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Is 5kw ASHP too small?


Roz

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Remember most heat pumps don't heat DHW and space heating at the same time, so no need to over size it to allow for both.

 

That does mean of course it can't do space heating continuously 24/7 so if it needs a constant 3kW of space heating that might be more like 4kW if it is running less than 24/7

 

My worst case heat demand at +20 inside and -10 outside is about 2.2kW  Mine copes nicely with the heating on for no more than 16 hours per day.

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2 minutes ago, ProDave said:

Remember most heat pumps don't heat DHW and space heating at the same time, so no need to over size it to allow for both.

 

That does mean of course it can't do space heating continuously 24/7 so if it needs a constant 3kW of space heating that might be more like 4kW if it is running less than 24/7

 

My worst case heat demand at +20 inside and -10 outside is about 2.2kW  Mine copes nicely with the heating on for no more than 16 hours per day.

ooooh good point

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There is very little in it tbh, cost wise.

I’d be tempted to take a punt on one of those cheap New Home units on eBay tbh, and if it lasts 10-12+ years then happy days. Depends if you’re going for RHI though and a suitable qualifying HP so the maths need to add up. 

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Just had a look at that newhome unit I linked to earlier (now that the welsh wizard said what he did above) and although I don’t know the name, it has a Toshiba compressor, soft start, digital lcd controller, cop 4.6  so sounds ok. Others (that know what their talking about) will be along shortly to confirm/deny ?

 

MAIN FEATURES

 
 

Heat Output : 12.5kw

 

Heating Power Input : 3kw

Voltage/Phase/Frequency : 220-240V/1PH/50HZ

Toshiba Rotary Compressor

Stainless Steel Heat Exchanger

Digital LCD Controller

Built-In Timer

Soft Start Technology

Refrigerant Type : R410A
Coefficient of Performance (COP) : 4.6

Water Flow Rate : 1.7 m3/h

Maximum Outlet Water Temperature: 60 °C

Hot Water Supply : 268 L/H

Sound Level : 52 dB(A)

Water Connections : 3/4 inch

 

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23 minutes ago, joe90 said:

So I guess it has an immersion built in to top up water temps?

 

Not necessarily, an output of 12.5kW at a C.O.P. of 4.6 implies an electrical input of 12.5/4.6 = 2.72kW. The remaining 0.28kW could be for things they do not include in the C.O.P. calculation (often pumps etc)

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1 hour ago, joe90 said:

So I guess it has an immersion built in to top up water temps?

That would mean a 3kW load on top of the max nominal operating scope of the HP, so ~2.5kW + 3kW, so would attract a makers label stating ~5.5kW if it had an immersion.

Theoretically lol. :ph34r:

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8 hours ago, Nickfromwales said:

At 7kW, it's less than a hob or decent shower? Showers don't soft-start either.

showers aren't inductive.

 

Bog standard 2hp induction motor may have a peak starting current of anywhere between 25A and 40A so soft-start is pretty relevant. Doesn't change the fact that the unit doesn't modulate tho...

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Soft start would remove the big inrush at start up that dims your lights and the DNO's hate, but would not allow the unit to modulate down to a lower power level.

 

It would be wise to clarify if possible which it has.  Most manufacturers are quick to highlight variable speed inverter drive.

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I just messaged the seller of the unit from Ebay to find its model number (in case anyone is interested) and this is their reply.

Hi,

The model number is : ESDAW-12.5-1PH

1499 pounds including delivery and VAT would be our best price.
 

Bods will be along in a mo to explain its pro,s and cons

 

link.... http://www.dreamheatpumps.co.uk/12.5kw_heat_pump_HOME.html

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14 hours ago, Wil said:

Is there any reason for me not to buy two of these instead of a 22kW GSHP and run them in parallel in a high heat requirement situation?

(Missing some context as this thread was about alternatives to a 5kW ASHP)

 

You'll need planning permission to install >1 ASHP

 

These don't appear to be inverter driven so the surge current and noise of two of them running could be quite high. 

 

You'd need to design the control system to run one or both depending on demand. 

 

 

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43 minutes ago, joth said:

(Missing some context as this thread was about alternatives to a 5kW ASHP)

 

You'll need planning permission to install >1 ASHP

 

These don't appear to be inverter driven so the surge current and noise of two of them running could be quite high. 

 

You'd need to design the control system to run one or both depending on demand. 

 

 

Yes, my apologies, got carried away when I saw the price and total thread hijack. Thanks for the answer though, I'll have to look into whether there's some sort of controller that could bring these in and out depending on demand.

 

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  • 1 month later...
On 18/08/2020 at 14:57, A_L said:

 

@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.
 

 

I have recently been recommended a Grant over  a Daikin and after taking all the advice on board originally and deciding to go for the 7kw Daikin, the Grant comes in a 6kw or a 10kw. The 6kw grant is a similar price to the 7kw daikin. It seems like I might be  on the edge of 6kw being ok according to these numbers... but is it risky? I don't really want to make another leap in price to a 10kw heat pump to be honest so I am wondering which is better?!

@ProDave @Nickfromwales I guess if you take off the DHW at the same time it would be OK but I am not sure if you agreed this was ok haha. The ebay heat pump worries meeee so havent spent much time looking at that!

Edited by Roz
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My own house has a lower heat loss than that. At -10 outside and +20 inside (not uncommon here) the heat load is a little over 2kW and my 5kW ASHP does it comfortably even allowing for it being off over night and doing the DHW as well.

 

Many heat pumps only do heating or DHW oe at a time. Mine is like that.  This allows it to put all of it's power into DHW and also to operate at a different temperature for DHW.

 

So in this case a 6kW unit is looking too small, you might have to run it 24/7 in the winter to stand a chance of coping.

 

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1 minute ago, ProDave said:

My own house has a lower heat loss than that. At -10 outside and +20 inside (not uncommon here) the heat load is a little over 2kW and my 5kW ASHP does it comfortably even allowing for it being off over night and doing the DHW as well.

 

Many heat pumps only do heating or DHW oe at a time. Mine is like that.  This allows it to put all of it's power into DHW and also to operate at a different temperature for DHW.

 

So in this case a 6kW unit is looking too small, you might have to run it 24/7 in the winter to stand a chance of coping.

 

Thanks Dave! But if the heat requirement is  3846Watts without the DHW added on, if they run them separately wouldn't that be fine? Sorry if I am getting muddled.

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2 minutes ago, Roz said:

Thanks Dave! But if the heat requirement is  3846Watts without the DHW added on, if they run them separately wouldn't that be fine? Sorry if I am getting muddled.

If the ASHP is rated at 5kW and you need 3864W then it will need to be doing heating 77% of the time just to get enough heat into the house. which means it will need to run 18.5 hours per day heating the house, and probably say 2 heating the DHW so it's not going to get much off time in the winter.

 

And that is calculated at 22 degrees delta T.  What if it's -10 outside?

 

What I am saying is you might not have much comfort zone.

 

Is there another source of heat such as a stove?

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Just now, ProDave said:

If the ASHP is rated at 5kW and you need 3864W then it will need to be doing heating 77% of the time just to get enough heat into the house. which means it will need to run 18.5 hours per day heating the house, and probably say 2 heating the DHW so it's not going to get much off time in the winter.

 

And that is calculated at 22 degrees delta T.  What if it's -10 outside?

 

What I am saying is you might not have much comfort zone.

 

Is there another source of heat such as a stove?

Gotcha. That makes sense (although we are now deciing between 6kw and 7kw, but I get your point)

 

Yes we will eventually have a woodburner. 

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Also, remember that the power output varies based on outside temperature and flow temperature.  So if possible get the datasheet and use the values on this rather than the "headline" number in the model name if possible when comparing with your requirements.

 

By way of example a "7kW" ASHP I'm looking at is:

- 7kW at 55C flow and -5C external temp.

- 10kW at 35C flow and 2C external temp.

 

 

 

 

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