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Boiler efficiency



  • JDHW
    JDHW Member Posts: 0
    This is going to be longish but I will try and explain what to do and why you are doing it.

    The thermostatic valves don't move very far - I would have said about 4mm but this does vary between brands so 2mm could be ok. You will find out later if there is a problem. Just try and move them up and down a bit with pliers with moderate force to see if they are stuck before you start.

    In a heating system hot water from the boiler is pumped through the radiators and the radiators emit heat into the room and in turn cool the water that is flowing through them. When the system was designed the radiator sizes should have been selected to more or less match the heat loss from the room they are in. And the boiler sized to match all the radiators. You only have one boiler and pump so the goal is to arrange for the water from the boiler to divide up and flow through all the radiators in just the right amount for each radiator and this is known as balancing.

    Because you have different sized radiators in different rooms each one will have a different flow requirement. To accommodate these different flow requirements each radiator has a lockshield valve (the ones with a domed cover you can't turn) that is intended to adjust the flow. If you pull the covers off these valves you will see a square or flat end that you can turn with an adjustable spanner (or pliers at a push). So you just have to adjust this valve until the radiator gets the right amount of water. You can't easily measure the flow through a radiator so what you do measure is the temperature drop across the radiator. If the flow rate through a radiator is too great the temperature drop will be small and if the flow is too small there will be a larger temperature drop.

    For UK systems the temperature drop should be 10c to 20c when the flow temperature is 80c-75c. So in theory all you have to do is measure the temperature across the radiator and adjust the flow until the temperature drop is about right and that is it.

    Unfortunately, the way UK houses are usually plumbed the radiators do not act independently. A single pair of pipes will run across the centre of the house (between the floors in you case I think) and then the radiators are connected across these pipes. Downstairs down the walls and then up from the floor upstairs. Because all the radiators are connected across this common pair of pipes the flows from the radiators are really competing with each other so adjustments to one radiator will affect the others. The process of balancing the system is a game of whack-a-mole. It is not impossible but just requires some systematic steps. Here they are:

    1) Get a note pad, a few sheets of paper, or a laptop and spreadsheet and allocate one sheet to each radiator in the house. Draw up 4 columns on each sheet with the headings:

    In Temp Out Temp Difference Adjustment

    As you go through the process you will complete a new line at each step.

    2) On one of the lock shield valves wind the valve closed (clockwise) then open and count the number of turns from one end to the other. This number of turns will give you a sense of how big to make each adjustment. From the fully closed position (fully clockwise initially set the valves as follows: For big downstairs radiators (like the one you had a problem with) fully open the valve. For smaller downstairs radiators 1/2 open the valve. For upstairs radiators open the valves about 1/3 and for really small radiators (towel rail) open the valve about 1/4. Make a note at the top of each radiator sheet your starting position.

    3) Turn up the room thermostat really high so the boiler will stay on all the time you make adjustments and let the system run for 1/2 hour to 1 hour. If you measure the hot pipe at a radiator near the boiler the temperature will come up to 75c-80c and stay about there. Make sure you have removed the thermostatic tops from the valves.

    4) Now measure the input and output temperatures at each radiator and enter on the sheets. For each radiator calculate the difference (hot - cold). You will see a range of temperature drops across the radiators because the flows will not be correct in the first instance.

    5) What you are trying to do is get the temperature drop across each radiator the same. So for the radiators with lower temperature drops you should close the valves a bit (clockwise) and for the ones with higher temperature drops you should open up the valves a bit (anti-clockwise). In the first stage you can make big adjustments of say 1 turn. Make a note of how big the adjustments made was.

    6) Wait about 20 mins and repeat the measurements as in (4) above. The differences now should be smaller. Repeat the adjustments using the same rules as (5) but this time make smaller changes say 1/2 turn.

    7) All you do is to keep repeating stage 6 and at each iteration the differences will get smaller and your adjustments should be smaller each time. You will soon get a sense of how big to make the adjustments to change the temperatures. After about 4 steps you should within 2c and that is good enough. You will find that the temperature drop across the radiators is then 10c-20c.

    If you find that that one radiator has a high temperature drop and adjustments don't make much difference the thermostatic valve is bad and needs freeing or changing.

    Another little problem is back to back radiators. If you have two radiators on either side of wall they main be piped with a common pipes from the mains. Tiny adjustments will make the heat ping-pong between these two radiators. You need to make fine adjustments!

    When you have competed these adjustments each radiator will be getting the right amount of flow from the boiler and the system will be balanced. You can reinstall the thermostat heads and you should be able to control the room temperatures with them.

    One final twist: Your wall thermostat is really measuring the temperature in one room and putting heat into the radiator without the thermostatic valve nearby. It is using the heat losses and gains in this room as a proxy for the heat losses in all the rooms. If this radiator is too big you will end up with the room with the thermostat being the right temperature and all the other rooms never getting warm enough. The trick here is to reduce the flow to this radiator (increase the temperature drop). This will cause the boiler to stay on longer and heat other rooms more.

    In terms of saving fuel the real driver to cost savings is the difference in temperature between inside the house and outside. Reducing temperatures saves money, turning down unused rooms will save money.

    Let us know how you get on!

  • Nic
    Nic Member Posts: 17
    edited February 22
    Wow! Thank you so much, that’s a great explanation. Can I ask two questions? The bathroom radiator only has two “tap handles”, am I right in assuming the right hand is the lock shield? 
    Also, if I balance the system, if I make the temp drop more across the radiators in the rooms we use less would this mean the radiators in the rooms we use more get more heat?
    I don’t want to sound patronising but that’s a perfect explanation. Thanks John I’ll try tomorrow and let you know 
  • JDHW
    JDHW Member Posts: 0
    You don't need to make the temperature drops different on the less used rooms. Flow through the radiator will be controlled by either the lockshield or the thermostatic valve. Lockshield sets the maximum flow and the thermostatic valve can only reduce it. So turn down the thermostatic valves in these rooms and the temperature drop will increase.

    There is no difference in terms of flow control between a manual valve or a lockshield valve they are just supplied with different tops. The key point is that the lockshield is a key control for your whole heating system balance and the manual valve is there for convenience/control by the user. You need to pick one to act as the lockshield, normally the one at the low temperature end. Then you need to make sure it is not touched after you balance the system. You could cut off the top of the "tap" rather than install a different valve. If this approach causes difficulties with the "local management" try a couple of turns of razor wire around it :smile:

    I think you bathroom radiator setup could be the root cause of your problems. If the two valves are fully open a significant fraction of the total boiler flow could be going through that one radiator. Bathroom radiators are quite small so you will find the when the system is in balanced the lockshield will be nearly closed.

    Good luck - don't forget to wrap black tape on the valves where you will be taking measurements.

    BW John
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