How to use Under Floor Heating more efficiently

Discussion in 'Home energy' started by guernseyguy, Nov 27, 2009.

  1. guernseyguy

    guernseyguy Frequent Poster

    Hi - I am interested in learning how to use our UFH system more efficiently. I have checked the key post on this topic and I note that a previous poster stated

    "The system works on the principle that the boiler is always on, thus keeping the underfloor pipes at a minimum temp. all day. Then in the evening or morning, the boiler kicks in at a higher temp and gives a blast of heat that quickly radiates through the room cos the floor is already warm, and doesnt have to be heated from scratch. Because of this, the room heats quicker than with radiators"

    In our case, we switch on our firebird boiler from about 4.30 every evening for the UFH. Therefore, we are trying to heat up floors that have gone cold. It takes quite a while (maybe 2 hours) for the rooms to reach a comfortable level of heat - even though our room thermostats are set to 24 / 25 degrees. Our oil bill is very high (€2200 for previous 12 month period).

    I am hesitant to have our boiler always on in case our fuel bill goes even higher and that our firebird boiler wouldn't sustain the pressure of being on all the time.

    Are we wasting too much oil in heating the floors from cold and would it be more efficient to have the boiler on all the time and set the room thermostats to a lower temp?

    Any advice would be appreciated.
  2. Leo

    Leo Moderator


    That's not how UFH is designed to operate. Do you have any other form of space heating? Is the house unoccupied for large periods of the day?

    Do you have a meter on your oil tank? If you could measure actual usage you would be able to determine which is suitable for your particular needs.
  3. guernseyguy

    guernseyguy Frequent Poster

    Leo, our house is a dormer bungalow with underfloor heating downstairs (5 zones), we have radiators upstairs. We have a fireplace in our main room downstairs but we don't light the fire very much. Our house is occupied most of the day/evening as my OH works from home. We have a digital device plugged in in our utility room that shows the level of the oil in the oil tank so we know when it is getting low to re-order.
  4. Disposition

    Disposition Guest

    I have underfloor heating downstairs heated by its own oil boiler. I have noticed in recent days that even if we turn on the heat at 8 a.m. in the morning that it is not much warmer by 6 p.m. when we return from work, foolishly I thought the house would be too warm! i am wondering if we need to check anything on our boiler or room stats?

    Floor temp this am was 15.4 C., when i turned it off last night it was 17C at 10 p.m. It seems such a waste to have it on all day with no one in the house.

    upstairs we have rads and they are so much easier to use and less costly on the oil.
  5. SuzyH

    SuzyH Guest

    We are having the same problems and our heating bills are crazy expensive & not sure if it is because we aren't using the system correctly.

    Anything I've read suggest that I leave it on as it takes between an hour & two to get going, but its really only the third hour that its on that the rooms are getting warm. But the good side is that in the morning we haven't lost the temp, its nearly like it drops just before we come home in the evening.

    Reading the websites, they are saying that the boiler can be set to a lower temp of 50 degrees, but does that mean that the radiators upstairs or the water doesn't get warm enough?

    Beginning to really really dislike this underfloor heating that everyone else seems to be so impressed with ... are we doing something wrong?

    HELP ...
  6. bertie1

    bertie1 Frequent Poster

    I have my stats at 18C in the living area & 16C in the bedrooms. The idea is the concrete warms up & releases the heat slowly. You should let the heating come on for a few hours during the day to keep the concrete topped up. If the room gets to the thermostat setting it will cut out. You are putting your boiler under even more pressure by expecting it to heat up all the zones at 4.30.. There is a lot of water in those loops.. If you are worried about the oil turn the heating off earlier in the evening the floor still hold temperature ( if you let it come on for a few hours during the day)
  7. forgotten

    forgotten Registered User

    Hi all.
    re underfloor heating, can i ask what type of insulation you have in the walls and roof as this might be where the problem is.
    no point in trying to heat a house if heat is escaping through walls and windows etc.
  8. Disposition

    Disposition Guest

    Just to update you after my last post. I had the plumber come and check the system and there was simple problem with a pump. He suggested that we leave the heat come on for two hours in every six. However, I just turn it when I get home from work as we have another heat source in the kitchen. It is now working well.
  9. threebedsemi

    threebedsemi Frequent Poster

    As a general point, combining underfloor heating (low temperature, ideally constantly on)combined with an oil fired boiler (high temperature, generally more suited to intermittent ‘burst’ heat) is not the ideal set-up. Combining underfloor heating with radiators in a house, where each demand different different temperatures to work correctly, is overly complicated to design correctly for the majority of contractors when one boiler is supposed to deal with both elements.
    The first step would be to carefully calculate the heat load for the house (include hot water requirement), and compare this to the output of the existing boiler. Oil fired boilers are only efficient when they run at close to maximum output, as this is the output at which the manufacturer generally guarantees their efficiency. So a 15kw boiler which is rated at 90% efficiency, but runs at 10kw most of the time, will certainly not reach 90% efficiency.
    Boilers are generally completely oversized in this country due to either a lack of understanding/correct calculations on the design side, or the usual approach of ‘bigger is better’. If you are installing a boiler, bigger is not better, and it would almost certainly be more efficient in most cases to install a boiler which exactly matches the load required, and install another ‘top-up’ heat source for the limited time for which it is needed.
  10. sydthebeat

    sydthebeat Frequent Poster


    what formula do you use to size boilers when compared to heat load??
  11. threebedsemi

    threebedsemi Frequent Poster

    I am involved on the building design side of things, and use the DEAP or PHPP software to calculate the heat and hot water loadings for a project. Generally (from a liability point of view) I leave the actual sizing of the boiler and design of the system to (ideally) a consultant mechanical engineer who we deal with frequently) or (less ideally) to the contractor.

    I would however always query a contractor who is (as an extreme example) recommending a 30kw boiler when my calculations indicate that the combined heating and hot water load is of the order of 15kw.

    I will try to get our engineer to create an account on this site and post some more detailed information!
  12. sydthebeat

    sydthebeat Frequent Poster

    thanks threebedsemi...

    i too am on the design side, but most of the domestic work we do does not have a mech eng involved. i have yet to find an applicable formula that takes into account the heat loss resistanc eof teh construction when sizing boilers. many contractors simply use old 'rules of thumb'. DEAP does suggest a formula but of course indemnifies it self by saying it shouldnt be used to size boilers.

    I too am amazed at how many oversized boilers are installed into dwellings under current regs. I am at a loss however when the contractor can can up a "btu's per cubic feet" rule of thumb and i cant direct him categorically to a formula that takes the envelope u values and air tightness result into account.
  13. villa 1

    villa 1 Frequent Poster

    There is a formula that is used to calculate the heat requirements for dwellings/rooms taking into account building fabric heat loss, ventilation loss and infiltration loss. Plumbing apprentices are taught how to calculate these losses during their apprenticeship training. There is also a hand held heat calculator that can also be used. Any good repetable time served plumbing contractor should be able to calculate the heating loads applicable to house heating design. It's not rocket science!! The bad practices of Celtic tiger construction led to oversized/undersized boilers-radiators. Times have changed and work practices will improve.
  14. Threader

    Threader Guest

    how to use underfloor heating more efficiently

    So back to the original question, what is the cheapest way to run underfloor heating? Should it be left on all the time with the room thermostats at a low temp or on for a few hours a few times a day?
    On a separate issue the thermostats seem to burn out, so some are disconnected. Not sure which is which but some of the rooms are too hot and some are pure cold. The broken thermostats fail one way and when they are disconnected it is the opposite effect. I forget whether disconnected switch on manifold means the heat is always on or always off.

    Anyway the question is:- on all the time or, how many hours on, every how many hours? Is the two hours on in every six hours the answer?
    I was getting away with three or four hours on a day until the cold spell.
  15. fmmc

    fmmc Frequent Poster


    Not suggesting this is the correct solution, but I've been wrestling with UFH for 18 months attempting to optimize fuel consumption with maintaining a comfortable temp in house at times we need it. Originally, I was running the bedroom zones for a few hours before bed & again for a few hours before getting up with a view to having them warm when needed. For the living quarters, I was similarly running for a few hours immediately before getting up and again in early evening with a view to having warm for late evening. The result was the boiler was effectively running most of the time, thus consuming lots of fuel & the house wasn't necessarily at the desired temperature as the slab wasn't receiving sufficient heat to give off heat when required. I then tried running all zones for 2-3 hour blocks just before waking in the hope of having warm rooms in morning, then running again in late afternoon or early evening with view to having house warm for returning from work or heading to the nest. This didn't work either in terms of consumption or having the house warm when necessarily required. What I've learnt (& should have known!) is that due to the very slow response times alluded to in earlier posts, is that UFH works & reacts very differently to rads, i.e. I can't turn it on immediately before I need it and expect a result. What I now do is use the stats to call for heat in all zones constantly for 5-6 hours between the hours of 2am and 8am. It appears then that the house is reasonably comfortable when the family get up in the morning as UFH commenced supplying heat 5-6 hours previously and furthermore as there was a constant supply of heat to the slab over a sustained period, it then releases heat slowly to the house for the remainder of the day. I'm open to other posters contradicting this but it appears to have worked for us and seems relatively fuel efficient. During the extreme cold spell, admittedly we did on occasion run for longer but generally, this process is working well. Hope it helps.
  16. TripMeUp

    TripMeUp Frequent Poster

    hey Fmmc...

    do you not have the UFH come on again at all during the rest of the day?

    From what you state, it seems you have UFH upstairs also? is that correct?
  17. onq

    onq Former user

    Hi Threader,

    In relation to your query, its horses for courses.
    There is no formula for perfect economical UFH installation or running.

    My understanding is that its better to have a smaller boiler running at capacity to keep a low level of heat on all the time - topping up with space heaters as required.
    This contrasts with a larger boiler capable of using massive amounts of fuel to raise temperature from cold occasionally, but running inefficiently the rest of the time.

    Putting UFH through or under a slab increases the thermal inertia [slow warm up time].
    Timber is an insulator the installing UFH under a timber floor reduces its effectiveness - similarly for a carpet.

    Although it seems cold to the feet, a stone or tile floor may be the best for more rapid warm up, laid on a lightweight screed [per manufacturer's instructions] set on foil-backed insulation turned up at the room perimeter to allow for expansion.

    However I am not expert in these matters and I usually refer all such queries to a competent Mechanical and Electrical Consulting Engineer.


    All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
    Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.
  18. fmmc

    fmmc Frequent Poster

    hey tripmeup,

    No,its just on for that 4/5 hour block in the morning with no additional "boosts" during the day. In my experience through messing with the system and trying different permutations & patterns, I've concluded that running it for an hour here and there makes no discernible different. In order to get enough heat into the slab so that it can release heat at a later time, it requires input of heat to the slab for a sustained period. My house is timberframe and appears to retain heat reasonably well. The temp @ 10am in the morning doesn't drop much by 10pm in the evening assuming outside temps are moderate (6-8 deg or higher). I hasten to add there is no science behind this, just my own views. Running on this pattern seems to heat the rooms I have tiled (living areas) to approx 19-20 degrees (when exterior temp is 3-4 deg C +) and bedroom areas which have wooden floors to circa 17-18 deg which we can live with. When the exterior temp drops closer to freezing, these interior room temps typically drop by a degree or so on average. I have no haeting at all upstairs yet. 2600 sq ft on ground floor and not using the converted attic as yet. Good luck with your experimenting. As far as I can see, there is no magic rule for optimising that works for everybody and it does require some tricking around with to find a solution that works best for you.
  19. TripMeUp

    TripMeUp Frequent Poster

    hey fmmc,

    thanks for the reply...

    Sounds very positive that you are only running it for the "4/5 hour block in the morning with no additional "boosts" during the day"....
    I absolutely agree that running it for an hour here and there makes no discernible difference..If u want to do that, then get Rads..!!

    I too have played around with different variables involving 4/5 hour blocks and am still experimenting...I have a well insulated house, so too find that there are no exterme drops when the heating is off....
    ...but by that token, you would then be safe leaving it on all the time too at say 18/19 deg as none of the stats will call for heat...;)

    Obviously, the extreme temperatures of the last month make some we have to factor this in..

    With regards to upstairs I went with Rads..I spoke to alot of people with UFH upstairs as well as down and most concluded that they would not go UFH upstairs again as the heat can be too much....but everyone to their own....

    I have an overall Digital control stat for the upstairs zone and thermo controlled radiators...I originally had the stat set at 20/21 deg from 4pm to 7pm and then at say 15/16 from then on as 20/21 would be too warm going to bed....I found though (good insulation effect I suppose) that when I went upstairs at 10/11, it was still at 20.5/21 presently playing around with this as well..!!

    I think (as you say) everyone eventually gets to a happy medium or an ideal setting for their set-up after a while experimenting..

  20. dub_nerd

    dub_nerd Frequent Poster

    fmmc -- good post and my experience is similar to yours. I have been running UFH for 8 years and spent a lot of time experimenting with its characteristics. I think there is probably going to be a lot of variance in people's experience because of the different ways that UFH can be installed. Mine is laid in screed on top of a heavy concrete slab with foam and foil-backed insulation underneath. I think the absolutely most important thing to realise is that the reaction time is INCREDIBLY slow -- on the order of many hours. When mine is up to temp, I find that I can change the temperature by about one degree per hour. But when it is completely cold, you could easily run it for 8 to 12 hours before the first barely noticeable heat is emitted.

    So, I would say the very first thing to do is to get yourself several thermometers and measure the reaction time of your UFH in different zones. And don't rely on a single temperature reading per room -- I find that on a cold night there is easily a four degree difference between the centre of a room and a spot below a window. There is also a substantial difference in reaction time between tiled floors and wood or carpeted floors. Wood floors are more insulating and need a corresponding higher temperature gradient to push the heat through. In my case I set the thermostat a couple of degrees higher in a room with wood floors compared to tiles.

    I regularly spend long periods out of the house. If I know I'm going to be leaving for a while, I would turn the heating off at least 12 hours before. In the same way that it takes that long to heat up, it's the same when cooling down. It's important to measure how fast the temperature changes, both up and down. That will give you an idea when you want your boiler on, or setback timer set, before the morning/evening periods when you want higher temps. As I said, my experience is that I can change the temp by one degree per hour. It would be futile to think I could change from 15 to 20 C by having the heating on for two hours before 8am. You HAVE to experiment with your own system though.

    Regarding boiler capacity and the suitability of oil to match UFH, I'm not sure what the fuss is about. The temperature of my boiler water bears no relationship whatsoever to the UFH temps. They are connected by an inlet valve and mixer valve. When the UFH needs warm water it opens the inlet valve. It doesn't matter what temperature the boiler water is at -- the UFH will just let in more or less, to achieve it's own desired temp by mixing with its cold water returns. You can set the temp of water going to the UFH ... up to a limit you can improve the response time by increasing the water temp at the inlet valve, however there is a max recommended temp and there is an automatic cutoff if it is exceeded. A problem I had for a long time was an intermittently faulty inlet valve which stuck in the open position, so the heating would appear to work for a couple of hours and then the auto cutoff would kick in and nothing else would happen. (There is also a balancing valve at the other end of your manifold, similar in function to the shield valve on a radiator, but I wouldn't go near this if you don't have to).

    Someone mentioned burned out thermostats. I think you probably mean burned out actuator heads on the UFH manifold. These are the little devices sitting on the returns of your UFH circuits that open and close the individual circuit valves in response to your zone thermostats. For some reason they do seem to fail from time to time. If you pull the head off (it's a little squeezy spring clip) the circuit will be permanently open. If you want it closed there is a little square nut that you can screw down. It's easier to see what's happening if you've got flow meters on your circuits -- a transparent perspex tube with a tiny sprung plunger that shows you if water if flowing -- but not all UFH manifolds have them. I got replacement actuator heads by ordering online from a company in Germany, and I now keep several spares. I think I've had two fail in eight years.

    (It's a long story, but when my house was built the heating was only half installed because of a dispute between the builder and heating subcontractor, which I only found out about when nothing worked in the first winter. Both the people who installed it and the distributor who supplied the system refused to have anything to do with sorting it out, so I've been learning how it works and maintaining it myself ever since. I can tell you from bitter experience that dealing with an east German company through Google Translate was an infinitely better and cheaper experience than any of the gangsters involved in the Irish end).

    Anyway, get to know your system. It's not that complicated at the end of the day. You will definitely save money by choosing the correct temperatures for each of your zones, and picking the right times to supply extra heating.