Outside heat pump element in the inside (well, so to speak): should this work?

[Garald]

Context: I am (as some of you have heard N times) in the middle of a major renovation of an apartment that amounts to most of a small house, with access to a small shared courtyard. Our plan was and is to install a heat-pump in such a way that there are no outside elements, so as not to annoy the neighbors (though we have looked at heat-pump models that are quiet enough that we could legally put the fans outside without breaking sound restrictions, if we got the coop's approval).

 

Now, manufacturers of heat-pumps that are designed for such circumstances are far and few in between - there's the German brand that has a very good reputation but whose products of this kind are both very expensive and very bulky; there's the little-known French brand whose products are still quite expensive (26k if one counts the installation - incidentally, the installer has ghosted us)... and that may be it.

 

Fortunately, my architect came across a vendor/installer in a building fair who believes that a tiny room facing the courtyard (really the coop's broken collective outhouse, which I just bought from the coop precisely to use for my heat-pump project) will be large enough to host the "outside" element (double fan, etc.) of a Yutaki S Combi 2.0 (11kW); the outhouse's door would be replaced by a ventilation grille. The inside element of the heat-pump would then go in the laundry room.

 

Do you think this should work? Are there any issues we should foresee or guard ourselves against? The "tiny room" has the following dimensions:

 

The ceiling height is 3m, minus a bit we will lose when we insulate. The courtyard is on the "north" side; the door that will become a ventilation grille is 100cm broad.

 

Here's the map of the relevant bit of the ground floor, for greater clarity:

 

 

(Ignore the LxPxH dimensions of the PAC - that would have been an Amzair monobloc; what matters here is the dimensions of the tiny room.)

 

With good luck, this somewhat unconventional setup of a Hitachi Yutaki S heat pump should be much cheaper than the alternatives; I would save about 13k eur (+taxes) in comparison to the Amzair.

Of course, Hitachi is also a good brand with an international reputation and so forth, no?

 

The question is: do I need good luck? Will there be serious issues (malfunction, loss of efficiency, etc.)  if we proceed in this fashion?

 

All right - discuss.

 

Edited November 30, 2022 by Garald
corrected typos

[JamesPa]

I don't follow the diagram but...

 

The outside unit of an ashp sucks in air at the outside ambient temperature and expels it several degrees colder.  The heat extracted is what gives an ashp the 'magic' property that it uses less electrical energy than the heat it delivers to the heating system.  For this to work the source of intake air needs to be 'infinite' and likewise the expelled air must be able to dissipate completely from the region of the intake.

 

So any 'confined' arrangement where the expelled air can get back to the intake will not work.  Similarly any arrangement where the expelled air ends up in the space to be heated won't work.

 

Does that answer the question?

[IanR]

  On 30/11/2022 at 03:14, Garald said:

Do you think this should work? 

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Unfortunately, it couldn't possibly work.

 

The data sheet for the heat pump states that the unit moves just under 3000 m³/h of air, and the room you are suggesting it goes in contains less that 4m³ of air. The ventilation grille won't allow effective throughput of fresh air so the unit will continually recycle the air within the room. I would expect that within a few minutes of the unit switching on to heating mode, and the air in the room being recycled up to 10 times a minute though the unit, the air temp would have dropped to a temp below its effective range.

 

Also:

 

  On 30/11/2022 at 03:14, Garald said:

 in the middle of a major renovation of an apartment that amounts to most of a small house,

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 &

 

  On 30/11/2022 at 03:14, Garald said:

Yutaki S Combi 2.0 (11kW)

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Have you done any heat loss calcs? If you are undergoing a significant renovation then there is opportunity to improve insulation and air tightness levels and with your description of the property as part of a small house, then the unit you are suggesting maybe larger than you need.

 

Although even a small unit couldn't go in the "tiny room" you describe.

 

Do you own the roof?  Could the outside unit go up there?

[PhilT]

  On 30/11/2022 at 03:14, Garald said:

so as not to annoy the neighbors

 

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The friend who recommended a heat pump to me has his located a few feet from the neighbour's back door and it's just not a problem.

The roof idea is a good one, as is any elevated postion, "Because cold air sinks relative to warm air, and because the ground radiates heat very efficiently during calm, clear nights, the temperature at or near ground level can often be several degrees cooler than the temperature at the 2-meter thermometer height" (weather .gov.uk)

[Temp]

  On 30/11/2022 at 03:14, Garald said:

The ceiling height is 3m, minus a bit we will lose when we insulate.

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Presumably you mean sound insulation because it wouldn't make sense to thermally insulate the outside unit.

[Garald]

  On 30/11/2022 at 04:00, JamesPa said:

I don't follow the diagram but...

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First of all, thanks to everybody for all the help. The diagram shows a space 115cm wide, behind what is now a 100cm-wide door opening onto the courtyard. It's the closed blue door here.

The plan would be to replace the blue door by a ventilation grille as tall as the door is now.

 

The slim rectangle in the middle of the diagram shows what the position of the air pump would be.

 

  On 30/11/2022 at 04:00, JamesPa said:

 

So any 'confined' arrangement where the expelled air can get back to the intake will not work.  Similarly any arrangement where the expelled air ends up in the space to be heated won't work.

 

Does that answer the question?

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My questions:

a) how is a 2.5m tall grille (say) different from an open door? (Note a sound-insulated heat-pump fan outside would presumably also be behind a grille.)

(b) If it is not, then how is the situation different from a heat-pump installed within a 120cm deep, 3m tall recess in a wall, with free space behind it?

(c) would installing a heat pump in such a recess be a bad idea?

[Garald]

  On 30/11/2022 at 08:34, IanR said:

 

Unfortunately, it couldn't possibly work.

 

The data sheet for the heat pump states that the unit moves just under 3000 m³/h of air, and the room you are suggesting it goes in contains less that 4m³ of air. The ventilation grille won't allow effective throughput of fresh air so the unit will continually recycle the air within the room.

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That's my question: what is the effective throughput of a door-sized grille? How does it compare to that of no grille (so that the 120cm deep "room" would in effect be an open recess in the courtyard wall)?

 

  On 30/11/2022 at 08:34, IanR said:

 

Have you done any heat loss calcs?

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Some, but not for the house as a whole. This is a three-floor apartment, and the attic ceiling may eventually be raised (giving a total indoors volume in the order of up to 550-600m^3; it's smaller now - one cannot stand in most of the attic). The heat-pump has to provide both hot water for heating and for direct hot-water usage.

 

We are insulating (nearly) everything well, but even so 11kW doesn't seem unreasonable.

 

  On 30/11/2022 at 08:34, IanR said:

 

Do you own the roof?  Could the outside unit go up there?

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That's a possibility we have considered. I can put it back on the table.

[Garald]

  On 30/11/2022 at 09:06, Temp said:

Presumably you mean sound insulation because it wouldn't make sense to thermally insulate the outside unit.

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Yes, but there has to be some (heat) insulation on the ceiling to protect the space above it well.

[ReedRichards]

How enclosed is the courtyard?  Is the air in the courtyard still or does the wind whistle through it?  Even if you can overcome the problem of getting sufficient air flow through a grille in your door, you could end up just cooling the courtyard if it is too enclosed.  That wouldn't make your heat pump inefficient and you wouldn't be very popular with the neighbours.

[IanR]

  On 30/11/2022 at 11:41, Garald said:

My questions:

a) how is a 2.5m tall grille (say) different from an open door? (Note a sound-insulated heat-pump fan outside would presumably also be behind a grille.)

(b) If it is not, then how is the situation different from a heat-pump installed within a 120cm deep, 3m tall recess in a wall, with free space behind it?

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a) it reduces the area of the opening. I don't believe it would work with an "open door", but it will fail quicker with a ventilation grille, as the cooling air inside the room will get mixed with even less fresh air. The sound insulated "boxes" that I have seen for heat pumps do not pull air from inside the insulated box, there is an inlet grille on one side and an exhaust grille on the other.

b) your situation is worse as the direction of air movement is not towards the opening, it's across the width of the "tiny room" with solid walls on inlet and exhaust side.

 

  On 30/11/2022 at 11:52, Garald said:

giving a total indoors volume in the order of up to 550-600m^3; it's smaller now - one cannot stand in most of the attic). The heat-pump has to provide both hot water for heating and for direct hot-water usage.

 

We are insulating (nearly) everything well, but even so 11kW doesn't seem unreasonable.

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If the planned insulation levels are high and you plan good air-tightness, then 11kW seems high to me.

 

While my own property is a recent conversion to very low energy losses, I'm at around 1650m³ and have a 500l DWH cylinder and find my 12kW heat pump over-kill. I didn't fully trust in my heat loss calcs that suggested an 8kW HP would easily manage, so went for the next size up. I've got away with it as I have a 200l buffer for the UFH, but my HP has a very easy life.

[Garald]

>How enclosed is the courtyard?  Is the air in the courtyard still or does the wind whistle through it?  Even if you can overcome the problem of getting sufficient air >flow through a grille in your door, you could end up just cooling the courtyard if it is too enclosed.

It's not exactly windy; it opens on one side (well, there is a low-ish wall) onto a neighbour's big garden.

 

 

My place is the one marked R+1+C ("ground-floor plus first floor plus attic"). The building on the other size of the courtyard is shorter than mine ("RDC+C" means "ground-floor plus attic").  RDC is an empty space on top of a ground floor, with a sad, forgotten soccer ball somewhere in it, as well as bits of random vegetation.

 

[Garald]

  On 30/11/2022 at 12:01, ReedRichards said:

 

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  On 30/11/2022 at 12:15, IanR said:

 

b) your situation is worse as the direction of air movement is not towards the opening, it's across the width of the "tiny room" with solid walls on inlet and exhaust side.

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I'm not sure I understand why that would be the direction of air movement - you mean air would have to get around the heat-pump fans? The inlet would be directly in front of the opening; there would be a solid wall more than half a meter behind the exhaust.

 

 

[SteamyTea]

What you have, in effect designed, is a fridge, with the door open.

This is how heat engines work.  They take a lot of low temperature mass and condense it into little high temperature mass.

Edited November 30, 2022 by SteamyTea

[Garald]

  On 30/11/2022 at 12:15, IanR said:

I'm at around 1650m³ and have a 500l DWH cylinder and find my 12kW heat pump over-kill. I didn't fully trust in my heat loss calcs that suggested an 8kW HP would easily manage, so went for the next size up. I've got away with it as I have a 200l buffer for the UFH, but my HP has a very easy life.

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Is there a whole-house heat loss spreadsheet around here? I have only seen the one for a single room.

 

Incidentally - how do indoors heat-pumps (such as Amzair and Stiebel-Eltron) manage then? Amzair in particular has a setup that isn't all that different from this one.

[Garald]

  On 30/11/2022 at 12:33, SteamyTea said:

What you have, in effect designed, is a fridge, with the door open.

This is how heat engines work.  They take a lot of low temperature mass and condense it into little high temperature mass.

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Here the open door would correspond to the walls on the sides, just as "heat" becomes "cold" in this example?

[IanR]

  On 30/11/2022 at 12:30, Garald said:

 

I'm not sure I understand why that would be the direction of air movement - you mean air would have to get around the heat-pump fans? The inlet would be directly in front of the opening; there would be a solid wall more than half a meter behind the exhaust.

 

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To be clear, this is what I have understood from your description, the direction of air movement being perpendicular to the opening and causing significant pressure drops and restricted air flow meaning more air will be recycled within the room than comes in through the opening.

 

 

  On 30/11/2022 at 12:35, Garald said:

Incidentally - how do indoors heat-pumps (such as Amzair and Stiebel-Eltron) manage then? Amzair in particular has a setup that isn't all that different from this one.

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You'll need to be specific about the model, the heat pumps I just searched from those brands were air-to-water monobloc external units, as are all the air-to-water monoblocs for domestic heating that I'm aware of.

Edited November 30, 2022 by IanR

[Garald]

  On 30/11/2022 at 12:46, IanR said:

 

To be clear, this is what I have understood from your description, the direction of air movement being perpendicular to the opening and causing significant pressure drops and restricted air flow meaning more air will be recycled within the room than comes in through the opening.

 

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Now you are making me wonder what that double line is, but no, my understanding (looking at the floorplan I also included) is that the opening is on the 100cm side.

 

 

  On 30/11/2022 at 12:46, IanR said:

 

You'll need to be specific about the model, the heat pumps I just searched from those brans were air-to-water monobloc external units, as are all the air-to-water monoblocs for domestic heating that I'm aware of.

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https://www.amzair.eu/nos-pompes-a-chaleur/pompe-a-chaleur-optim_duo-2/

[IanR]

  On 30/11/2022 at 12:50, Garald said:

https://www.amzair.eu/nos-pompes-a-chaleur/pompe-a-chaleur-optim_duo-2/

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I've not come across these before, but they manage with discreet inlet and outlet ducting to fresh air and avoid any mixing, plus they are low power. Hopefully they are very well sound insulated, otherwise the household will wake up every time the compressor spins up.

 

 

 

With regards to your set up, the double-line suggested the door opening. But even still, as drawn, I don't feel it would work, but, put the outlet duct up to the door opening and seal it against the grille so that all exhausted air goes outside and then have an inlet grille in the upper half of the door opening, open to the whole volume of the room, and it could work without too much pressure drop that effects efficiency. But this still requires an air change in the "tiny room" every 4 seconds or so. The smaller the heat pump the better for this set up.

Edited November 30, 2022 by IanR

[Garald]

  On 30/11/2022 at 13:06, IanR said:

But this still requires an air change in the "tiny room" every 4 seconds or so. The smaller the heat pump the better for this set up.

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Would the fan of the unit as it exists ensure such a quick air change?

[Garald]

  On 30/11/2022 at 12:33, SteamyTea said:

What you have, in effect designed, is a fridge, with the door open.

 

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Would this still be true if one proceeded as IanR suggested?

 

 

  On 30/11/2022 at 13:06, IanR said:

But even still, as drawn, I don't feel it would work, but, put the outlet duct up to the door opening and seal it against the grille so that all exhausted air goes outside and then have an inlet grille in the upper half of the door opening, open to the whole volume of the room, and it could work without too much pressure drop that effects efficiency. But this still requires an air change in the "tiny room" every 4 seconds or so. The smaller the heat pump the better for this set up.

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Just out of curiosity - would this be more effective than having the inlet duct up to the door opening and the outlet open to the whole volume of the room? (Or is it that the cold air would then be more in contact with the rest of the house?

[SteamyTea]

  On 30/11/2022 at 13:42, Garald said:

Would this still be true if one proceeded as IanR suggested?

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Only if there is little resistance from any ducting/vents.

 

What you need to do is make sure that many tonnes of air can pass the the ASHP every day.  Any restrictions will lower the CoP.  ASHP units are large because of the size of the fan and heat exchanger (which is just the same as a car radiator) and fan.

Increasing the fan speed (mass air flow though the heat exchanger) would work , but would make the unit noisy as it forces even more tonnes of air though the heat exchanger.

If you only have one place (the doorway) to introduce and expel air, the efficiency is going to be reduced.  This efficiency reduction is two fold, one there is less air passing though, and the other is that half the air is cooled air, and half is warmed air.  This reduces the overall temperature difference of the ambient air.

Any ducting, vents, twists and turns (posh name for vortices) is going to reduce efficiency.

 

[IanR]

  On 30/11/2022 at 13:32, Garald said:

Would the fan of the unit as it exists ensure such a quick air change?

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Yes, it's the fan speed that is moving 3000m³/h, which leads to the requirement of 12.5 air changes a minute

 

The issue is putting a restriction in its path, ie. the ventilation grille. The smaller the cross-sectional area of the opening, the faster the air flow required to shift the 12.5 air changes a minute through it. That need to speed up the air flow adds resistance to the air path and makes the fan less efficient. ie. less air volume moved for the same electrical power used.

 

The lager the inlet ventilation grille the better. The upper half of the door, with a louvered grille and perhaps a "wide" insect mesh (6mm x 6mm min) should be fine.

 

  On 30/11/2022 at 13:42, Garald said:

Just out of curiosity - would this be more effective than having the inlet duct up to the door opening and the outlet open to the whole volume of the room? (Or is it that the cold air would then be more in contact with the rest of the house?

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I went with the exhaust duct sealed to the door as there is more detritus at ground level (leaves etc.) which won't enter the room if the inlet is higher, and also that the exhaust air starts at a small cross-sectional area (the diameter of the fan), and it seemed inefficient to me to allow it to expand into he room, and therefore slow down, before it then needs to speed up again as it "squeezes" through the ventilation grille. Just my engineering judgement, I'd have to model it to know if it actually made any difference to the pressure drop.

Edited November 30, 2022 by IanR

[SteamyTea]

  On 30/11/2022 at 13:54, IanR said:

I'd have to model it to know if it actually made any difference to the pressure drop.

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Would need to know of there may be an 'over temperature' problem.  This could come about if the refrigerant gas gets too hot, or too high a pressure because there is not enough air though the heat exchanger for the fan speed.

 

I think it is heading for problems.

[Garald]

Time to resuscitate the fan-on-the-roof plan, it seems. I can't remember why it was set aside; given that we will have plenty of skylights, it won't even be all that difficult to access. Of course architects do seem to hate it when one has to go back to an option they themselves considered in the past... (I'm extrapolating from a sample N=1.)

 

(I don't own the outer surface of the roof, but the coop is very reasonable.)

Edited November 30, 2022 by Garald

[Dan F]

Vaillant do a ducted heat-pump which you can put in a room like this and duct air intake and exhaust.  Another client of our main contractor has just installed one.  Ducting needs to support 400 m3/h, tank is 270L. But i'm pretty sure that the output (kW) is really quite low and that it only support heating the cyclineder, not for anything else. https://www.vaillant.co.uk/for-installers/products/arostor-domestic-hot-water-heat-pump-58880.html

 

I don't know what heat output you need, but could you combine an Arostore for DHW with a all-in-on MVHR / Exhaust air heat pump unit?  Or an arostore for DHW with hot/cold A2A air-con(s).