Combi KW Rating (High or low?) - Fit Deaerator and Extra Expansion Vessel?

Discussion in 'Plumbers' Talk' started by ScrewThings, Aug 12, 2020.

  1. ScrewThings

    ScrewThings New Member

    Hi All,

    I am looking to replace an Ideal Classic FF350 in a 4 bed detached house, 1 main bathroom with a bath and electric shower, also en-suite with a thermostatic shower that is currently pumped (knackered) from a DHW cylinder (essex flange weeping). F&E and CWS tanks in the loft, gravity fed system. Only wife and I, mostly take baths and unlikely to use the bath showers at the same time.

    There are 13 rads, 3 of which are towel rails. 1 rad in an extension is tall vertical designer style rad, the rest are standard sized Stelrad dual or single convector rads. Upstairs Rad pipework is all copper 15mm and the drops to downstairs are 10mm flexi plastic. All power flushed last year, and looking to have done on new install.

    Looking to replace with a combi to solve pump and cylinder problems, and some weaker flow to extension kitchen taps, not interested in an unvented cylinder, had combis in previous houses so aware of limitations.

    Water pressure measured 3 times now from same outside tap at different times by different people. This has varied from 14-16 Litres a minute. Will assume lowest value, so 14L/min incoming mains flow.

    Had several quotes - most engineers seem happy to fit the Ideal Vogue Max 40kw Combi (12yr warranty appeals, if you can claim on it) to give me the best potential hot water flow (16.4L/min). This seems logical to me that the boiler has more headroom for pushing more water flow than I have coming in (rather than less) but....

    A couple of the engineers disagreed and said would be better going with a lower rated kw boiler of 30kw-35kw max. Advice here was 40kw boiler when running the hot tap (or thermo shower) due to flow rates being lower than boiler can deliver, would cause hot water to pulse hot and cold as the boiler would constantly cycle on and off,as the flow isn’t high enough to make use of it properly (and empty the heat exchanger). Trying to find out if this is a genuine concern or not.


    A couple of other suggestions coming out of the quotes I was hoping for opinions:

    * Fit a Spirotech MV2 de-aerator to remove air and microbubbles.
    Seems like a good idea for a relatively minimal cost.
    * Fit a 15L Expansion vessel (additional to one inside boiler).
    Really unsure on this one. Only 1 engineer suggested it.

    So with the above info in mind, what do you guys knowledgeable guys think?

    Thank you for any help and advice you can give on this matter!
     
  2. The Teach

    The Teach Screwfix Select

    Well,without being able to 'walk through' your property :).

    Minded to fit an open therm compatible 32kw combination gas boiler. Central Heating controlled by an open therm room thermostat.
    Run a new gas supply if necessary.
    Fit a quality central heating filter in an accessible position.
    Spiro mv2 ?. Fit de-aerator only if the system looks like it will be slow to naturally vent air.
    15L expansion vessel mention, would need the existing system radiator & pipe volume worked out to confirm yes or no. No is a good
    guess.
    An expansion vessel fitted under the boiler on the cold water mains supply,very highly recommended.
    An excellent boiler when ideal made gas boilers.
    ;) interesting.

    Many more considerations,everyone will have differing ideas ;)
     
  3. ScrewThings

    ScrewThings New Member

    Thank you for your response Teach.

    From my research so far and suggested on all quotes anyway was a magnetic system filter (believe the Vogue Max comes with one), limescale filter and shock arrestor (again cheap enough and seems like a good idea).

    Most have said flush with chemicals, but only 1 specifically said a powerflush with a kamco machine, so would have to check that was a powerflush and not just a mains job if using the others.


    In terms of the hot water requirements and the warning we had on the 40kw model, is that definitely likely to hamper us then as I have been told by 1 engineer?. I actually have some quotes where the 40kw is less than the 32!

    My old Bristan Prism Thermo shower for example is specced to draw @ 1.5bar - 8.1 litres /min (rainfall type shower head and fixed column). At 2bar it can do 9.5 L/min so both of these are well under what even the 32kw could deliver. Theres a strong chance I might replace the thermo shower for a different or faster flowing one, and possibly the electric shower in the main bathroom for a mixer in the future.

    To a novice like me, it seems like going with a 40kw will give us a bit more scope for the future, and more DHW headroom (it can shift more than my mains can provide, whereas the 32kw is slightly less than my mains). I suppose it depends which quote I go with, as some have said to go with the 32kw anyways.

    I was just wanted to make sure I wasnt setting myself up for a problem down the line. I cant afford to have a new system fitted, and then have to replace it again in the future if it was undersized or oversized.

    I want to make it the best it can be, get it sized right, and hopefully installed right, and then have it serviced annually as you are meant to, so that it can last as long and work as well as it can for its lifespan.
     
  4. Bob Rathbone

    Bob Rathbone Screwfix Select

    Combi boilers have a modulating flame and most have a variable speed flue fan. This keeps efficiency up even at low loads. Fit the largest boiler to suit your hot water needs, the central heating will sort itself out on most houses.
     
  5. Mike83

    Mike83 Screwfix Select

    The main issue with oversizing a combi is meeting the heating demand.
    A 40kw boiler may only be able to modulate down to say 10kw minimum.
    This would mean during a low heating demand or when the system heats up the boiler will start to cycle on and off reducing efficiency.
    The lower output a combi can provide on heating mode the better for efficiency.
     
  6. Mike83

    Mike83 Screwfix Select

    1BD40D7E-8010-4870-9E4C-733E662B985D.jpeg
    these are the modulating figures.
    Both 32&40kw models seem decent for your sized property.
    Don’t be concerned about issues with the hot water. Oversized combi is fine with this.
    You can just set your maximum hot water temperature on the boiler and it shouldn’t go above this.
     
  7. ScrewThings

    ScrewThings New Member

    Thank you Mike - so it sounds like apart from costing me a bit more on gas consumption and perhaps not being quite as efficient (tho still far better than my old classic) the 40kw isnt going to cause major problems with the DHW side, which is what i was wanting to clarify, but equally, the 32kw is probably fine for the job as well.

    As i said, Ive had just a many quotes with the 40kw being cheaper than the 32kw I found it a bit odd.
     
    Last edited: Aug 13, 2020
  8. Mike83

    Mike83 Screwfix Select

    The dhw minimum flow rate on both boilers (32&40) is the same 2lpm.
    This is pretty standard so there’s no reason that a 40kw would cut off more than a 32kw.
    As long at the minimum flow is maintained the boiler will modulate to maintain temperature and stay light.
     
  9. sam spade

    sam spade Active Member

    You seem to be concentrating on the HW performance and ignoring the CH side.

    10 radiators, at an average of 1.5kW each is 15kW. Add 3 towel rails at 500W each gives a total CH requirement of 16.5kW. As you haven't said anything about feeling cold in the winter, I will assume that the rads are sized correctly. So any of the three Vogue max Combi boilers will be able to meet your requrement when it is very cold. But that is only about 5% of the year. What happens when it gets warmer?

    The '26' model can modulate down to 3.7kW, which is equivalent to an external temperature of 17C, while the '40kW version can only modulate down to 5.7kW. equivalent to a outside temperature of 14.5C. When the external temperature is above 17C or 14.5C the boiler will have to run in on/off mode to maintain the required temperature, which is less efficient. So, considering the CH requirement only, the '26' model would be the better choice.

    Now consider the HW performance of the '26' version. The spec states 10.6 litres/min at 35C rise and 12.4 lpm at the 'specific rate' (however Ideal define that) which is a 30C rise. Showers and baths should not be more that 40C for safety reasons, so there is no point in heating the water 60C only to mix it with cold to bring it down to a safe working temperature. (It's different if the water is stored in a cylinder as prevention of Legionella has to be considered.) With a 14lpm flow a 26kW boiler will raise the temperature by about 26C, or from 14C to 40C, Cold tap water is normally about 15C.
     

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