Is 5kw ASHP too small?

[Roz]

Hi all. I have been planning to get a 5kw ASHP but our plumber suggested we get a design done by the plumbers merchant here as it was free. It seems they are suggesting a 7kw pump. 

 

the house is about 65 square metres split over two floors. We have a telford tempest 200L heat pump cylinder to go in, and underfloor heating in the ground floor, nothing upstairs. We will also eventually have a woodburner downstairs for occasional use

 

There is one main bathroom with bath and shower over, and then a second shower room in a garden studio, which will only be used very occasionally. 

 

I've attached our draft sap - I actually can't remember what plan we had for heating at the point this was done, not sure if ASHP was on the cards then, I think we has just said we would use oil. Can anyone shed any light? Budget is quite tight so I don't want to size up the ASHP if it isn't necessary.

thank yooou!

EAC 040-SapDesignDraft-TrevathanBarn,Redruth (1) (dragged).pdf

[joe90]

Well I have a 5KW ASHP no calcs, passive esk build, my house is bigger than yours my DHW tank is also bigger and it copes just fine. I found my ASHP on Ebay dead cheap, still new so took it as a punt and it paid off. Others will be along shortly (who know what their doing) and talk figures ?.

[Roz]

Just now, joe90 said:

Well I have a 5KW ASHP no calcs, passive esk build, my house is bigger than yours my DHW tank is also bigger and it copes just fine. I found my ASHP on Ebay dead cheap, still new so took it as a punt and it paid off. Others will be along shortly (who know what their doing) and talk figures ?.

Thanks Joe, I had seen this from you before and had made me confident in the 5kw one, I guess the difference is our insulation won't be as top notch as yours. I forgot to re-mention we are doing a barn conversion  . I haven't seen any second hand ones I have felt confident in yet, and time is ticking so might be just going new!

[joe90]

No idea about these but first hit on Ebay

 

https://www.ebay.co.uk/itm/NEW-HOME-AIR-SOURCE-AIR-TO-WATER-HEAT-PUMP-HEATER-12-5KW-RRP-2099/353168398563?hash=item523a7a18e3:g:JF8AAMXQU6tQ9u81

 

technical bods here will tell you if it’s a “good un”. 12.5 kW!!!!!!!

Edited August 18, 2020 by joe90

[BotusBuild]

I would recommend getting someone to take the SAP doc and work out what size of heat pump you require - fit the wrong size and they can be expensive to run. Joe may just have got lucky ?

[joe90]

1 minute ago, BotusBuild said:

Joe may just have got lucky

you may be right but at £850 it was worth a try. I based my (finger in the air) assessment on what @Jeremy Harris did. I don’t have a degree in anything but good “common sense” (oops wrong thread ?)

[BotusBuild]

£850 - I would have as well ?

[ProDave]

A quick scan through the SAP (section 8 ) tells you your worst case heat loss is just a shade over 2kW, about the same as my house. And mine is running fine from a 5kW ASHP running under floor heating downstairs and only in the bathrooms upstairs.

[Roz]

17 minutes ago, ProDave said:

A quick scan through the SAP (section ? tells you your worst case heat loss is just a shade over 2kW, about the same as my house. And mine is running fine from a 5kW ASHP running under floor heating downstairs and only in the bathrooms upstairs.

Thanks ProDave, does yours do DHW in the bathroom too?

[ProDave]

3 minutes ago, Roz said:

Thanks ProDave, does yours do DHW in the bathroom too?

Yes via a Telford 300L Heat Pump cylinder.

 

Two bathrooms, kitchen and utility room. 3 people in the house.

 

We have run out of hot water on a couple of occasions.  Because the water is only heated to 48 degrees by the ASHP it will use more "hot" water than it would if it were hotter to run a shower.  and given our showers are capable of something silly like more than 15L per minute, you can empty the tank in half an hour.  And when the ladies get washing and conditioning their hair, two such showers can use up that half an hour.

 

to guard agains that I have a Steibel Eltron instand water heater in line with the HW tank output https://forum.buildhub.org.uk/topic/8020-stiebel-eltron-dhc-e-810-instant-water-heater/?tab=comments#comment-136666

 

 

[Conor]

For comparison, I was planning a 5kw ASHP for our 300m² build (majority of heating on ground floor only), bit upsizing to 7kw as there isn't a huge price difference and unit won't be running as "hard" as a 5kw.

 

[Roz]

1 minute ago, ProDave said:

Yes via a Telford 300L Heat Pump cylinder.

 

Two bathrooms, kitchen and utility room. 3 people in the house.

 

We have run out of hot water on a couple of occasions.  Because the water is only heated to 48 degrees by the ASHP it will use more "hot" water than it would if it were hotter to run a shower.  and given our showers are capable of something silly like more than 15L per minute, you can empty the tank in half an hour.  And when the ladies get washing and conditioning their hair, two such showers can use up that half an hour.

 

to guard agains that I have a Steibel Eltron instand water heater in line with the HW tank output https://forum.buildhub.org.uk/topic/8020-stiebel-eltron-dhc-e-810-instant-water-heater/?tab=comments#comment-136666

 

 

Thanks! Do the tanks 'top up' the temperature though, so you have the feed from the ASHP but you can store it at a higher temperature? Then mix this hot with cold to get a usable temperature. 

I am not sure if that's an efficient thing to do but I had thought that would get us around the small tank we have 

[Roz]

Just now, Conor said:

For comparison, I was planning a 5kw ASHP for our 300m² build (majority of heating on ground floor only), bit upsizing to 7kw as there isn't a huge price difference and unit won't be running as "hard" as a 5kw.

 

That's good to know! I've just gone back to the plumber actually to get the price difference for the 5kw so I can actually compare. I know the list price, but with their package discounts it gets confusing. 

[Roz]

Also @ProDave and anyone else that know the answer I guess. I got a Telford tank on recommendation from this forum actually, but I might be starting to regret it. Apparently the merchant or Daikin have told our plumber that we need a 'third party kit' because of the cylinder not being Daikin. It's probably a bit premature to ask this question because I am waiting to find out what this kit involves, but at the cost of £200 or £250 it's making it seem like I should have got the Daikin water tank, which seemingly would have ended up a similar price according to this list I have from the merchant.

 

Has anyone else experience with a 'third party kit' and what it involves?

 

EDIT: Google gives me this picture, still don't really know what it is 

 

Edited August 18, 2020 by Roz

[ProDave]

My LG heat pump had an "optional kit" for a "sanitary water tank" (not my description)  It was included FOC with my heat pump.

 

It consisted of the temperature probe on a long (but not long enough) length of flex, and a box containing a contactor and a MCB.  That box is wired to control the immersion heater in the tank, so if you want to, you can program the ASHP to do a legionairs cycle toheat the tank water to 65 degrees once a week.  I have that disabled.

 

So I could have installed the tank with just the temperature probe, which is a plain old thermistor.

[ProDave]

13 minutes ago, Roz said:

Thanks! Do the tanks 'top up' the temperature though, so you have the feed from the ASHP but you can store it at a higher temperature? Then mix this hot with cold to get a usable temperature. 

I am not sure if that's an efficient thing to do but I had thought that would get us around the small tank we have 

No I just have the one tank, heated by the ASHP to 48 degrees.  For much of the summer, surplus solar PV lifts the temperature a lot higher than that.

[joe90]

I also heat to 48’, with a buffer tank (not sure I would have a buffer again tho) fir the UFH Which is blended down by the UFH manifold to 25’. Also 48’ to the DHW tank (300litre like @ProDave) Which is more than adequate. Just remembered we have guests and I have not switched to the lower stat to give me more hot water, but we have not run out yet!.

Edited August 18, 2020 by joe90

[Roz]

2 minutes ago, joe90 said:

I also heat to 48’, with a buffer tank (not sure I would have a buffer again tho) fir the UFH Which is blended down by the UFH manifold to 25’. Also 48’ to the DHW tank (300litre like @ProDave) Which is more than adequate. Just remembered we have guests and I have not switched to the lower stat to give me more hot water, but we have not run out yet!.

what does the buffer tank do and why wouldn't you have one? Ive been told we need one but not sure if that was in reference to the ASHP.

 

So I am confused, if you send 48 degrees to the DHW but you want to get more hot water out of it, can you top up the tank to a higher temperature, then use less but hotter water and blend the temperature at the tap? If you want the tank to go further. Not sure if this makes sense

[joe90]

56 minutes ago, Roz said:

what does the buffer tank do and why wouldn't you have one? Ive been told we need one but not sure if that was in reference to the ASHP.

 

So I am confused, if you send 48 degrees to the DHW but you want to get more hot water out of it, can you top up the tank to a higher temperature, then use less but hotter water and blend the temperature at the tap? If you want the tank to go further. Not sure if this makes sense

I was advised to use a buffer tank to stop the ASHP from short cycling (on off on off on off). Whereas others here have found running the UFH direct not a problem but you will need a bit more antifreeze/inhibitor.

 

@ProDave and i (and probably others) found that 48’ is hot enough to wash dishes, it’s  painful to hold your hand under that temp water so hot enough for bathing, showers etc, so we have larger tanks as we don’t blend much. You can top the temps up with solar or immersions if you have a smaller tank and need to blend but will be difficult to match immersions to ASHP unless the ASHP is timed otherwise once the tank temp is below your 48’ the immersion will kick in and you need the ASHP do get it up to 48’ and immersions or solar to whatever temp you require. Also a DHW tank at higher temps will have higher losses.

[Roz]

1 hour ago, joe90 said:

I was advised to use a buffer tank to stop the ASHP from short cycling (on off on off on off). Whereas others here have found running the UFH direct not a problem but you will need a bit more antifreeze/inhibitor.

 

@ProDave and i (and probably others) found that 48’ is hot enough to wash dishes, it’s  painful to hold your hand under that temp water so hot enough for bathing, showers etc, so we have larger tanks as we don’t blend much. You can top the temps up with solar or immersions if you have a smaller tank and need to blend but will be difficult to match immersions to ASHP unless the ASHP is timed otherwise once the tank temp is below your 48’ the immersion will kick in and you need the ASHP do get it up to 48’ and immersions or solar to whatever temp you require. Also a DHW tank at higher temps will have higher losses.

We are running the underfloor direct too, we've been given this diagram but we don't have the radiator section

We didn't have space for larger than 200L unfortunately. Will see how we get on with water consumption, there are only two of us
006_2016 - Hydraulic - Monobloc Dual Setpoint Rads & UFH.pdf

[PeterW]

2 minutes ago, Roz said:

We didn't have space for larger than 200L unfortunately

200/250/300 all share the same diameter base at 580mm - as long as you have the height (1120/1330/1650) then there is no limitation I can see. Once you go to 400 then the 710mm diameter is usually the limiting factor about getting them through doorways. 
 

The other consideration is the expansion tank that will be 10% of the tank volume - on a 500 litre this will be a floor standing 50 litre tank which is pretty substantial.  

[A_L]

 

@Roz

 

1) Your fabric heatloss is 95W/K and your ventilation heat loss 62W/K (in January) so a total of 152W/K

2) With a 22°C difference outside to inside then your total heat requirement is 22*152 = 3344Watts

3) It would be usual to oversize the boiler by 15% giving 3344*1.15 = 3846Watts and add 3000watts for DHW giving a boiler size of 6344Watts

4) Casual gains are estimated at 483Watts but probably safer not to subtract these.

I would say from these figures a 5kW heat source is to small.
 

 

[joe90]

4 hours ago, joe90 said:

Others will be along shortly (who know what their doing) and talk figures ?.

I told you someone ( @A_L ) who knows what they are doing would be along ?

[Nickfromwales]

29 minutes ago, PeterW said:

200/250/300 all share the same diameter base at 580mm - as long as you have the height (1120/1330/1650) then there is no limitation I can see. Once you go to 400 then the 710mm diameter is usually the limiting factor about getting them through doorways. 
 

The other consideration is the expansion tank that will be 10% of the tank volume - on a 500 litre this will be a floor standing 50 litre tank which is pretty substantial.  

You can get 'slimline' UVC's which retain the smaller footprint, but the OP actually has their UVC so needs to work with what they've got  

For 400L, 500L and above I always put two smaller separate wall mountable expansion vessels in ( 2x35L for a 500L is my go to size as you can never have too much expansion, and the uplift is pennies ). That gets things up off the floor, or even up to a second storey or attic is not unheard of / completely permissible.

Agree this is a candidate for a 7kW HP so as to add to the longevity and speed up the UVC recovery times a little.

 

@Roz

Just have a boost switch for the immersion and set that to heat up when you think you'll need extra 'volume'. 30 min / 60 min / 120 min boost settings are the norm, and are fool proof too as you don't have to go back afterwards and switch it off if you forget. Link 

[Roz]

Thanks everyone! It is actually vertical space that was the issue anyway, it's going under a sloping roof. I was hoping the expansion vessel could go up in the top of a wardrobe space (basically further up the sloping roof) about a metre away horizontally, but not sure our plumber knew what I meant so I don't know if this is OK @Nickfromwales ?

 

Great thanks Nick, I will mention a boost switch to him, I think I thought this would just be an option with the programmers/controllers so good to know you need an extra thing!

 

and thanks @A_L and @Nickfromwales for confirming 5kw might be too small, better to play it safe then.