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Combi or system boiler for new build with UFH?

Discussion in 'Plumbing and Central Heating' started by Ruggers, 22 Aug 2020.

  1. Ruggers

    Ruggers

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    Advice required for what type of boiler would be recommended for a 2 story detached self build. It will have an upstairs and downstairs bathroom, wet UFH downstairs with radiators upstairs. The boiler and tank if required will be located downstairs.

    I've used a combi boiler for the past 13 years that i installed, a Baxi 33 platinum, it's been a brilliant boiler, never needed anything so I'd like something as reliable as this. I know SAP will determine a lot of it from what i've heard but you can recommend your preference models over the poor ones offered.
     
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  3. oldbutnotdead

    oldbutnotdead

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    With 2 bathrooms you'll have a better chance of decent hot water performance with a cylinder of some sort. Nothing stopping you having a cylinder on the heating side of a combi, you could use the combi hot water for kitchen & cylinder water for the 2 bathrooms.
     
  4. muggles

    muggles

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    A system boiler and cylinder would definitely be the preferred setup. Go for a boiler capable of providing hot water priority and two flow temperatures - a high temperature flow for fast cylinder heating, and a lower flow temperature for the space heating. Size the radiators to run at 60°C flow / 40°C return for best efficiency - standard radiator calculators don't do this so you'll need to apply a conversion factor.
     
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  5. ianmcd

    ianmcd

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    it all depends on your HW requirements, modern combis have great HW setups but all depends on your usage, any combi will happily feed your CH requirements
     
  6. Ruggers

    Ruggers

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    I wasn't aware you could split them this way. Someone will be doing the boiler instal, flue and commisioning. I just want to know what the best options are available before i make contact with trades. Taking the unvented cylinder away from it, what's the main differences between the combi and a system boiler? combi is DHW priority over heating and can't do both at the same time.

    There will be approximately 75m2 of UFH and upstairs will have 7/8 radiators. It's unlikely we will run 2 showers at the same time as we manage now with a bath and separate shower but I suppose it's best to cover the use of two in use.

    Does the hot water priority mean that while someones running a shower/bath the heating goes off like a combi would and no 3 port valve?
    Could you recommend a boiler that has the features you mention so that i can have a read into it. I've just had a quick read, are you referring to Delta T50 for the flow and return? I've always ran my combi C/H at 75 degrees C, is 60 now the optimum setting?
     
  7. oldbutnotdead

    oldbutnotdead

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    As you said, combi can't do heating and dhw simultaneously. Most combis have more moving parts (so more things to fail). Combi is a good space saving solution for small houses with single bathroom. To my mind there's not much point having 2 bathrooms if you can only use 1 at a time.
    Another thing to consider- get your dynamic water flow and pressure tested before you make any decisions about DHW- combis and unvented can only deliver what they're given through the main, if your supply is at the minimum service obligation (10 litres per minute at 1 bar) then both combi and unvented will be very disappointing, you'd be better with a vented 250 litre cylinder & header tank and shower pumps.
    EDIT Unvented can only be installed and worked on by a subset of RGIs, they require (afaik) annual maintenance again by suitably qualified persons. Worth knowing the cost of said ongoing specialist work before you commit
     
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  8. denso13

    denso13

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    Not really, you can have the relevant unvented qualifications without being an RGI.
     
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  9. oldbutnotdead

    oldbutnotdead

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    Ah, I sit corrected :)
     
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  11. muggles

    muggles

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    Hot water priority means the full power of the boiler is used to reheat the cylinder as required, but you would have a 3-port valve installed (or two two-port valves). So yes, the central heating would go off briefly, but experience shows that it's such a short period of time that nobody notices. The advantage over a combi is that you get stored hot water and higher flow rates (assuming your incoming mains is up to the higher flow rates of course). Intergas boilers are great for this, and in your highly insulated new home the Xclusive might well suit you (set up in system boiler mode of course). The suggestion for sizing radiators for 60/40 is to more closely match them to your UFH temperatures - running the system at a lower temperature will put less stress on your UFH mixing valve, and cooler is more efficient anyway. The absolute key is to ensure at least a 20°C differential between primary flow & return temperatures for heating, and to ensure the return temperature never exceeds 50°C, so you could run at 70/50, it'll just mean your UFH has more work to do blending down to a lower temperature
     
  12. JohnD

    JohnD

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    very reasonable. some of them are electric-only, and some are on oil boilers. I know somebody who has a biofuel burnace.
     
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  13. ianmcd

    ianmcd

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    Does it burn his ace ?? :rolleyes:
     
  14. JohnD

    JohnD

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    stable waste and woodchip.

    Oh, I see.
     
  15. wannocks

    wannocks

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    If its a new build it should be very well insulated. This would make any combi massively oversized for the heating load causing cycling and inefficiency.
    The lower you can run the radiator temperatures the better. Modern controls can let the boiler run at a higher temp for fast HW recovery and a lower temp for efficient heating.
     
  16. Ruggers

    Ruggers

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    So replying to all answers with more questions, the Walls are cavity construction, 100mm wall insulation, 150mm floor and around 300mm in the loft i believe, I'll also be installing MVHR.

    1. I was going to fit TRV’s on all the upstairs radiators if required and have an upstairs portable, or landing wall mounted thermostat. Downstairs with the UFH i was thinking individual room stats are a good idea?

    2. How do i get the mains dynamic flow and pressure tested, can this be done from the road or does it require the house built and mains line to stop cock installed first? If so SAP may have already specified a boiler by then?

    3. Do system boilers require a more servicing than a combi boiler, do the tank components require annual servicing also?

    4. Why would a system boiler with hot water priority need a 3 port valve, when would it ever be in the middle position to feed both C/H & the cylinder.

    5. To get the return temperature 20 degrees less than the flow, is this controlled by the radiator outlets or just naturally from the design of the system pipework? Where are the temps taken from, boiler flow and returns or each radiator pipe? I read years ago about getting the radiators to have an 11 degree drop in temp. from the inlet to the outlet for efficiency.

    6. Does having a boiler heating temperature set at 60 instead of 75/80 not take a lot longer to heat the house with the water being cooler? My current house is set high, i may lower this if so.

    Hopefully I can take away the comments & do some more research.
     
  17. oldbutnotdead

    oldbutnotdead

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    1 Upstairs knock yourself out. Wherever you put the thermostat should be heated by a rad without a trv (otherwise you get trv and thermostat squabbling)
    UFH No, single thermostat & balance the zones at the manifold.
    2 If the meter etc has been installed at the road you can get a guide there (the size & length of pipe from meter to house will have a negative effect. Up to 30 metres, 25mm will be fine. More than that start thinking about going bigger.
    3 Boiler service probably costs the same for combi or system. System is a much simpler beast (no diverter valves, pump usually external). Not certainabout unvented maintenance.
    4 For rapid cylinder recovery the boiler will go to max output temperature. Zone valve(s) will prevent the rads getting too hot.
    5 Radiator sizes. Think @muggles explained that earlier(do your heat loss sums then size rads for 60 deg not 75 deg)
    6 If the rads are sized for correct heat output at 60 degrees then fine, otherwise less fine. Try it come winter
     
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