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Author Topic: Why aren't AHSP located in the loft?  (Read 5158 times)
Tombo
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« Reply #15 on: August 26, 2015, 12:21:51 PM »

There is a UK based  non-Navitron manufacturer who makes ASHP's for the loft which only heat the DHW.  I think Noise and vibration is their main concern, rather than chilling the loft because the energy requirements for DHW are quite modest.
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pdf27
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« Reply #16 on: August 26, 2015, 12:23:21 PM »

Typical loft volume = 0.5 x 5 x 8 x 3 m3 = 60m3
Mass of air in loft = 60 x 1.2 = 72 kg (ish)
Specific heat of air in loft = 72 x 1010 J/K = 72.7 kJ/K

A 5 kW heat pump will extract = 5 x 60 x 60 = 18,000 kJ/hour from the loft space
This means that the temperature in the loft will (assuming no large external heat sources) drop by roughly 247 degrees per hour (18,000/72.7 - just over 4 degrees per minute). Of course, the heat losses into the loft from the house and outside world will go way up before this happens, and the COP of the pump will crash. Either way, even for a very small heat pump the amount of energy stored in a hot loft is trivial.

If you want to harvest this energy, just fit a small extractor fan to the loft and put it on a thermostatic controller. If the loft is warmer than the house and the house is below temperature, turn on the fan to blow air from the loft into the house and you'll rapidly extract all the useable heat at minimal cost.
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Countrypaul
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« Reply #17 on: August 26, 2015, 07:28:43 PM »

Typical loft volume = 0.5 x 5 x 8 x 3 m3 = 60m3
Mass of air in loft = 60 x 1.2 = 72 kg (ish)
Specific heat of air in loft = 72 x 1010 J/K = 72.7 kJ/K

A 5 kW heat pump will extract = 5 x 60 x 60 = 18,000 kJ/hour from the loft space
This means that the temperature in the loft will (assuming no large external heat sources) drop by roughly 247 degrees per hour (18,000/72.7 - just over 4 degrees per minute). Of course, the heat losses into the loft from the house and outside world will go way up before this happens, and the COP of the pump will crash. Either way, even for a very small heat pump the amount of energy stored in a hot loft is trivial.

If you want to harvest this energy, just fit a small extractor fan to the loft and put it on a thermostatic controller. If the loft is warmer than the house and the house is below temperature, turn on the fan to blow air from the loft into the house and you'll rapidly extract all the useable heat at minimal cost.

A 5kW heat pump will be more likely to extract 4kW from the (assuing a COP of 4) the other one coming from the electricity used to power it!, so about 14,400 kJ/hr from the loft space. The reuslt will be near enough the same though  Grin
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pdf27
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« Reply #18 on: August 27, 2015, 07:33:34 AM »

True - was doing that calculation in a hurry and as a result was slightly sloppy about how I did it.
It does raise an interesting question though - does the COP of an air-source heat pump drop on windless days? You're certainly going to have a lot more air to play with, but if it's working hard and there isn't any wind to move the bulk air about I do wonder if you'd see a measurable local cold spot around the house and so a reduced COP.
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M
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« Reply #19 on: August 27, 2015, 07:54:57 AM »

True - was doing that calculation in a hurry and as a result was slightly sloppy about how I did it.
It does raise an interesting question though - does the COP of an air-source heat pump drop on windless days? You're certainly going to have a lot more air to play with, but if it's working hard and there isn't any wind to move the bulk air about I do wonder if you'd see a measurable local cold spot around the house and so a reduced COP.

That's a fascinating question. Would the convection effect resolve this? Would convection be seriously hampered by obstacles such as solid walls / fences all round the garden?

Whilst bored many years ago, I took a helium party balloon and attached a small plastic bag which I then added salt to to get the balloon to a neutral state. When the heating came on, within minutes (perhaps seconds) the balloon rose in front of the rad, stopped short of the ceiling, crossed the room to the doorway, dipped down without touching the wall, and went through the doorway and up the stairs. Incredible to watch.

Taught me two things:

1. Having the radiator on a wall facing the doorway is a bad idea, especially if you have cats that like doors to be open or ajar.

2. Replacing that rad with a slightly smaller one, then adding an even smaller rad to the room in the opposite corner meant the room warmed up in a fraction of the time. Sorry, not quite true, the room warmed up a little faster, but the air warmed up much, much faster, giving the occupants of the room comfort in a very, very short time period.

Mart.
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Just call me Mart.     Cardiff: 5.58kWp PV - (3.58kWp SE3500 + 2kWp SE2200 WNW)
Countrypaul
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« Reply #20 on: August 27, 2015, 09:24:19 AM »

True - was doing that calculation in a hurry and as a result was slightly sloppy about how I did it.
It does raise an interesting question though - does the COP of an air-source heat pump drop on windless days? You're certainly going to have a lot more air to play with, but if it's working hard and there isn't any wind to move the bulk air about I do wonder if you'd see a measurable local cold spot around the house and so a reduced COP.

A very good question as Mart said.  I could quite readily see that the COP might drop creating a local  low temperature area depending on where you live. If in the bottom of a flat valley I imagine this could be significant, but at the top of a hill or on the side of a hill this might not be significant as the cold air could still escape easily.

If we assume that ir contains about 1kJ/m3/K (lets keep the maths simple) then a 5kW ASHP running for 1 hour (assume a COP of 4) needs 3600*3 m3 of air, so 10800m3, or   a box of air about 22mx 22m x 22m which doesn't actually seem that big - have I got he maths right?

Of course if the air is dead still, the energy needed in the house is likely to be lower sine there is much less cooling due to wind.
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dhaslam
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« Reply #21 on: August 27, 2015, 01:00:27 PM »

ASHPs  air  supply is limited by the ability of the fan to move air.   The fan in my 5kW ASHP, according to the manual,  moved  500 cfm  which is  850  cubic metres   per hour.    The more efficient ones move a bit more.    In cold weather it typically produced about 2 kW  of which about one half  would have come from the air  and that would have meant the air was cooled by  about  four degrees.    That means that when the air  was below about 4C the  moisture would freeze  causing the  grill on the  exit side of the fan to freeze up and reduce air flow.      There may be some heat gain from  the phase change  but that would be after passing  the heat exchanger.   
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DHW 250 litre cylinder  60 X 47mm tubes
Heating  180,000 litre straw insulated seasonal store, 90X58mm tubes + 7 sqm flat collectors, 1 kW VAWT, 3 kW heatpump plus Walltherm gasifying stove
mike7
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« Reply #22 on: August 29, 2015, 01:40:07 AM »

The idea is not so completely daft:-

Suppose that the attic floor is reasonably well insulated and that the roof is not insulated at all. Extracting heat from the attic air will of course lower its temperature which, in the absence of sunshine, will have been at or close to ambient. The lower temperature will mean that the roof will now act as a heat collector from external air at a rate determined by the area of the roof, its U value, and the temperature difference. It may well be that in some circumstances those parameters allow a worthwhile rate of heat extraction at a still-good-enough COP. If the attic is effectively airtight then the air it contains will be dehumidified just once and there will be no condensation – and there’s the bonus of no icing of the evaporator, BUT if humid air is able to enter, then certainly there is going to be more opportunity for condensation. Noise has been mentioned – my experience is limited to small aircon AAHPs which seem to me to be very quiet and thus unlikely to pose a problem.

So it all depends.
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M
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« Reply #23 on: August 29, 2015, 08:50:40 AM »

I've thought of a solution. On cold days and nights, just place a patio heater in front of the ASHP ..... problem solved.  Undecided

Mart.
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Just call me Mart.     Cardiff: 5.58kWp PV - (3.58kWp SE3500 + 2kWp SE2200 WNW)
mike7
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« Reply #24 on: August 29, 2015, 06:18:09 PM »

Some figures: My attic roof is 18m x8.5m, so area is 153m2. Clay tiles and roofing felt, so U value could be 4 W/m2K (probably higher). Suppose permissible temp drop is 3C. That would result in a heat transfer into the attic from outside of 153 x 3 x 4, ie 1836W.
Attic floor area is 108m2, U value maybe 0.2, so additional heat loss to attic from below due to lower attic temp would be 108 x 3 x 0.2 ie. 65W.
A COP of 3 would use 918W and deliver 2.754kW or a net input (after subtracting the 65W extra loss) of 2.689kW. Not a lot, but enough to take a fat slice off the bottom of my heat demand curve over the year.

Why bother? The outside unit is not pretty. You need planning permission to have one outside on a listed building, also for an unlisted building if you already have one and want a second. Sheltered position for the pump and evaporator may mean longer service life/less maintenance. Installation might be difficult otherwise. Fewer defrost cycles.

Edit - When first posted I muddled up the figures for a COP of 3 with those for a COP of 4 - hopefully correct now.

 AFAIK the COP is the ratio of heat delivered to energy input, so the 5kW HP referred to earlier giving a COP of 4 would use 1.25kW electricity and extract 3.75kW from the air.
« Last Edit: August 30, 2015, 10:19:53 PM by mike7 » Logged
Worcester
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« Reply #25 on: September 01, 2015, 09:17:53 PM »

As Tombo has mentioned , there are actually systems that kind of do what the OP suggested.

They're call exhaust air heat pumps.

Take the heat out of the top of the building and then exhaust outside.

Usually used only for heating DHW, and extracts are placed at the top of landings, or if a warm room in the roof space, and the like.

I can think of at least 4 manufacturers of such units, interestingly enough, none of which are RHI approved .. yet, as they are considered an ineligible technology type, 2 of them have got an MCS Certification, though from experience that doesn't mean a lot...

In industrial sites it's also quite common to duct the air out of the server room or another process area that is generating heat and use that as at least part of the input to an ASHP,

Some MVHR units also have ASHP's built in to them for the same reason.

So... though you wouldn't actually place an 'ordinary' ashp in the loft for the reasons specified, the principal of using any wasted heat from there isn't necessarily a bad idea.

However the best thing to do would actually to insulate the loft so the heat never gets there in the first place and reduce the heat load on the property Smiley
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Worcester Renewables - Installers of Navitron Technology
Stochengberge
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« Reply #26 on: September 02, 2015, 08:32:57 PM »

I have a saying that I impart upon all of my guys at work, "there is no such thing as a silly question, just one that you don't ask". Ie, I would much rather they ask than carry on blindly making a horrendous c0ck up which could have disastrous (or even fatal) consequences. If they keep asking the same question or otherwise try and "extract the urine", that is another matter, but I have a good team that are willing to listen / learn.

This saying borne in mind I was somewhat alarmed at the response that the poster received from some on a generally friendly forum.

And then to prove the point there follows an informative series of posts on how you could get a microclimate around the property using an ASHP and a cracker from Mart giving an excellent demonstration of the importance of sizing and siting your radiators. Although that one did demonstrate that perhaps Mart really has too much time on his hands!

From a silly question, much useful information was gleaned...

SB.
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On the North Downs of Kent with 3.2kWp facing 12' west of south @ 33', 36 x 58mm Thermal tubes on an east / west split, 300ltr triple coil DHWC and an 8kW to water WBS.
M
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« Reply #27 on: September 02, 2015, 10:57:31 PM »

Although that one did demonstrate that perhaps Mart really has too much time on his hands!

From a silly question, much useful information was gleaned...

SB.

Yep, you got me, can't argue with that.

Regarding questions I was told a simple rule by a boss of mine many moons ago, who had been told the same rule by her boss, before:

If you don't know something, then just ask.
If you still don't know, then please, just ask me again.
But if you F'ing ask me three times ........

Mart.
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Just call me Mart.     Cardiff: 5.58kWp PV - (3.58kWp SE3500 + 2kWp SE2200 WNW)
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