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Heating and cooling: ASHP and gravity / vented hot water supply

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    • CommentAuthorDur
    • CommentTimeFeb 9th 2022
     
    Good morning

    Apologies, as ever, if this has been covered before but I can't find anything much.

    As part of an extensive renovation, we are wanting to fit an ASHP but we have fairly feeble water pressure/flow and so have a 100 gallon header tank in the loft currently feeding a traditional vented copper hot water tank with 22mm pipes on the main runs all in as new condition. So while not much else in the house works as yet, we do have a decent shower and wouldn't be happy with a shower at mains flow/pressure.

    Our potential supplier /installer would like to fit a 300 litre high gain unvented tank which I think is a pretty standard approach. He has looked to see if there is a vented equivalent but to no avail.

    So I am hoping for some help on potential options that maintain the efficiency of the purpose made unvented tanks but give us the advantages of the old fashioned system ie good flow and the ability to have two showers running and no dip in flow if someone fills the kettle.

    I guess we could have a pumped pressure set using the loft tanks as break tank/ reservoir but that is more hardware and complication (and we lose water in a powercut).

    Are there any other approaches we can look at?

    Thanks!
    • CommentAuthorEd Davies
    • CommentTimeFeb 9th 2022
     
    Why does the supplier want to change the tank? Bigger tank? Or bigger coil? Why would either of those help with a HP?
    • CommentAuthorDur
    • CommentTimeFeb 9th 2022
     
    Posted By: Ed DaviesWhy does the supplier want to change the tank? Bigger tank? Or bigger coil? Why would either of those help with a HP?


    The old tank is very old (uninsulated type) copper and is actually currently jury rigged to give us hot water so we definitely need a new tank to a decent insulation spec. The 300 litre volume and the high gain coil is so we can keep the hot water temperature as low as possible to help the HP efficiency.
  4.  
    Having the same discussion with an installer. There is a valid point that HP-compatible tanks have much larger coil areas (low DT as Dur said) and different temperature sensors.

    But it really seems to come down to liability - the HP installers all complain about being called back to installations where they integrated a hi-tech new heat pump into an aging CH system. When something stops working a year later, the householder blames the HP installer, who has to hurry back and diagnose it.

    They would prefer to fit a complete new system as a black box, with a cylinder that is supplied and warrantied by the HP mfr, comes with all the pumps and valves and HP controller pre-plumbed and pre-wired and will definitely work without any thought processes. This seems cheaper for them overall, and hence for me.

    I have promised that I won't phone them if my existing cylinder doesn't work, but they don't seem convinced!

    Dur, any reason why the unvented cylinder cannot be fed water from your gravity tank? As far as the cylinder is concerned it's just a pipe that supplies cold water, it doesn't care where the water comes from. The unvented bells/whistles such as accumulators and psv will be unnecessary but will probably come bundled in the price.
    • CommentAuthorDur
    • CommentTimeFeb 9th 2022
     
    Posted By: WillInAberdeen
    Dur, any reason why the unvented cylinder cannot be fed water from your gravity tank? As far as the cylinder is concerned it's just a pipe that supplies cold water, it doesn't care where the water comes from. The unvented bells/whistles such as accumulators and psv will be unnecessary but will probably come bundled in the price.


    Well this is the bit that I am not sure about. From my limited understanding, an unvented system uses a pressure vessel to deal with expansion and an old vented system vents above the header tank level to do the same thing
    I don't know that you could set up an expansion vessel to work properly with only 5 or 6 m head. Maybe but I just don't know. Maybe you could actually just take a vent pipe off the hot water feed out of the unvented cylinder, lag it well and run up to the loft.
    It could be that I simply don't understand the system well enough and this is plain wrong!
    • CommentAuthorDur
    • CommentTimeFeb 9th 2022
     
    I spoke to a tank manufacturer and it sounds like it might be possible to get a vented tank with the right spec - they are just not a standard item.
    I will report back if/when I know more...
  7.  
    The expansion vessel for an unvented tank is fitted on the flow between the non return valve and the tank. If your installer insists on an expansion vessel rather than the alternative I am suggesting (he should know better) then fit the vessel and set the preload pressure to match the height of the water in your loft header tank. (10m = 1 bar), otherwise in place of the expansion vessel fit a vent/overflow pipe up to the header tank as you have now. (and insulate the pipe)

    Done as above you will be able to use the standard offering 300 litre high gain unvented tank which will probably be better than a non standard item which will almost certainly have a longer lead time and probably cost more.
    • CommentAuthorDur
    • CommentTimeFeb 9th 2022
     
    Thanks Peter. It sounds like there will be a solution one way or another.

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