ASHP Sizing

[DenkiJidousha]

Quoting from another thread, 

So too small is bad, too big is bad, want it to be just right...

 

Our first quote at 12kW may be fine. Still waiting for the rival firms' quotes, will be interesting to see if they size it the same.

[joe90]

Well I just guessed, working on what other people had for similar builds and the fact a 4kW one came up on  Ebay and it works just fine .

[Triassic]

On 24/10/2019 at 16:39, joe90 said:

Well I just guessed, working on what other people had for similar builds and the fact a 4kW one came up on  Ebay and it works just fine .

I’m currently looking for an ASHP,  who did you get yours off, was it a lone ebay seller or a company EBay seller?

 

Any got any pointers to a good ASHP (for heating and cooling) for sale, something around 7 KW.

[joe90]

9 minutes ago, Triassic said:

I’m currently looking for an ASHP,  who did you get yours off, was it a lone ebay seller or a company EBay seller?

 

Any got any pointers to a good ASHP (for heating and cooling) for sale, something around 7 KW.

 

Just an Ebay seller, not a company (I think he was a professional Ebayer, buying and selling stuff). As long as it’s an invertor driven unit, most are re badged stuff, mine is a Kingspan re badged from a Carrier unit like @JSHarris bought. I think it’s luck, I bought mine 18months before I needed it. It was new, still shrink wrapped and on a pallet so worth a punt as it was so cheap. I don’t know about this make https://www.ebay.co.uk/itm/Ecotec-7-KW-Air-Source-Heat-Pump/163912346034?hash=item2629efe9b2:g:9BcAAOSwZH1dmJ0H

 

you need to be quick, or notice the seller and make an offer if it does not sell.

Edited October 27, 2019 by joe90

[PeterW]

Samsung 5Kw Gen 6 unit for less than £2k

 

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F223220316591

 

You would need to get a control unit for it, but that’s about £400

[SteamyTea]

2 minutes ago, PeterW said:

Kw

kW

[DenkiJidousha]

Our first quote was 12kW, second 8.5kW and the third looks likely to also be ~8kW (waiting for paperwork).

 

Having been watching the half-hourly gas usage via the smart meter, and disregarding the combi-boiler's spikes in usage for hot water, I think the lower figure makes sense.

[SteamyTea]

It is normal to oversize an ASHP, so you may well find that the 12 kW is the better fit.

Under the old RHI rules, the ASHP had to supply enough energy 99% of the time before any suplimentary heating is required. 

Was a sensible rule.

[gc100]

Good timing this thread. The supplier I have chosen done some detailed calcs for our house and came back with a 8.5Kw system (160m2). However I then pointed out that the room volumes are much larger than a normal house due to valued ceilings all all rooms bar 2. His response was:

 

Quote

We always have 10% factor built in and the loss is within the design requirements for the 8.5KW.   However it is tight and the recovery time on a 300 cylinder with an 8.5KW will be excessive (over 1.5 hours). That in mind I would go for the 11.2KW but let me know your thoughts.

 

 

So it seems I'm on the edge of both systems. For me its not the extra initial outlay (its not much more to be honest), its more the long term running costs/maintenance - what do you think would be best?

 

[joe90]

Well I didn’t do any figures on ASHP size fir our passive esk build I just took a punt at a 4kW that I found cheap on EBay ( @Jeremy Harris had one about that size and that was good enough fir me ? ) and it’s fine, never seems to defrost, does all its required and our build is about 240sq m.

[gc100]

1 minute ago, joe90 said:

Well I didn’t do any figures on ASHP size fir our passive esk build I just took a punt at a 4kW that I found cheap on EBay ( @Jeremy Harris had one about that size and that was good enough fir me ? ) and it’s fine, never seems to defrost, does all its required and our build is about 240sq m.

 

fair enough! Thing is ours is not passive - its a barn conversion and because of some really stupid planning laws we need to retain/rebuild some really heat zapping walls and I suspect the best we can do will be standard regs levels of heat loss.

[SteamyTea]

13 minutes ago, gc100 said:

suspect the best we can do will be standard regs levels of heat loss.

Are you going for UFH and can you add extra insulation there. Get a tripple win that way.

I would go for the bigger ASHP. They are not quite like cars where you take an everyday hit for the few holiday trips or towing holiday.

What modulation is all about.

[gc100]

5 minutes ago, SteamyTea said:

Are you going for UFH and can you add extra insulation there. Get a tripple win that way.

I would go for the bigger ASHP. They are not quite like cars where you take an everyday hit for the few holiday trips or towing holiday.

What modulation is all about.

 

Yes my gut tells me the same, just wondered what other peoples experiences might be. Again we can't go with too much insulation on the existing slab as we'll run out of head space where the roof meets the walls (its not very big!). Though my main loses will be windows and ceiling saying all this. My fear at the moment, is that there will be a lot of cold bridges tying the cracked walls to the new inner structure for stability purposes and I will have more heat loss than calculated in SAPs. I am planning to make air tight as possible at least. 

[SteamyTea]

You can get ply and plaster board covered in aerogel. Thin and a bit better.

Why can't you dig down deeper to fit more insulation just under the pipes, and some insulation around the external edges.

[gc100]

10 minutes ago, SteamyTea said:

You can get ply and plaster board covered in aerogel. Thin and a bit better.

Why can't you dig down deeper to fit more insulation just under the pipes, and some insulation around the external edges.

 

You mean adding to the existing walls with plasterboards and aerogel on the cavity side? (FYI the existing walls will be used to clad the building, there's then a small cavity and then the main structural stud walls which will also hold the roof. These sit on new steel re-enforced lintels that sit on top of the existing slab. The existing walls will be tied into the new internal stud/structural wall ). We can dig down as this would basically be removing the existing slab which again is a no-no for a conversion.

 

[SteamyTea]

49 minutes ago, gc100 said:

on the cavity side?

Yes.

Seems strange that the existing floor\base\foundation can't be modified. Almost as if they want these old barns to fall or burn down.

[JulianB]

Evening All,

 

Thought I'd revive this topic as I'm looking at sizing my ASHP! I've used JHarris's workbook to estimate the heat demand for our old cottage with proposed extension (I've weighted the wall U values from each type of wall construction to maintain the book's simplicity). I've assumed that I'd be able to get to an ACH of 2.5 (or better), and am targetting internal temps of 21degC. Based on minimum daily OAT stats, it looks like Jan will be the greatest CH demand. Using the same methodology as pg1, this gives me a 6kW ASHP requirement, without any safety margin, or any overhead for DHW (& assuming the ASHP is on for half the day). The question is, what should I be apportioning to DHW, given that currently I have no other means of heating this up (other than the immersion?). Also could I ask if it is common place for people to hook their ASHP up to a storage tank, which would then feed either CH or DHW, and avoid the unit switching on and off more regularly? I also quite like the idea of circulating the UFH at specific times of the day to help exploit solar gain in parts of the house.

 

Any suggestions on units people have found to be reliable / good performers would be greatly appreciated!

 

Heat loss through the year, using Met Office historic climate data for the location
	January	February	March	April	May	June	July	August	September	October	November	December
Mean daily OAT for each month (deg C, from Met Office) =	4	4	6	7.6	11	13.8	16.4	16.2	13.6	10.2	6.6	5
Mean daily delta T (deg C) =	17	17	15	13.4	10	7.2	4.6	4.8	7.4	10.8	14.4	16
Mean minimum daily OAT for each month (deg C, from Met Office) =	1.3	2.4	5.2	4.3	7	11	13.9	13.2	10.4	7.5	4.3	3.7
Minimum daily delta T (deg C) =	19.7	18.6	15.8	16.7	14	10	7.1	7.8	10.6	13.5	16.7	17.3
Days in month =	31	28	31	30	31	30	31	31	30	31	30	31
Mean OAT daily ventilation heat loss energy (J) =	2807805	2807805	2477475	2213211	1651650	1189188	759759	792792	1222221	1783782	2378376	2642640
Mean OAT daily ventilation heat loss power (W) =	780	780	688	615	459	330	211	220	340	495	661	734
Mean OAT daily fabric heat loss power (W) =	1818	1818	1631	1481	1163	901	658	676	920	1238	1575	1725
Total daily heat loss power for average OAT (W) =	2598	2598	2319	2096	1622	1231	869	897	1259	1733	2235	2459
Minimum OAT daily ventilation heat loss energy (J) =	3253751	3072069	2609607	2758256	2312310	1651650	1172672	1288287	1750749	2229728	2758256	2857355
Minimum OAT daily ventilation heat loss power (W) =	904	853	725	766	642	459	326	358	486	619	766	794
Minimum OAT daily fabric heat loss power (W) =	2071	1968	1706	1790	1537	1163	892	957	1219	1491	1790	1846
Total daily heat loss power for minimum OAT (W) =	2975	2821	2431	2556	2180	1622	1217	1315	1706	2110	2556	2640
Monthly heat energy input (kWh) for average OAT =	1933	1746	1725	1509	1207	887	646	667	907	1290	1610	1829
Monthly heat energy input (kWh) for minimum OAT =	2213	1896	1808	1840	1622	1168	906	978	1228	1570	1840	1964

 

 

[Ed Davies]

11 hours ago, JulianB said:

The question is, what should I be apportioning to DHW, given that currently I have no other means of heating this up (other than the immersion?).

 

A commonly used rule of thumb is 3 kWh/person/day. Adjust to suite shower/bathing habits - more if anybody habitually takes half-hour showers with boiling water at flow rates comparable to Niagara falls or whatever.

 

11 hours ago, JulianB said:

Also could I ask if it is common place for people to hook their ASHP up to a storage tank, which would then feed either CH or DHW, and avoid the unit switching on and off more regularly?

 

That's common with oil or gas boilers but not such a good idea with ASHPs because that would mean that the ASHP would always be running at DHW temperatures whereas normally you want it running at lower CH temperatures to get a better CoP.

[JulianB]

11 hours ago, Ed Davies said:

 

A commonly used rule of thumb is 3 kWh/person/day. Adjust to suite shower/bathing habits - more if anybody habitually takes half-hour showers with boiling water at flow rates comparable to Niagara falls or whatever.

 

 

That's common with oil or gas boilers but not such a good idea with ASHPs because that would mean that the ASHP would always be running at DHW temperatures whereas normally you want it running at lower CH temperatures to get a better CoP.

 

Thanks for that!

 

I've read that anything above 45degC really penalises an ASHP's CoP - I'm thinking to either use it as a buffer and then use an additional source to get it to the typical 60 odd degC, or potentially contemplate DHW @ 45degC (I've read that some have no problem with this temperature?) ...legionnaire disease aside

[PeterW]

1 hour ago, JulianB said:

 

Thanks for that!

 

I've read that anything above 45degC really penalises an ASHP's CoP - I'm thinking to either use it as a buffer and then use an additional source to get it to the typical 60 odd degC, or potentially contemplate DHW @ 45degC (I've read that some have no problem with this temperature?) ...legionnaire disease aside

A 300 litre tank will go from 45c to 60c on a 3Kw immersion in about 70 mins. That gives you a lot more useable hot water and if you do that using E7 then it will cost about 20p or thereabouts. 
 

With an unvented cylinder there is very little risk of legionnaires so it’s not a variable you probably need to consider. 
 

One downside of using a 45c tank temperature is a lot of the newer showers need a minimum 50c for them to operate - worth checking but can be fixed with an immersion boost. 

[ragg987]

1 hour ago, JulianB said:

I've read that anything above 45degC really penalises an ASHP's CoP - I'm thinking to either use it as a buffer and then use an additional source to get it to the typical 60 odd degC

The higher the flow temperature, the lower the COP. But you risk overthinking it.

 

I use my ASHP to heat DHW to 50C and usually get a COP of 3.5 to 4, depending on external temperature.

 

I have not topped the temperature in the 3 years using direct electrical, though granted it gets to about 70C on bright sunny days courtesy of solar PV diversion.

[ragg987]

31 minutes ago, PeterW said:

One downside of using a 45c tank temperature is a lot of the newer showers need a minimum 50c for them to operate

Can't see why this would be. Even if you heat your water to abovee 50C, as you use the water in the cylinder it will drop lower and lower. So is the shower not going to work?

[PeterW]

2 minutes ago, ragg987 said:

Can't see why this would be. Even if you heat your water to abovee 50C, as you use the water in the cylinder it will drop lower and lower. So is the shower not going to work?

yes - some of the new digital showers need a minimum 50c hot supply. 

[ragg987]

7 minutes ago, PeterW said:

yes - some of the new digital showers need a minimum 50c hot supply. 

There's no way I would be buying one of those. I mean even if I had a full tank of hot water, but below the magic 50C, they would not work. Barking.

 

I'm surprised the EU has allowed this, it makes ASHP not usable.

 

Our thermostatic Hansgrohe is fine with lower temperature water.

[JulianB]

8 hours ago, PeterW said:

A 300 litre tank will go from 45c to 60c on a 3Kw immersion in about 70 mins. That gives you a lot more useable hot water and if you do that using E7 then it will cost about 20p or thereabouts. 
 

With an unvented cylinder there is very little risk of legionnaires so it’s not a variable you probably need to consider. 
 

One downside of using a 45c tank temperature is a lot of the newer showers need a minimum 50c for them to operate - worth checking but can be fixed with an immersion boost. 

 

Thanks for that! On E7 it does sound like a worthwhile route to top up with immersion...Are people using any particularly smart controllers on their immersion to take into consideration factors such as outside air temperature, as presumably passed a certain threshold it pays you to top up the final delta T with ASHP rather than immersion?

 

Interesting point re. newer showers...very odd that anymore would design in something like that! I would be keen on exploring solar PV, but I'm not sure that we have the budget for it at the moment, and we're in a conservation area + listed building so I'm not really holding my breathe (plus we've got a few trees around us...see attachement..photo taken from the west).

 

7 hours ago, ragg987 said:

The higher the flow temperature, the lower the COP. But you risk overthinking it.

 

I use my ASHP to heat DHW to 50C and usually get a COP of 3.5 to 4, depending on external temperature.

 

I have not topped the temperature in the 3 years using direct electrical, though granted it gets to about 70C on bright sunny days courtesy of solar PV diversion.

 

I would happily take 3.5-4 CoP... I don't suppose you have any recommendations on ASHP brand/model...as I'd be keen to have a look at some sort of COP vs flow temperature chart to begin to understand their relationship (although I don't suppose they are 'real life' numbers, once you factor in your particular annual temperature/humidity trends?)