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Heating and cooling: My journey with ASHP under the BUS scheme

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    As I ask a lot of questions I thought I'd give back a bit for others and use this thread to document my ASHP installation under the BUS Scheme (£7500 grant): https://www.gov.uk/apply-boiler-upgrade-scheme/what-you-can-get

    In my last 4 bed Victorian terraced house I installed my own Mitusbish Ecodan 8.5KW Heatpump and Kingspan cylinder back in around 2014 having minimal knowledge. I ran a mix of rads and UFH mostly run at about 35c 24/7 for the heating season. In the 9 years the only fault I had was one sensor and a zone valve. Other than that it worked brilliantly.

    Moving in to our 4/5 bedroom detached house (1960s cavity construction ~200m2 floor area) earlier this year my first thought is to replace the 24 year old system boiler and rads as part of the renovation which will include an extension, cavity insulation, EWI system and upgraded windows.

    I calculated my heatloss as approximately 7kw assuming 0.17 U-Value for walls and 1.4 U-Value for windows/doors. U-Values were claculated using:
    https://www.changeplan.co.uk/u_value_calculator.php

    Heat loss calculation was done using MCS spreadsheet available here:
    https://mcscertified.com/wp-content/uploads/2021/04/MCS-Heat-Pump-Calculator-Version-1.10-locked.xlsm

    I then contacted 2 local companies to me who offer the BUS scheme using the MCS link here:
    https://mcscertified.com/find-an-installer/

    The first company came out and said I need a 8.5kw system and quoted £4000 (after the £7500 discount) to supply and commision a Samsung 8.5kw and 210l cylinder which I thought was excessive.

    The second installer looked at my calcs and did their own and we agreed we need ~10.5kw to cater for DHW and the proposed extension and potential loft conversion. Their recommendation is a Vaillant 12.5kw, 300l matching cylinder and a buffer tank. Cost to me after the grant is ~£2000. He can supply cheaper units but recommended the Vaillant as better weather protection in our environment.

    On site he had advised no buffer tank needed but this was on the quote so their may be some savings to be had and I may ask for a smaller cylinder (only 4 people live here). So will chase this up this week. He also kindly provided radiator sizes which again more or less matched my calcs give or take aprt from 2 rooms which I actually think my figures are right.

    All of these prices are for myself to rip out the old cylinder, system etc. get power supply to the 2 new locations and get the cylinder signed off. Also upgrade all the rads and plumbing.

    So the rads and pipework are being installed/laid out this week so will update as we go.

    Questions
    1. Preference on heat pump manufacturer?
    2. Thoughts on buffer tanks? Radiators will be oversized anyway and UFH in 50-60m2 area
    3. Size of cylinder for 4 people in a 4/5 bedroom house?
    4. With all pumps now being inverter type is a 12.5kw going to be an issue if my heat loss ends up being less? (with better insulation etc)
    5. Controls? Just use one stat and TRV's on all upstairs rads?
  2.  
    Any thoughts on my queries all?

    I asked the supplier and his comments were:

    1. Vailant have better protection in a coastal environment
    2. He suggested this will be used as a volunteer as opposed to a buffer
    3. He recommended 300 litres as the price difference between 200-300 is marginal
    4. Didn't ask him
    5. Didn't ask specifically but he said the vailant controls are the best on the market with data readily available from the system

    Thoughts?
  3.  
    Any thoughts on my questions?

    Would a Samsung Unit be better value than the Vaillant? Apparently it will be ~£1k cheaper

    Any thoughts on tank size?
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    2 volumiser is better than buffer

    3 doesn't really matter, 8-12kW heat pump can reheat even a small cylinder faster than you can use it

    4 the 12kW vaillant apparently modulates down to a minimum of 5.8kW (according to discussion on OEM forum). When the heat load is less than this, eg with more insulation and on milder days in autumn, it will cycle on and off which is less efficient.

    5 use the manufacturer's controls throughout. Heat pumps don't like on-off type stats, especially TPI, they prefer compensating controls which gradually reduce the flow temperature if the house is too warm rather than shutting off the heat pump and starting it again later.

    4 12.5kW
  5.  
    Sorry last line above was a typo, I can't edit it out.
  6.  
    Thankyou for your input

    Modulating down to 5.8kw is that going to be a problem? Should I be asking for a smaller model?

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