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Heating and cooling: Boiler output and sizing....Am I stupid?

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    Ok sorry if this is a dumbass question...but I need a definitive answer and I know I can get it here.

    I hope this makes sense. This is all theoretical BTW.

    I have two boilers one 24kw the other 52kw (the KW amount is unimportant, they just need to be different)
    All things being equal, that is cold water feed temp, ambient temp, same flow rate etc., which would burn more gas, (and therefore be more expensive to run) to run a shower for 5 mins at say 40% delivering similar amounts of flow.

    Essentially are boilers like cars? Big engine cars cost more to run than small engine cars. However the big engine cars are capable of greater torque and higher speeds.

    Or not?

    I am looking at a higher output boiler to buy (Possibly an Andrews Fastflo) I would like to have the potential to run 3 showers and 1 bath. But realistically one shower will be used most of the time. Will I be using more gas to run 1 shower with a higher output type water heater/boiler, than a smaller output combi?

    Many thanks
    Tim
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      CommentAuthorSteamyTea
    • CommentTimeSep 22nd 2012
     
    You need to find out the efficiency at different output temperatures and flow rates, the power and torque curves if you like.
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      CommentAuthornigel
    • CommentTimeSep 22nd 2012
     
    If you want to run that much from one boiler you would be better off with an storage based system so that the peaks of usage reduced. That way you can have a much smaller boiler. The other advantage of a stored system is that you can have a pumped primary meaning hot water is circulated near to point of use so you dont have to draw off much water before it is hot. Also easier to add solar at a later date.

    Most gas boilers modulate their output to match demand but there are limits to that. So potentially they would both use the same amount of gas but if usage is not within the modulation limits of the boiler then it would be less efficient.
  4.  
    Hi,
    As mentioned it's difficult to be exact as there are so many variables. I would be tempted to use the smaller boiler and run the main shower and bath off this. The less used showers I would consider fitting electric showers. Although the electric showers will cost more to run overall it is possible that it will be the most economical way to do it. I would also check that the main water feed is capable of dealing with the flow rate if everything is being used.

    Richard
  5.  
    Great thanks for the response.

    Nigel...I am planning on some sort of store. I do/will have solar. I was looking at an Andrews fastflo to take DHW water from the store (via some sort of heat ex.) This boiler modulates??? to account for the incoming temp of the DHW. However the FASTFlo model comes as a 52 kw.

    I havent decided on a store yet (HELP!!!) and I am still considering a stove with back boiler....though this looks less likely as I look at the costings and the overall benefit of this addition.

    As for heating Ive got 100m2 of UFH which I still need to work out!!

    I have 3 young/teenage kids and 2 showers and a bath. It is a newbuild /restoration project and I am designing a system. (or struggling to design a system)

    Nigel, from what I understand having a pumped primary is the equivalent of having a large heatloss loop in the loft constantly being pumped with assocated electrical and heat loss expense. Is this the case or am I being to critical?

    HalcyonRichard I would not consider an electric shower. I hate them sorry!!!

    Tim
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      CommentAuthornigel
    • CommentTimeSep 22nd 2012
     
    You would have to check if the andrews is compatible with solar input. Some do not work with preheated water.

    I assume you are going to have two boilers 24kw for heating and the andrews for dhw.

    Is the 24kw the right size for heating. If so I would save the cost of the andrews and put in a decent size cylinder, either indirect or a thermal store. If you configure the heating as DHW priority then you should not need extra boiler capacity for DHW.

    The pumped primary should be insulated, set to very slow circulation speed and set on a timer so it runs only at the times when DHW is most used.
    Then it comes down to the heat loss in the circuit versus the heat loss and cost of water wasted.
    • CommentAuthorCWatters
    • CommentTimeSep 22nd 2012
     
    If you want to run multiple showers and other outlets at the same time then first job is to check the flow rate available from the water main. If that can't deliver enough then some form of stored water system will be needed.
    • CommentAuthoralec
    • CommentTimeSep 23rd 2012 edited
     
    For that demand of hot water you will need some store of water with the right pressure and flow rates...

    boiler sizing should be calculated to work out the heat loss of the building, a system of hot water priority should be specified on the boiler.

    Many times you actually end up specifying the boiler for the hot water coil size....or stick with the correct size for the building, enhancing boiler efficiency with a slightly undersized boiler for the cylinder.

    Condensing Boilers are most efficient at low temps so compensation controls are a must to ensure that comfort is not compromised. Almost all manufacturers offer these....

    Also think about your hot water usage.. A viessmann 222 19kw will give 150 l of hot water in the first 10 minutes...and run one (sensible flow regulated) shower for ever.....

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