ASHP Sizing

[andy]

Hi all

 

I'm struggling to match the PHPP data in the screen shot below with the sizing requirements for an ASHP.  If I have a peak heating load of 6.6kWh/m2 for December for a floor area of 142m2 = 937.2kWh, how do I marry that with working out the ASHP requirements (with an appropriate overheard added just in case)?

 

I am quite sure I'm missing something blindingly obvious here but any education (as ever!) gratefully received ?

[ProDave]

So December needs 973.2KWh that is 31.39KWh per day or an average power 24/7 of 1.3Kw

 

You probably don't want to run the heating 24/7 so assume it is on half the time it will need to produce 2.6Kw of heat for that time.

 

A 5KW ASHP should do you comfortably.

[PeterW]

More scientifically you need to look at the low for the month. So if you get the Met Office data for your area and look at the min OAT figure, you’ll see that January may be colder than December. 

 

I would expect to get to an average, there is a minimum that is 30-40% lower than the mean so you will have at some point a loss of 60KWh on a day. Running an ASHP for the 7 hours of the E7 period (assuming no solar) means you need around 8.3Kw/h input heat, so a 9Kw ASHP would do this with no peak usage. As @ProDave has said though, a 5Kw would work however there is no consideration for hot water provision. 

 

@JSHarris spreadsheet is much easier to use btw ..!! 

[ProDave]

My observation from my own ASHP is that is spends typically 4 half hour busts each day doing hot water, so it is "out of service" as far as heating is concerned for that period.

 

Yes you do need to consider what energy regime and if you want to do it all over night on E7 then the output for that period will need to be higher.

 

And good point for allowing extra for particularly cold spells.

[PeterW]

I’d be inclined to just leave it on in really cold periods as there is no point in being cold in a house ..! Think I worked out that the 9kw I installed recently is using about £1.12 per day on E7 to keep at 19.5c, but that is all overnight heating.

[Jeremy Harris]

We've only been running our ASHP overnight on E7 since early January.  Despite some pretty cold (for here) weather the house has maintained a fairly steady temperature of around 22°C.  It seems that with the slab being charged up overnight it can easily maintain the house at a comfortable temperature all day.  I've not accurately worked out the cost, but would guess that the worst case has been around £0.60 per day (at E7 off-peak rate), with quite a few days when the heating hasn't come on at all.

[andy]

See it was really simple and makes perfect sense now!!  Thanks all, I think a 9kW ASHP would cover worst case scenario and running it at a lower demand should give a better CoP I hope.

 

I will be having PV and battery storage (later) so would need to do the maths about E7/10.

[jpinthehouse]

Hi

Im just at the stage of chosing and having air source sized also

First size ive had was 12kw, our house is 300sqm exterior,3 floors, integral garage and up graded insulation, so how can i get the system specked.

Rather than start a new topic does anyone know where to get 32 mm twin pipe underground insulated pipe to get from house to air source in the garden

Thanks

John

[PeterW]

19 minutes ago, jpinthehouse said:

Rather than start a new topic does anyone know where to get 32 mm twin pipe underground insulated pipe to get from house to air source in the garden

 

How far do you want to go ...?? You can make your own using 110mm corrugated ducting, 22mm Hep2O and and 22mm x 19mm Nitrile rubber insulation. You can wrap it in pallet wrap to slim it down a bit and it will slide through. Add a couple of 20mm ducts for cables and you’ll be fine. 

[andy]

@jpinthehouse I was just looking at that too - I'm not saying this is the best pipe to use, it seems to be a good spec:

 

http://www.ufhn.co.uk/shop/product/uponor-ecoflex-thermo-twin-pre-insulated-pipe

 

Having just seen @PeterW replying though perhaps my advice is overkill!  I think I'd have about 10m to run under the slab and I have to be 1000% sure there won't be any future issues...

[ragg987]

6 hours ago, andy said:

I think a 9kW ASHP would cover worst case scenario and running it at a lower demand should give a better CoP I hope.

I found running the pump fully modulated provides best COP - usually 30% of max capacity is the lowest speed. So running at max capacity to take advantage of the shorter hours for E7 will result in lower COP - both because it is colder at night and because you are running at full power. This may not matter as E7 is "half-price" - your bills should still be lower.

 

I prefer to run it at a fixed set temperature 24x7, so I can get away with a smaller pump and run it at lower power / higher COP. Plus, I have PV so running in the daytime can give me free electricity on sunny days. I do not have E7, our consumption pattern (bearing in mind said PV) does not makes it worthwhile as daytime prices are higher. As a reference, I have 7kW ASHP with a 330m2 house built to PH standards. This provides both space and DHW.

[joe90]

28 minutes ago, ragg987 said:

does not makes it worthwhile as daytime prices are higher.

 

I have just found that Octopus supply lecky (in our area) on E7 and the day rate is cheaper than I am paying currently for single rate ( so I am changing and putting timers on the ASHP)

[andy]

Interesting - https://octopus.energy/static/consumer/documents/agile-report.pdf

[ragg987]

1 hour ago, joe90 said:

 

I have just found that Octopus supply lecky (in our area) on E7 and the day rate is cheaper than I am paying currently for single rate ( so I am changing and putting timers on the ASHP)

In my area octopus are 18p e7 day rate Vs single rate 15p. I have not found any supplier whose e7 day rate is cheaper than their single rate.

[joe90]

Yes @ragg987, but with green energy I am paying 32.09p day rate and 21.29p per unit., like you Octopus is 21.13 day rate and 15.77 single rate but 18.7 day rate with 10.45 night rate on E7. I will have to do the maths to see if E7 is worth it. I need an energy monitor.

[joe90]

12 hours ago, ragg987 said:

I have not found any supplier whose e7 day rate is cheaper than their single rate.

 

Ah, not what I meant , I am saying Octopus E7 day rate is cheaper than I am paying with “green energy” for single rate!

[Jeremy Harris]

For us Bulb has a slight edge over all the others for E7, at 15.729p/kWh day rate, 8.148p/kWh off-peak rate and 20.44p/day standing charge.  Octopus Go offers a much cheaper off-peak rate but needs a smart meter plus only has a 4 hour off peak rate, which isn't really long enough for us, a it can take 5 to 6 hours to charge the slab up with heat overnight in cold weather. 

 

Not sure if/when Bulb will be putting up their prices, though.  If they do then Economy Seven Energy look to be the next best, at 17.903p/kWh day rate, 8.76p/kWh off-peak rate and 20.11p/day standing charge.

[TerryE]

As the regulars will know, we are still using a Willis Heater and won't be installing the ASHP until the late summer.  This seems to draw 2.88 kW. The E7 window is enough to keep our house at temperature unless the outside temp is consistently below about 5-6 °C and what I've been doing is to leave a small oil filled radiator on an E7 timer which outputs another ~kW into the upstairs airspace, and this is enough to keep the entire house cosy using E7 alone, though the heating algorithm will occasionally top up during the day if needed. So for us a 7×3 kWh target would still leave maybe 30-40 heating days where we would need a peak rate top up. 7×4 kWh would drop this peak overflow down to single digit days a year.  However, when you factor in the CoP of an ASHP,  the marginal annual cost of this overflow into peak rate is still extremely small compared to the upfront ASHP costs.

 

As someone who has yet to install their ASHP, and especially when I factor in payback periods, keeping the install complexity and cost low is more of a driver than squeezing the last ££ out of the running costs.  For example, as I know that my slab will happy take up 4kW over a 7+ hour heating period, I do wonder whether I even need a buffer tank, and instead go for just using a PHE to couple the ASHP and UFH circuits. 

Edited March 4, 2019 by TerryE

[joe90]

7 minutes ago, TerryE said:

and instead go for just using a PHE to couple the ASHP and UFH circuits. 

 

Why would you need a PHE ?

[Ed Davies]

5 minutes ago, TerryE said:

…and instead go for just using a PHE to couple the ASHP and UFH circuits. 

 

Why keep the circuits separate? Cost of the PHE and an extra pump vs the cost of a few more litres of antifreeze.

 

[Ed Davies]

Snap.

[TerryE]

9 minutes ago, joe90 said:

Why would you need a PHE ?

 

Need to think this one through. Hummm. I feel a conversation with Jeremy coming on ?

 

Edited March 4, 2019 by TerryE

[ProDave]

Just wire the ASHP in series with the Willis heater. Then you can use either.  As already noted, just a bit more antifreeze.  It needs to be about 25% to withstand temperatures down to -10

[DenkiJidousha]

Had to google that, PHE = plate heat exchanger, which can be used to transfer heat into the hot water tank (especially with a low temperature heat pump), versus using a traditional coiled pipe running though the bottom of the tank (with a high temperature heat pump)?

 

Getting this back to the original topic, ASHP sizing, I'm looking at replacing our mains gas combi boiler which provides domestic hot water (DHW) and under floor heating (UFH) to our three bedroom two story approx 125m2 mid terrance house, built about 2005 with reasonable insulation (EPC rating C). We have 5kW of solar PV and a time of use electricity tariff.

 

I'm still chasing local installers to get multiple quotes, but the first one back suggests (before doing an on site visit) a 12kW monoblock HT heat pump (space will be tight so the monoblock makes sense), no thermal store for the UFH, and a 250 litre slimline cylinder or SunAmp for the hot water. I'm wondering if a 12kW ASHP is too about right or too big, and what the trade offs are.

 

Too small a heat pump: Have to leave it running more hours a day (problematic with time of use electricity tariffs), or in extreme cases would need supplementary heating.

 

Too large a heat pump: Wasted capital cost. Would only be on in bursts, making it less efficient (short cycling issues)? Would this strengthen the case for a thermal store for the heating side (again higher capital costs)?

 

In terms of data, I have about a year's worth of hourly gas usage via a Loop Energy Saver (since replaced with a smart meter). Looking at peak daily usage, and peak hourly usage, and counting on the morning and evening hot water demand spikes to be spread out with the storage, I think a 12kW heatpump should be more than up to the demands - perhaps even overkill.

Edited October 23, 2019 by DenkiJidousha
Added keyword short-cycling

[PeterW]

13 minutes ago, DenkiJidousha said:

Had to google that, PHE = plate heat exchanger, which can be used to transfer heat into the hot water tank (especially with a low temperature heat pump), versus using a traditional coiled pipe running though the bottom of the tank (with a high temperature heat pump)?

PHE is overkill here as they are good for multiple sources or where it’s not a “clean” source. You would still need a coil on the tank as otherwise you’ll need a Thermal Store and that needs much higher temperatures. They also have more moving parts and you need an additional pump.