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System boiler kw sizing and expansion vessel

Discussion in 'Plumbing and Central Heating' started by LozM, 11 Oct 2021.

  1. LozM

    LozM

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    Hi,

    After a bit of a saga with my plumber (posted before), I need to chose boiler size as soon as possible, the plumber is not going to do any heat loss calculations or do anything to work out accurately so I am on my own. Appreciate this is not what should be doing but after some level of finger in the air guidance. The plumber had said he thinks the baxi 824 (24kw) system boiler is fine for property, annoyingly they don't do a 28 in that range. Could go for a 28kw baxi megaflow but an older boiler with less of a guarantee. There is a Baxi platinum but that only comes in a 32kw model. My preference is for the 824 24kw boiler if can cater for the house (there will be no further additions once renovation is complete).

    Here is summary of the house:

    1955 detached house with cavity wall insulation, it is 4 bedroom two of which added to new building regs so well insulated. House is not particularly cold in that there is only one room where radiator is on full in winter.

    At the end we will have 13 radiators, many of them quite small, have done some calcs and KW of them all seems to add up to on 12kw - seems low as guidance seems to be 1.5kw per rad but a few of them are towel rails and small radiators. Worse case guess would be 16kw.

    We will also have 2 zones of under floor heating first zone 1 is 13m squared and second zone 2 is 24m squared using 16mm pipe.

    We have 3 showers but 2 electric so only one will be running off tank, did consider combi but would be long way from where water needed and decided on the system approach with unvented tank in centre of house.

    We will be going for a 210 litre indirect unvented tank from gledhill - StainlessLite System Ready Indirect | Gledhill - https://www.gledhill.net/products/unvented-cylinders/stainlesslite-system-ready-indirect/, the manual lists the 210 tank having a primary coil rating of 20.5 kw - not sure if reading that correctly as cylinder calculations normally allow 3-5kw.

    Another thing that has been mentioned is that the 824 has an integral expansion vessel of 7L - is that enough for 13 radiators of varying sizes (6 are doubles) and the approx 37m squared under floor heating with 16mm pipe ? Seems a lot of people mention fitting a separate expansion vessel but never come across that before with friends systems. The 28 kw and upwards boiler comes with a 10L integral tank.

    Any thoughts greatly appreciated.....

    Lawrence
     
  2. Swwils

    Swwils

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    Get a new plumber and hire an actual heating engineer.

    You want as bigger radiators as you can accommodate and run them as cool as possible with a deeply modulating boiler like a viessmann. This will also work well with your "biggest possible radiators" aka underfloor heating.
     
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  4. LozM

    LozM

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    The plumber is tied in with the builder I am using but tempted to see if I can get a heating engineer that knows what they are doing round to advise on the best setup for the plumber to install,

    Many thanks,

    Lawrence
     
  5. Swwils

    Swwils

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    Yes.

    And make sure they know what they are doing and don't give you a system designed to operate at 70 degrees.

    It should really be designed for 50 or lower. Possibly even with a future heat pump in mind, that will require larger radiators to be cost effective.
     
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  7. muggles

    muggles

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    You're not obliged to use the builder's installer. It's your house.

    Go to www.heat-engineer.com and pay the £12 to get access to the full heat loss calculator. That'll give you the most accurate result possible. As a rule of thumb I use 1kW per radiator, so 13kW in your case, but the calculator will give you a proper result. 80% of UK homes need less than 10kW.

    As others have said, get the radiators sized correctly for a condensing boiler. Ideally 60°C flow temp / 40°C return temp, or lower if you have the wall space. Condensing boilers don't automatically condense - they need to run at low temperatures and be set up correctly.

    Look at boilers which can do hot water priority to provide fast heat up of your cylinder and low temperature heating, preferably with OpenTherm controls capability. Joule Cyclone cylinders are far superior to Gledhill...
     
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