Is it effective or not?
In this Paper firstly we consider the problems of designing underfloor heating systems, showing why they are hugely and unnecessarily expensive to install, and then explain why they are also expensive to operate – much more expensive than conventional systems.
The implications of the higher thermal mass in the floors are then considered and the ‘best’ heating systems for houses explained and shown.
We have seen in another Paper that solid timber flooring can be laid over Underfloor Heating without any problems at all, but that doesn’t address the basic question – is underfloor a good way to heat a building?
Tranquility has underfloor laid throughout but it is never used. Why? Because it is extremely inefficient. It costs more to run and emits more carbon than the alternative. If I want the house heated for an evening I have to turn it on at least 24 hours earlier. If the sun then comes out or the weather warms or plans change, all that heat is wasted. Too much hassle and horribly inefficient.
I too was persuaded during the design process it was the most efficient way to heat the building, but as Tranquility was designed and engineered to require very little additional heating, there were clear difficulties right from the start.
With an estimated annual heating cost (in 2004) of £40, I was astonished to be quoted £24,000 to install a system. Madness! £24,000 to emit £40! Asking how that made sense, I was greeted with a very long silence and told the design software has no options for different heat loads!
I was fortunate enough to have a Sponsor company who asked if I knew how to design a system for the house as they couldn’t. Having said “Yes” the layouts and installation solutions were left to me which resulted in only 660m of pipe being used rather than apparently the 2,600m I was told they would have used. Some of the rooms are piped at 900mm centres, some at 600mm, 450mm, 300mm and 150mm – to discover how each worked. I have also used different solutions to spread the heat. The fact is that every room heats perfectly no matter what is laid beneath it, a bit of a surprise.
The ‘One size fits all’ problem
No matter how much heat you need there is one underfloor heating solution – 150mm pipe centres (for 15mm or 16mm pipes) with loads of pipe, lots of circuits and a very high installation cost. It makes no difference what temperature you want the room at or how much heat any individual room needs – the solution is always the same. Odd when you understand that properly designed alternative systems calculate, room by room, how much heat each one needs and a heat emitter installed to supply that. Why should underfloor be any different?
In a world running out of resources, it is also a very retrograde step to use a single consumptive solution which of necessity has to provide for the biggest heat loads ever wanted for a room with that floor area. Every design solution is therefore over engineered – by definition.
For those with really old cottages, or houses that leak heat horribly, it is one thing, but if you have worked to improve the thermal performance of the building why should you spend so much to install such an over-engineered solution? Consider:
- Double glazing loses less than half the heat of single glazing.
- Filled cavity walls lose a fraction of what unfilled cavities do, and all solid walls are much worse. 2ft wide stone walls are not efficient but they do have huge thermal mass.
- Well insulated roofs (350mm is optimal) lose a tiny fraction of what un-insulated ones do.
- Very insulated ground floors can lose next to nothing (mine score zero).
- Internal walls lose ‘nothing’ while external walls depend on their insulation value as above. Rooms with one external wall therefore need very little heat compared with those with 3.
- High ventilation rooms (bathrooms) lose much heat which other rooms (halls) don’t.
- Higher temperature rooms (sitting rooms) need more heat than lower temperature ones (bedrooms).
- A two storey room needs a lot more than a normal one.
How is it then that underfloor designs are the same everywhere? It must be clear the layouts produced are the quick and easy design solutions making the installation hugely more expensive than necessary and using the maximum material.
The only thing underfloor heating design sort of takes account of is room size, as the floor area is proportional to that.
How does heat move?
In order to know how best to work with heat we need to understand some scientific facts – but don’t worry – we are not going technical but it is important.
- Heat will always move from a warmer material to a cooler one no matter what the material is – solid, liquid or gas.
- It only moves in 3 ways:
Conduction: Heat will simply move through any material from warmer to cooler parts of it no matter the direction which is important. There is a simple test you can do to show this. Boil a kettle and taking a saucepan (thicker bottomed will demonstrate better) pour some in. Quickly put your hand underneath and you will feel it warming, but be careful in case it gets quite hot. The heat moved from the top surface to the bottom very quickly didn’t it! Conduction at work.
Radiation: All the heat arriving on Earth is radiation coming all the way from the sun, but hold your hand in front of a hot radiator and you can feel horizontally radiated heat. Lift a boiling saucepan (DON’T spill anything) and hold your hand close underneath. Feel the heat? That is radiated downwards so heat radiates in all directions equally.
Convection: Now we do have a heat transfer that only goes upwards but it is the only one that does. If you know of lava lamps (in which coloured oils move upwards from the light bulb), that demonstrates it, but for a bit of fun (and to teach any children) put some cold water in a glass jug, casserole or bowl that can be heated directly (Pyrex can be). Very carefully dribble a little food colouring down one side and let it settle on the bottom on one side, or better still get a tiny bit of potassium permanganate crystal from the chemist and drop that down one side. Then warm the water mostly on the side with the colour and watch what happens. Convection at work which shows it only goes upwards.
What does this mean for underfloor heating?
A lot, because the heat source is not in the room – it is under it. The heat radiated and conducted from the pipes goes down as strongly as it goes up, but down isn’t of any use unless it can then be made to come back up. This may come as a surprise as we tend to think heat only goes up, but what about the convected heat that does go up?
No heat can be convected off the heat source – the pipes – because they are not in the room and not in a free moving air space. The air immediately above the floor will convect upwards when it has been warmed but not before, so as the heating is turned on, depending on the elements in and under the floor, nearly all of it is being used within and under the floor
I could consider all the technical specifications but that would take a lot of space, so a few points:
- The heat being radiated downwards can mostly be sent back up if there is a shiny reflective surface somewhere below the pipes. Mirror stainless reflects some 99% but only while it is a clean surface. The shiny side of aluminium foil would reflect over 90%, but if any shiny surface degrades over time (e.g. dust) and cannot be cleaned, the performance rapidly degrades too.
- As the sub floor warms it too radiates heat, so a reflective surface needs to be underneath that. It is complex because as the foil warms up, it will also radiate heat away, so it needs to be in the coolest place. There is advice around to use a foil backed polyurethane foam as an ‘in floor’ insulant but the gas in the foam can migrate which, when gone, halves its insulation value and it is very expensive. Cheaper to buy polystyrene and use kitchen foil.
- You don’t want two reflective layers as any radiation between them keeps bouncing off them until it gets past one.
- If there is an air void under the floor, this will be ventilated to outside, so all the heat reaching that will be lost.
We can therefore see that all the mass in the floor will be heated, even if it is a solid concrete slab under the insulants, and the longer the heating is on the greater will be the heat absorbed down there. Heat will escape from the sub floor if the outside walls are even within 4 metres, so don’t think it will all come up again.
Underfloor Heating Factors
There are some important things to know when you use underfloor heating:
- The system is designed to be turned on in the autumn when heating is first wanted, and off in late spring. This means the boiler (if used as the heat source) will be on all the time! It is impossible to consider heating continuously with a permanently fired boiler to be efficient. In all honesty that is about as far from the optimal solution as it is possible to get.
- Rooms are not normally individually thermostatically controlled.
- It is unresponsive, taking time before serious heat is emitted if turned on from cold – i.e. on and off – but there is always a long time lag.
The floors are a significant thermal store, absorbing a lot of heat before emitting. So long as the heating is permanently on it won’t matter, but that isn’t how best to heat any space.
To maintain the temperature in a space: heat in = heat lost. (Heat in includes solar gain).
The heat lost is proportional to the temperature difference between inside and out.
The most effective heating system must therefore heat only the spaces needing heating; only for the time they need heating and only raise them to the temperature needed.
To be efficient, a heating system must apply heat quickly and not emit it when not needed.
Every deviation from that increases the energy used; the cost to the consumer and the amount of carbon emitted.
How well can an underfloor system do that?
So to be efficient, a heating system must apply heat quickly and not emit it when not needed.
How well can an underfloor system do that?
To a little extent it depends on the way it is installed but with certainty none can do it well and none is designed to:
- In order to cool the house through the night, the heating would have to be turned off hours if not half a day or a day before the evening. First you would have to know how long it took to cool down and heat up, and then activate the change that far ahead of when the temperature change is wanted. But even knowing that, the heat emitted slows down as the floors cool down – and speeds up as the floors warm up. It does not stop when you turn it off or start when you turn it on. Clearly that makes it impossible to vary room temperatures, not just effectively but at all – except long term changes by altering the set temperature. [Note: IF there are thermostats in every room linked through to flow control valves on each circuit – then rooms can be set to run at different temperatures but these will be constant].
- This produces an interesting if unwanted effect.
The detailed explanation is in Appendix 1, but briefly the floors emit least heat when the house needs it most, and most heat when the house needs it least. It almost perfectly amplifies the problem. Mad.
Effectively the heat wanted at 6 a.m. may be coming from the floors at 2 p.m. No matter what the delay – it is doing exactly the wrong thing.
- Many installations (if not most) don’t have individual room thermostats, so as will now be clear these are especially inefficient, but even with separate thermostats, all the above points apply.
- Now let’s take the situation where the weather has improved and heating isn’t really required, but bear in mind that the sun makes us feel warmer so when it is out we may be comfortable with a slightly lower temperature. Do we turn the heating off? With underfloor heating the answer is no. And even if it is turned off – all the heat in the floors continues to come into the house – but given it takes so long to get heat back out again generally it is left on.
We can therefore see that underfloor heating is necessarily and by definition, very inefficient. Starting from scratch, you would never design a heating system like this.
What about energy efficient buildings?
All the factors above apply but more so because the heat required is much more variable, often going to zero. Conventional systems can and do respond but underfloor heating systems simply cannot.
Further, the more efficient the house, the bigger the inefficiency of an underfloor system. This is so sad for people who have spent much time and money improving the performance of their houses only to end up with the worst heating system possible for them because of unqualified advice.
Heating cost and cost effectiveness
Clearly having a heating system running continuously is extremely inefficient, not least because the boiler and pipe runs from it to where the heat is needed are constantly hot and constantly losing heat. However, if you use a condensing boiler – beware again. You may be assuming the boiler is running at the quoted efficiency figures of around 90 to 94%. Unfortunately not, as those figures only apply when the boiler is running in condensing mode which it only does when the flow and return temperatures have a big difference between them. With the systems running 24/7 this will almost never happen so you will have the same efficiency as conventional boilers, BUT condensing boilers have a very much shorter life and are much more expensive
Depending on how it is designed and used, the annual cost will vary considerably but I am told by some who have it that it costs about 25% more per annum than previous systems – which of course doesn’t surprise me. I feel very sorry for the buying public because the information in this Paper is not generally known and as with nearly all products – ‘Buyer Beware’ – don’t trust what you are told by anybody selling products.
What with the heat supplied by a Heat Pump?
Underfloor heating with a GSHP and (engineered flooring) is a constantly marketed ‘solution’ but is the worst option of all, as many have found out. Every element of this is Red – or the opposite of Green. It saves no operating cost (or carbon) compared with gas if gas is available, and a very small amount if oil is used, but costs a very significant additional amount of money to install – see the White paper which explains this in detail: Ground Source Heat Pumps. The cost of installing this combination is between substantial and huge, not good news when the annual operating cost will go up as will the amount of carbon emitted.
The Impact of Climate Change
From the point of view of this Paper we needn’t assume the climate is changing – just ‘What if it is and significantly?’ From other Papers on this web site I argue and show that it is, and that we have no options but to drive for a zero carbon economy and not some futuristic modest reduction in emissions. Given how inefficient these underfloor systems are, when we understand the scale of the climate problem, they should be the first to be simply permanently turned off. We have one customer who has installed such a system (though an electric one) and he has already found he cannot afford to use it. I am trying to help him with a viable alternative.
I am constantly concerned about the amount of money being invested in very short term projects, and for the investors it is really bad news if an expensive system will be used for just a few years.
Why is it so commonly fitted?
I don’t know, but it is frustrating for those of us working to reduce global energy and carbon. Call it fashion or the latest ‘must have’, but only a few years ago domestic wind turbines were in every exhibition, and even the official advisory groups were recommending we fit them. Now where are they? Gone, and sadly those who fitted them either have them as proof that many things don’t work, or have removed them. The same is happening with Ground Source Heat pumps and will happen to the other products that are of no value. The Papers on this web site are written without fear or favour representing a completely honest and unprejudiced analysis.
But there is another reason. I used to lecture in a centre that was meant to be an independent advisor for the self-build industry and always to a packed hall. One Sunday morning after arriving to give my talk I was asked/told to moderate/remove my section on Ground Source Heat Pumps as one of their members sold them. I asked if I was being shackled and was told “Really, yes”. It took me less than 20 seconds to tell them I would not lecture unless I could tell the truth, and as they wouldn’t budge I walked out and haven’t been back. Sad that the profit of a single member should go above honesty, integrity and the pockets of their customers. I also offered my work to the publisher for the Building Research Establishment, but to no avail even though the BRE was still fitting roof turbines at that time. I lectured in Earls Court again to a big audience, but offering to speak the following year I was told I had to pay to be an exhibitor as only exhibitors could lecture. Clearly the notion of integrity has no place in business.
It is very difficult to challenge what seems to be conventional wisdom especially when promulgated by most in the building industry (who earn a lot by installing these things), and thinking through why it isn’t efficient is quite a task isn’t it!
So what is the solution or best option?
The System: ‘Best’ is unquestionably a system that can deliver heat quickly where wanted and when wanted, so a hot water based system it is. This gives the lowest cost; most efficient and least carbon emitting option, so then using the best local energy option for heating the water is the simple choice.
I hear you saying “not radiators again” but of course there are different ways to emit the heat. I also hear you say “I haven’t the wall space and I don’t like the look of them” and on both issues I so agree. But skirting heating options aren’t great so I set about solving both problems – wall space and visual impact. It is possible to use small hot water coils with the heat being circulated by fans blowing through the coils, and these can be at floor level – for example under kitchen units.
Having a long standing belief we can always live with beauty around us if we choose to, the ‘heat emitter’ challenge was interesting. Let’s cut to the chase and show what I have used:
Design is personal so thankfully we don’t all like the same things, but I find the above truly great to look at AND they use little wall space. And I still love looking at all of them.
We seem to think we must use the same everywhere, but I only repeat one radiator design here as you can see. Visitors seeing the two vertical red tubes (kitchen) usually don’t know what they are, which says something about the visual image, and most say they love them. The stainless finish (office) is as supplied, but all the others were professionally coated in Stroud. Going vertical with style solves both the space and visual impact problems, but these radiators are more expensive per kw.hr of output. However, the saving from the installation cost of an underfloor system swamps this additional cost and it is your choice.
To further show how we can have beautiful things around us even if they have to be functional, here are some rolling pins, chopping boards and a dining table.
So to answer the original question –
Underfloor Heating is not an efficient system to install in a house.
The house thermostat is controlling the flow valves, so as night comes on; the external temperature drops – the heat lost goes up – so the valves open up. By about 6 in the morning the heat lost is greatest so the flow is maximised though the night and the floors reach their highest temperatures. But this is just when the external temperatures are rising with the morning light and when the heat demand is reducing.
These systems push out most heat when least needed and least when most needed! Or technically they amplify the natural temperature range. And of course the same is true of changing sunshine levels and weather changes. As the heat needed drops there is a long delay before the systems respond, and if the lag is 8 hours the house will be doing exactly the opposite of what you want – and you pay for that.