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General: BTU Calculators

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  1.  
    • CommentAuthorkristeva
    • CommentTimeMay 15th 2022
     
    Hi all,

    Can anyone recommend an online BTU calculator they used to purchase radiators that have proved fairly accurate?

    I've noticed many online calculators seem to give widely varying results. For my small bedroom I've had figures ranging from 2200 to 5000. I know some take into account more details than others, like window size, spaces above and below.

    I also found this 'old school' advice on a forum last night:

    30 odd years ago I was told to calculate the cubic feet of a room,and multiply by 5 for living rooms,and 4 for bedrooms.With double glazing and better insulation this is probably over adequate BTU sizing,and the thermostatic valves will control the temperature.For example a bedroom 13 feet x 12 feet x 7.5 feet = 1170 cubic feet,needing a radiator of 4680 btu.

    Maybe this is a safer bet. Cheers.
    • CommentAuthorSimonD
    • CommentTimeMay 15th 2022
     
    I would not base radiator sizing on a 30 years old rule of thumb, and you are right in that the online versions will vary significantly depending on the assumptions made by the calculator - which you need to know. I've tried to attach an open document spreadsheet which I made up a few years back that essentially duplicates the heat loss calculator provided in the Domestic Heating Design Guide. It's a bit clunky but works okay. However the forum doesn't allow this document for upload. If you whisper your email I'l send a copy across to you.

    For annual consumption you'll need your regional Degree Day Factor. In the sheet I've just put the value for Heathrow as a default.

    With respect to ACH, there are some typical assumed rates ranging from as much a 3.0 per hour for a living room in an old house to 0.5 in new builds. There are obviouslt copious notes to go with the inputs but without these you can probably still arrive at a fairly reasonable figure.

    The caveat is that I've copied and pasted this sheet from a larger more comprehensive spreasheet I had developed, so please do check the formulas have all copied across - I think they have and they work okay.

    HTH
  3.  
    5000 btu is approx 1.5 kW So apply the test of reasonableness - is 1.5 kW as a heat source enough for room depending upon the length of time your boiler runs / day and how often you need to heat the room quickly. Also if the room is upstairs then heat will drift upwards reducing the size needed to maintain the required temp.

    Don't get too hug up on calculating the exact size of rad. needed unless you are stuck for space and very tight on money. Put in a rad. that will cope in the worst case and put on a TRV to take care of the rest of the time.

    Just IMO
    • CommentAuthorowlman
    • CommentTimeMay 15th 2022
     
    I agree with PiH let the TRV take care of the job.
    • CommentAuthorkristeva
    • CommentTimeMay 15th 2022 edited
     
    Posted By: Peter_in_Hungary5000 btu is approx 1.5 kW So apply the test of reasonableness - is 1.5 kW as a heat source enough for room depending upon the length of time your boiler runs / day and how often you need to heat the room quickly. Also if the room is upstairs then heat will drift upwards reducing the size needed to maintain the required temp.

    Don't get too hug up on calculating the exact size of rad. needed unless you are stuck for space and very tight on money. Put in a rad. that will cope in the worst case and put on a TRV to take care of the rest of the time.

    Just IMO


    Yes, I agree Peter, I only went down this road because I found someone a few days back selling a couple of brand new traditional column radiators at a very decent price. The stated BTU fell in line with my previous calculations (run a year or so ago) but when I went to double check those calculations I started getting figures almost doubled, which seemed odd.

    My original plan was to utilise modern radiators, and in this scenario you're advice makes perfect sense as the difference between 2000 btu and 7000 btu is often negligible in price.

    Are there any negatives in oversizing the rads (combi boiler)?
  6.  
    https://mcscertified.com/mcs-launch-new-improved-heat-pump-calculator/

    This one does room-by-room heat loss calcs and you can tweak the flow temperature to see what BTU size radiator would be needed.

    Remember to push up the capacity if you don't want to run the heating 24/7 in the coldest weather, so you will have capacity to 'catch up' if the heating is turned off for part of the day/night.

    Oversized radiatiors allow lower flow temperature, which means better condensing efficiency for a gas combi boiler.

    Beware that radiators capacity is often quoted at a very high flow temperature, 50degC greater than room temperature. You're unlikely to want to run the rad that hot and so in practice you won't get as much heat from it as the label says.

    Often a double panel radiator is not much more £ than the same size single panel, and gives lots more heat. If you are replacing existing rads, you might save labour if you can avoid moving the plumbing.
    • CommentAuthorSimonD
    • CommentTimeMay 16th 2022
     
    Posted By: Peter_in_HungarySo apply the test of reasonableness - is 1.5 kW as a heat source enough for room depending upon the length of time your boiler runs / day and how often you need to heat the room quickly.


    So how do you know unless you know the room losses? However long you run the boiler, if the rad is undersized, it's going to inadequately heat the space.
  8.  
    Posted By: SimonD
    Posted By: Peter_in_HungarySo apply the test of reasonableness - is 1.5 kW as a heat source enough for room depending upon the length of time your boiler runs / day and how often you need to heat the room quickly.


    So how do you know unless you know the room losses? However long you run the boiler, if the rad is undersized, it's going to inadequately heat the space.

    look at similar rooms in the house or if different rooms then mentally adjust the size. With calculators giving such a large range as quoted above a test of reasonableness is going to give a guide against the calculators. However as I said above err on the large side and let a TVR do its job.
    •  
      CommentAuthordjh
    • CommentTimeMay 16th 2022
     
    Posted By: Peter_in_Hungary
    Posted By: SimonD
    Posted By: Peter_in_HungarySo apply the test of reasonableness - is 1.5 kW as a heat source enough for room depending upon the length of time your boiler runs / day and how often you need to heat the room quickly.


    So how do you know unless you know the room losses? However long you run the boiler, if the rad is undersized, it's going to inadequately heat the space.

    look at similar rooms in the house or if different rooms then mentally adjust the size. With calculators giving such a large range as quoted above a test of reasonableness is going to give a guide against the calculators. However as I said above err on the large side and let a TVR do its job.

    Or turn the heating off in that room and use an electric heater to heat it for a while. Then it's very easy to measure how much heat is needed.
    • CommentAuthorMike1
    • CommentTimeMay 16th 2022 edited
     
    Posted By: WillInAberdeenOversized radiatiors allow lower flow temperature, which means better condensing efficiency for a gas combi boiler.
    And better compatibility with a future heat-pump replacement for the gas boiler.
    • CommentAuthorSimonD
    • CommentTimeMay 16th 2022
     
    Posted By: Peter_in_Hungary
    Posted By: SimonD
    Posted By: Peter_in_HungarySo apply the test of reasonableness - is 1.5 kW as a heat source enough for room depending upon the length of time your boiler runs / day and how often you need to heat the room quickly.


    So how do you know unless you know the room losses? However long you run the boiler, if the rad is undersized, it's going to inadequately heat the space.

    look at similar rooms in the house or if different rooms then mentally adjust the size. With calculators giving such a large range as quoted above a test of reasonableness is going to give a guide against the calculators. However as I said above err on the large side and let a TVR do its job.


    In other words, stick one finger in the air and take a best guess while holding your other hand behind your back with your fingers crossed that what is likely to be a completely uneducated and unexperienced guess is going to be right.

    By logical conclusion we might as well just chuck in large radiators and large boilers just in case.....which is funnily what we've got in the general marketplace.


    Posted By: djhOr turn the heating off in that room and use an electric heater to heat it for a while. Then it's very easy to measure how much heat is needed.


    So if its the middle of June, or an unseasonably warm February, just wait until the weather gets cold - and how cold?, then take a trip to the shop, or order a heater or two for a single use...

    Might as well go to CIBSE and buy the pdf version of the Domestic Heating Design Guide for £20 and do some relatively simple sums.

    The MCS linked calculator WiA linked to above is based on the same calculations as the Domestic Home Heating Guide which even better is free.

    Posted By: Mike1And better compatibility with a future heat-pump replacement for the gas boiler.


    Not if you just take a guess it migh be about right. Even designing a heating system using gas boiler at 55 degrees flow temperature requires larger radiators than designing the same system using a heatpump. So if it is going to work, it does need some planning.
    • CommentAuthorMike1
    • CommentTimeMay 17th 2022 edited
     
    Posted By: kristevaCan anyone recommend an online BTU calculator they used to purchase radiators that have proved fairly accurate?
    Looks like the answer is no.

    I used to manually calculate the heat loss for each room & the heat transfer between them to derive the rad requirements. Now normally 16mm UFCH pipework @ 150 centres & calculate the heat source.

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