Underfloor heating temperature

[Jamieharnett]

In the summer I’ve laid geocell and installed underfloor heating with a lime screed on top around 70-100mm. Then limestone tiles on top laid with adhere Cal.

I’ve never lived with underfloor heating before and have done the usual turning the heating on 2-3 hours before you would normally however the house doesn’t seem to be getting warm as such, I’ve just noticed on the blender valve the water is set to 40c is this correct? Do I need to leave the on cycles on for longer?

Kind regards

Jamie

 

[88v8]

From what I've heard, ufh is not really suitable a standalone means of space heating. It provides baseline background heat and needs to be supplemented by spot heat such as rads.

It's like a big storage heater. One can't switch storage heaters on and off. Due to the heat mass, the rate of temperature response is too slow.
That's why it's not suitable as standalone space heating - it needs to be left on all the time.

Hopefully someone more expert will be along soon.

Ivor

 

[MikeG]

Forty degrees is higher than normal, by a few degrees. Underfloor heating is a slow system. Mine takes some 5 hours to raise the house to its maximum temperature (although the heating is only on for a part of that period), and it was over 6 hours in my last house. I set it to come on at 1pm to have the house warm for the evening. It will go off after an hour or two, but the temperature keeps rising long after the heating has gone off. It's all about thermal mass. DON'T mess with the temperature of the circulating water.

This, incidentally, is why I always advise my clients to avoid mixing underfloor heating and radiators on the same floor level. If you can't do it everywhere downstairs, it's best not to do it at all (and of course, it simply doesn't work upstairs).

 

[MikeG]

From what I've heard, ufh is not really suitable a standalone means of space heating. It provides baseline background heat and needs to be supplemented by spot heat such as rads....

Sorry Ivor, but this is very wrong, and it's a mistake that is repeated here from time to time. It absolutely does not need to be left on all the time, either.

The confusion is that there are two things called underfloor heating. One of them is the electric blanket laid under bathroom tiles, and it is this which really shouldn't be thought of as space heating, simply as a way of warming the floor. Warm water underfloor heating, set in screed above insulation, is an extremely effective way of heating a house. It creates a beautifully even heat throughout the ground floor, and can raise the temperature to whatever a radiator system can (although houses with UFH tend to be heated to somewhat lower temperatures because you don't get the temperature differentials across a room creating draughts).

My bona fides: I've lived in houses heated by UFH for about 25 years now, and as an architect, I've specified them in many dozens of houses over the years. I really wouldn't ever live in a house heated any other way, and most of my clients who've lived with UFH would say exactly the same thing.

 

[Flyfisher]

I don't really think of UFH as significantly different to any other central heating system, as far as the basic principle of having heated surfaces radiating heat are concerned.

Most people (not PPUKers obviously :wink: ) have become used CH systems based on steel radiators and high water temperatures and there are many online design guides and calculators for determining the required size of radiators for a given room size and a target indoor temperature. All fine and good.

Now consider using cast iron radiators instead of steel ones. For a given size, cast iron rads are far heavier because they contain more metal, which means they have a greater thermal mass, which means they are less responsive than steel radiators. Thus, they take longer to heat up but also take longer to cool down. But if they are fed with the same water temperature as a steel radiator they will be just as effective in heating the room space, albeit it a slight time lag, which is easily adjusted for by switching on the heating a little earlier than with steel radiators and also switching it off a little earlier. But the amount of heat delivered from the boiler (or whatever) into the room in question will be the same whether steel or cast iron radiators are used. So far so good.

Now consider doubling the size of the radiator (cast iron or steel, it doesn't matter). If the heating is switched on for the same times, more heat will be delivered to the room in question and the room temperature will be higher than required. To reduce the room temperature there are two main options; switch the heating on for less time or reduce the temperature of the radiator water. Both options result in less heat being 'delivered' to the room in question and the room temperature will go back to the required temperature. So, the smaller the radiators (of any material) the higher their temperature must be in order to raise a given room volume to the desired temperature (think of an open fire - compared to a typical radiator it has a far smaller surface area radiating heat but it is hot enough to heat a large room). But, small are sources of high temperatures gives rise to greater temperature gradients (and drafts) across a room, whereas a large area source of a lower temperature means less of a temperature gradient.

Extrapolate this simple principle and UFH is little more than a very large radiator, so large in fact that running it at radiator temperatures would make the room intolerably hot (and uncomfortable to walk on!). The answer is to run it at a much lower temperature. This DOES NOT make it a less effective 'radiator' because the larger surface area compensates for the lower temperature and the required room temperature is still achieved. Extrapolate this even further and imagine the walls and ceiling also being connected into the heating system; in this case the operating temperature would hardly need to be any higher than the target room temperature! Plus, there would be essentially no temperature gradient within the room, so no draughts!

A typical UFH system will also have a high thermal mass, the extent of which will depend on the amount of floor material being heated. A UFH system heating just the tiles on a bathroom floor will have far less thermal mass than a 100mm limecrete/concrete slab with tiles/flags on top. But this thermal mass will not affect the basic action of the floor to act as a 'radiator' of heat, it will only affect the time lag between heating up and cooling down - exactly the same principle as the difference between a 'fast-responding' steel radiator and a 'slow-responding' cast iron radiator - and this is easily compensated for by adjusting the times at which the heating system is switched on and off.

In my experience, it takes a little longer to learn how an UFH system responds but once learned it is no different to a 'normal' CH system. In our case, we've found that our UFH works well with a four hour adjustment, ie we switch it on and off about four hours earlier than 'normal'. Incidentally, our UFH is set to operate at 45 C.

I wouldn't go as far as MikeG in stating that UFH and conventional radiators should not be mixed. Our kitchen extension has UFH but the adjoining sitting room (open to the kitchen area) has a suspended floor and a cast iron radiator plus a wood burner. The kitchen UFH heat spills into the sitting room and on its own will ward off the chill even on the coldest winter days, although the sitting room radiator is also necessary to make the room comfortable, so the two systems combine quite well. With the wood burner alight we don't need the radiator, so again different systems can be mixed. Having said that, I imagine that UFH across both rooms would probably be better in terms of giving more even and less draughty heating, but I don't consider our mixed system to be bad enough to consider not having the UFH.

We installed the UFH system because it was in a new extension and the incremental cost was relatively tiny. We had no previous experience of UFH and regarded it as a sort of experiment with no downside - if it hadn't worked we'd simply not use it and wouldn't have lost much. But we've been very pleasantly surprised and now wish it was possible to install thoughout the house, though unfortunately it's not practical for the rooms with suspended floors. However, we still have one large room awaiting refurbishment and it has old pamments laid on earth and below the level of the outside ground, so could be a candidate for raising to help with some damp issues and installing UFH at the same time . . . but that's another project (isn't there always something else!!).

Apologies for the long post - I can ramble on and get carried away!


Back to Jamie's basic issue, I'd agree with MikeG. Don't mess with the UFH water temperature (at this stage anyway) and try switching it on much earlier. I'd say four hours at least, MikeG suggests five hours or perhaps longer. Basically, you need to spend a few weeks experimenting and learning how your particular system behaves. Perhaps one approach might be to simply switch on the heating at say 6am on a saturday morning (or weekday if you're in all day) and leaving it on constantly until the room temperature gets up to your target temp? Take notes of the temperature each hour and gradually build up a 'profile' of how the system behaves. Maybe take notes of the outdoor temperatures as well as, depending on the insulation characteristics of your house, this will also have an effect on how the system behaves. But do persevere and let us know how you get on.

 

[Jamieharnett]

Wow! Thanks for all your time.

Today I decided to run the heating all day before it was only set for a couple of hours in the morning and evening.

I think that long cycles will be the way forward, maybe the lack of curtains and draught proofing oh and the old extension with no UFH/CH are all contributing to the cold house.

Supppose I just need to get a move on and complete the remainging works! Ahhhh

Many thanks

Jamie

 

[Flyfisher]

A lack of curtains and draughty windows etc will certainly affect the effectiveness of the heating!

The main thing (at this stage) is whether the extended heating on times got the room up to your target temperature and how long it took.