeries, low lkad on system, high insulation and good level of thermal mass in screed, which also benefits from solar gain on sunny days.

I think modern houses that have insulation on the inside suffer from a lack of thermal store in floors and walls, so residual heat doesnt get retained. My guess is the efficiency of heating does depend on lifestyle, maybe if you are home a lot, a house with a significant thermal store would be mor eefficient.