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Heating and cooling: Radiator and pipe sizing for heat pump

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  1.  
    • CommentAuthorEuskalElle
    • CommentTimeDec 30th 2015
     
    I want to see if I can replace my oil fired central heating boiler with a heat pump. I understand that for this to be effective and ecological, I need to choose a pump with a 45° or 50° water delivery temperature. So, step 1 is to work out the heat loss calculations, step 2 to size the radiators, step 3 to size the pipes. Then see if the existing system is practicable to use / change / adapt.

    In theory this is fine: in practice, steps 1 to 3 are a bit challenging for me! Can anyone point me to resources, preferably online, where I can find out how to make these calculations? After asking a local renewables energy rep to design a system, I do have wall mu values, which helps. (The rep told me that a direct boiler replacement with a 60° system would be just fine. I believe not: at 60° I doubt whether it would either have heated the house, saved any energy cost or reduced the carbon footprint)

    The system I'm looking at is air to water: I live in the SW of France so this is, I believe, a practical option with the new generation auto-de-icing pumps.
    • CommentAuthorDarylP
    • CommentTimeDec 30th 2015
     
    EuskaElle,

    Beware...more than 35deg output will push your CoP down...
    I suggest you look at your current energy demand, and see how to reduce that?
    More insulation, better glazing, improved air-tightness, then look at the HP....

    However what is your ErDF connection rated at, 12kVA, or are you on 3ph? That may be the deciding factor?

    Good luck:smile:
    • CommentAuthorchuckey
    • CommentTimeDec 30th 2015
     
    if your existing system is correctly designed. Then it would be designed for a water temperature of 70 degs, i.e. 50 degs over your designed house temperature, so you have 50 degs of heating. If your new water temperature is 35, i.e. 15 degs over the house temperature, you need 50/15 times as much radiator surface. and you need 50/15 as much as hot water, i.e. more flow. So if the pressure drop in your existing pipes and your existing pump can cope with this, just crank up the pump speed. However if the water flow gets too fast then the system will be noisy, under these circumstances , larger diameter pipework would be required.
    Frank
    • CommentAuthorEd Davies
    • CommentTimeDec 30th 2015
     
    Maybe you can work backwards from how much oil you use now.
    • CommentAuthorMackers
    • CommentTimeDec 30th 2015
     
    I can't see it being feasible as a direct replacement unless your doing other fabric upgrades.

    Look up BSRIA guide to mechanical engineering calculations or the CIBSE concise guide for guidance.

    A quick Google will also turn up many resources of his to pipe size.
    • CommentAuthorEuskalElle
    • CommentTimeDec 30th 2015
     
    When the heating system went in when the house had no double glazing or draught-proofing, and is capable of heating the house very quickly. By the time the heat pump would go in, there will be double glazing, draught proofing and a good level of insulation. At a guess, heat load will be reduced by a factor of 2 or 3, given the state of the 30 to 100 year old windows the glazing will replace. Being prepared for long heating runs, ie, all day rather than morning / evening, I suspect the current system won't be far off, maybe with larger radiators if the pipe sizes will stand it, and with smart control valves.

    Supply is currently 1ph 9kva but can be lifted to 12kva. Given the (oversized) oil burner is 25kw and I can go to a 12kw heat pump, I have hope :)

    A further possibility is that the lounge has a 12kW woodburner, due for replacement, sited not far from the boiler. I am interested in using this to add heat to the CH system from a back boiler. Here, dry oak logs are cheap and plentiful, but 0.6m long, meaning a large fire or no fire. I like the fire, so need to lose the heat usefully. Ideally, therefore, I'd like a back boiler and heat pump feeding into the system. If I do this, I could safely reduce the requirement on the heat pump itself to serve the house in mid-winter. I'm not sure how the two systems running at different temperatures would interplay: maybe I'd have to use the heat pump as a pre-heater to the back boiler supply, keeping the firebox a bit warmer and adding temperature as well as calories to the CH feed.

    But whichever way, I want a designed system that I know will work before starting, which I can contract to an installer to put in, otherwise I will not be able to attract any low carbon funding for it.
    • CommentAuthorDarylP
    • CommentTimeDec 30th 2015
     
    EE,

    If you 'only' have a 12kVA connection, you will be pushed to run a 12kW HP along with any other devices, such as washing machine, dishwasher, cooker etc....?
  8.  
    I had exactly same requirements 4 years ago and went for a Daikin HT ashp which is actually two heat pumps in series , meaning it can deliver my central heating at 70degC using existing pipes/radiators with my actual real world Cop around 2.5.

    It was designed as direct replacement for oil boiler.

    I'm still very pleased and have saved good amount of CO2 and £. Been 100% reliable so far.

    If of interest, Search GBF you might find my posts about it.
  9.  
    A 12kw heat pump uses about 4 to 5 kW of electricity, so in principle a 12kVA supply is plenty. However the instantaneous current at startup can be quite high depending on inverter design so best to check with local installer of your preferred model.
    • CommentAuthorEuskalElle
    • CommentTimeDec 31st 2015
     
    WillinAberdeen, does a COP of 2.5 improve the carbon footprint? With a power station efficiency of 0.3 and a boiler efficiency of over 0.8, don't you need a COP in excess of 3 to save carbon? This is why I'm trying to get the delivery temperature down.
    • CommentAuthorDarylP
    • CommentTimeDec 31st 2015
     
    Mains elec in France is predominately nuclear, so you take your view on the CO2 'content' of that.....altho' wind/renewables inputs are on the increase....

    Yes, you will need to keep the delivery temps down to circa 35 deg for max CoP.

    In the S-of-F I imagine your ambient air-temp is not going to go too low, so you may well achieve CoP 3 or more in the heating season?

    Cheers:smile:
  12.  
    Elle, using UK grid CO2 intensity, a CoP of about 2 is sufficient to improve on my previous non-condensing oil fired boiler. If it was displacing a gas boiler then a CoP about 3 would be needed to save CO2, but there is no gas supply where I live. For UK this means ashps may only be really appropriate for off-gas-grid houses. You mentioned that you currently have oil, but not whether you have access to mains gas.

    If you do the sums using French (or Scottish!) grid intensity, you will get different conclusion and ashps may be appropriate more widely.
    • CommentAuthorEd Davies
    • CommentTimeDec 31st 2015
     
    Posted By: WillInAberdeenIf you do the sums using French (or Scottish!) grid intensity, you will get different conclusion and ashps may be appropriate more widely.
    French generation is predominantly nuclear and hydro so the COâ‚‚ intensity is very low - perhaps low enough that you could argue for resistance electric heating without the embodied energy of a heat pump (ie, trade off the HP embodied energy against that for the power stations). No idea how that would work out.

    But, it's not the overall intensity that matters; it's the marginal intensity: the amount of CO₂ caused by additional use. I'm not sure where in France that comes from, how much is via ramping up gas generation [¹] and how much via the interconnectors. I suspect quite a bit is via the interconnectors, either actually importing electricity or just exporting less so making the neighbours burn more coal, oil and gas so really it's the European (marginal) intensity that matters.

    Whatever, I wonder if a heat pump operating at sensible temperatures into the existing radiators wouldn't be sufficient most of the time and that, for cold spells, either running the heat pump at less-efficient high temperatures or supplementing with resistance heating might not be the best deal.

    [¹] Looking quickly at the weekly graphs here: http://www.gridwatch.templar.co.uk/france/ it seems like gas peak follows on days when the wind doesn't help out enough. But that's based only on a quick look at an odd week when industry and commerce probably aren't following their normal cycles.
    • CommentAuthorEuskalElle
    • CommentTimeDec 31st 2015
     
    The ambient temperatures here are about 6° higher than the UK, which does help enormously. And in the end, I would be happy with some back-up heating, either the woodburner or fan heaters: the cold spells here tend to be short anyway.

    CO2 intensities must decline over the years, so perhaps wanting a COP of better than 3 is being a bit purist, but it seems to be a practical objective, and anything less would not show an energy cost advantage. We are off the gas grid here but heating oil is cheap, and the oil burner is less than ten years old and condensing. Logs are cheap too. I'm looking for energy cost savings to offset the capital costs of the system, together with eco-finance, which on a personal basis I wouldn't take unless I was sure that my carbon footprint actually was reducing.

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