ASHP size

[redtop]

Not sure f this is right place, but attached is my SAP design report. Struggling to work out hat size ASHP I need for space heating?  We were going too use a gas boiler but are going down the ASHP route now,

 

cheers all

Full_SAP_Calculation_ROWETT-5242-19_revised_2019-04-30_11-08-13.pdf

[MikeSharp01]

That is a relatively low SAP rating for a new build and will demand quite a big ASHP. What dies it need to do, UFH / Rads... DHW.

[redtop]

UFH and hot water.

[puntloos]

I was wondering about ASHP sizing too. Not lacking any experience it seems prudent to over-size them just so they 'don't have to work too hard, and become noisy'. 

Or is that way too grossly oversimplified?

[nod]

We decided against ASHP 

We rely on ground floor UFH and lots of insulation 

I think we have wasted our money on 9 radiators for the five beds and three bathrooms

A year in and we still haven’t used them 

 

Final Sap score 88

[ProDave]

3 hours ago, redtop said:

Not sure f this is right place, but attached is my SAP design report. Struggling to work out hat size ASHP I need for space heating?  We were going too use a gas boiler but are going down the ASHP route now,

 

cheers all

Full_SAP_Calculation_ROWETT-5242-19_revised_2019-04-30_11-08-13.pdf 280.74 kB · 12 downloads

Total Heat Loss 174 W/k

So at +20 inside and -10 outside total heat loss = 174 * 30 = 5.2KW

 

You are probably looking at something in the region of an 8-10KW ASHP

[MikeSharp01]

3 hours ago, ProDave said:

You are probably looking at something in the region of an 8-10KW ASHP

Don't forget to add a bit if you have a young family or anybody else who takes loads of showers / baths as DHW will be a key factor over and and above heating the building. 

[SteamyTea]

13 hours ago, ProDave said:

So at +20 inside and -10 outside total heat loss = 174 * 30 = 5.2KW

-10°C external temperature is a very rare event.

The old MCS had a rule that the ASHP, without supplementary heating, should cover 99% of all heating needs.  Easy to hit down here in the far South West, not so easy to hit 'up North'.

10 hours ago, MikeSharp01 said:

Don't forget to add a bit if you have a young family or anybody else who takes loads of showers / baths as DHW will be a key factor over and and above heating the building. 

If you work on 100 lt per person per day, then add 2 kWh/day.person.  That should cover losses as well.

The usual metric for hot water seems to be 150 lt/day.person, but if we are going to be serious about saving energy, then reducing the amount used is the easy option.

Now remember that the ASHP will not do both the space heating and the DHW at the same time, so allow no more that 6 hours for water heating (fits in nicely with the E7 window).

So say you have 6 people all wanting a shower, using up the full 100 lt each, then that is 600 lt and 12 kWh, so a 10 kW HP is just ticking over when spread over a few hours.

You will need a very large water tank.  Realistically you would use a 300 lt cylinder, but the HP is still only running twice (once in an E7 period, and once not).

But I suspect that most of the time there will be less people, and less water used, and the weather will be warmer.

And then the MVHR can help reduce ventilation losses (not looked at your SAP but assume it is in there, if not, work it out).

Fitting PV could cover a lot of the DHW if timed/managed correctly and is probably the easy and cheap way to make a big saving.

Edited December 28, 2019 by SteamyTea

[Gone West]

1 hour ago, SteamyTea said:

The usual metric for hot water seems to be 150 lt/day.person

150l/day is for all water, not just hot water.

[MikeSharp01]

1 hour ago, SteamyTea said:

If you work on 100 lt per person per day, then add 2 kWh/day.person. 

That makes sense, but as @PeterStarck says the 100l might be a bit high, so I think that means you get:

 

Worst case day ASHPKw = (174 * 30) + (n * 2)

Average case day ASHPKw = (174 * 10) + ( n * 2)

 

Where n = number of people at home

 

Substituting n = 2

Worst Case is: 9.2Kw (Where temp delta is 30)

Average Case: 5.7Kw (Where temp delta is 10 as is roughly the average case for the UK across the year, so might differ in your actual location - Cornwall for you!)

 

You can assume that there will be very few days in a decade when in most parts of the UK where the temp difference will be 30 degrees, and even fewer for your location, so you already have a good safety factor. This all means that for 2 people a 10Kw ASHP should be enough for even the worst case and on a normal day it will be working at about 62% of its max capacity. If you want to get to 50% you need to somewhat heed @SteamyTea's thinking about where you are as critical.

[SteamyTea]

11 minutes ago, PeterStarck said:

150l/day is for all water, not just hot water.

Yes, it was early.

Even dropping to 80 lt of 48°C (lower than a lot of people think is necessary) is not going to change the 2 kWh figure much.  It is a rule of thumb only.

7 minutes ago, MikeSharp01 said:

where you are as critical

Can get good quality data from the Met Office.

[DamonHD]

Also degreedays.net.

 

Rgds

 

Damon

[joth]

15 hours ago, puntloos said:

I was wondering about ASHP sizing too. Not lacking any experience it seems prudent to over-size them just so they 'don't have to work too hard, and become noisy'. 

Or is that way too grossly oversimplified?

I seem to understand ASHP follow Goldilocks sizing rules.

Too small they will work too hard and make loud noise, but too large it will not be able to put out low enough power so end up short-cycling on & off which reduces efficiency and is also the most annoying noisy (stop/start clicking). Get the size right and  it will just make  constant low hum. Fortunately modern inverter driven designs have a wide operating window so being a bit too oversized is less of a risk these days (all AIUI)

 

 

[SteamyTea]

11 minutes ago, joth said:

make loud noise

Not really with a properly installed one.

They are not like cheap air conditioning units that hang off the side of developing world walls, or UK pubs.

[MikeSharp01]

27 minutes ago, joth said:

I seem to understand ASHP follow Goldilocks sizing rules.

Get a good inverter controlled unit and you can control the output quite finely and, if you get reversible one and add some control valves / pumps etc, you can cool the slab in the summer.

 

[joth]

1 hour ago, SteamyTea said:

Not really with a properly installed one.

They are not like cheap air conditioning units that hang off the side of developing world walls, or UK pubs.

Even a barely large enough one that is always running at maximum capacity? That will surely be noisier than a correctly sized one. That was my point

 

 

1 hour ago, MikeSharp01 said:

Get a good inverter controlled unit and you can control the output quite finely

Exactly what I concluded too, thanks ?

 

[ProDave]

22 minutes ago, joth said:

Even a barely large enough one that is always running at maximum capacity? That will surely be noisier than a correctly sized one. That was my point

?

 

Most of the noise from cheaper ones appears to be a large "clunk" as they start up, and then bits of poorly designed casing rattling when they run.

 

Ours does none of that.  Just a quiet little "squeal" as the inverter starts up and then really all you hear is the compressor purring away and the noise of the air moving through the fan.

 

I have said it before, I doubt our ASHP is any noisier than the burner of an oil fired boiler, yet people are happy to have that burner noise inside their house, but complain at the noise of an ASHP that is outside. That does not make sense to me.

 

We have a 5KW ASHP for a house that needs just over 2KW of heat  at -10 outside (which is not a rare occurance up here) so the heat pump can heat the house with it only being on half the time.  That allows plenty of time for it to be heating the DHW, or to be off.

[MikeSharp01]

39 minutes ago, ProDave said:

and the noise of the air moving through the fan.

....and on out into the pine forests, across the glen, glancing off the mountains and onward to the Lochs and thence the sea - bucolic or what... 

[redtop]

thanks for all the replies.  I suspect the u values for the house are right however we are trying to improve the air tightness to less than 1; not sure if that will make much of a difference. most of the time there will be two of us, but we do have fairly frequent visitors which can push it to 6 including water hungry teenagers.

 

In any case it sounds like I should aim for around 10KW or slightly less and a 300L water tank

[SteamyTea]

1 hour ago, redtop said:

including water hungry teenagers.

Give them a lesson in environmental science by drawing straws.  The short straw get to bath last.

A they say up North 'that will learn them' or 'hard lines'.

Unless they are from Yorkshire, then give them a sticky stick, they know which end to pick up.