Slightly mad thought about split a2w system

[Beelbeebub]

OK, another random thought. 

 

Currently there is general anxiety about the size, visual and acoustic footprint of the external parts of heat pumps. 

 

Usually they are a big white box sat outside the building (cough octopus cosy cough) 

 

Air to Air systems are a little smaller as they don't need the air to water heatexchanger etc and split A2W are essentially a a2a outside unit with an internal heat exchanger.

 

But the inside units of a2a units (the boxes on the wall) are small and quiet, because of their location. They tend to use crossflow fans and are very compact. 

 

It occurred to me the heat exchange capability of the inside unit must be similar to the outside unit. Usually 2-3kw per head unit. 

 

What if (in essence) used the form factor of an inside unit as the external heat exchangers and what is effectively the outdoor unit was inside? 

 

As we would be going to water, the inside unit woukd replace the bulky fan and coil with a PHX and circulation pump. 

 

So inside we woukd have a boiler sized (and probably sited) unit with a compressor, PHX and pump. 

 

Outside you would have a wall mounted head units, say 2 for a 6kw system. Coloured to blend in. 

 

So in essence my pitch is: why not use crossflow fans and compact coils for the outside unit? 

[Beelbeebub]

If you really wanted to be radical, whay if each "head unit" was entirely self contained with a small compressor and the inside unjt purely held the controller, a PHX and pump, maybe a booster heater.  Then each external unit woukd be capable of producing (say) 3kw, and you just double, triple, quadruple up for 3,6,9 and 12 kW demands 

 

This woukd proably add some cost through duplication, but that might be offset by economies of scale as the same outside unit is being produced in larger numbers. 

 

The really bug advantage thought would be that your modulation woukd be genuinely huge. 

 

In shoulder months you could run a single unit at low power (say sub 1kw for a 3kw peak unit) and then just spin up extra units as the demand increased up to however many you needed, say 12kw.

[SteamyTea]

Have you not just described a split A2WHP system, but with a smaller external fan unit.

 

I think the problem is physics, you would need a large airflow externally, which may generate more noise, and not be very efficient.

[JohnMo]

Cannot see why that would not work, also if you use the refrigerant to heat the cylinder, (divert it before the plate exchanger) that is around 80 degs.

 

Cascade control schemes are nothing new, my ASHP is already pre-programmed for it.

[DanDee]

The internal 3kW a/a runs the refrigerant at 35-40C vs 20C indoor air, so dT of 15-20C, the outdoor 3kW unit has a dT of 3-7C, thus the air flow difference between the two would make the indoor unable to harvest the heat. So we a need a split with everything inside with just the evaporator coil+fan outside, but the same coil and fan size like now just in a more compact case(without the compressor and HEX+bits).

The GSHP already sits everything inside.

 

https://www.ochsner.com/en/ochsner-products/air-source-heat-pump/

[markocosic]

1 hour ago, Beelbeebub said:

So in essence my pitch is: why not use crossflow fans and compact coils for the outside unit? 

 

People buying heat pumps, for the most part, have plenty of space. It's only if you have a stupidly small house in a stupidly small plot with a stupidly high heat loss (UK...) that size is an issue.

 

And big fans are efficient and quiet...so...that's what you get.

 

Split units with JUST the fan/evaporator outside is the direction you'd go if you wanted to minimise the bulk of the outdoor gubbins.

 

But that puts the moderately noisy (unless you go to some effort) compressor indoors and it's not *that* much bigger to plonk it outdoors so outdoors is what you get.

 

The yanks do top-exist units with the fan on top and the motor/compressor gubbins inside the coil. I suspect these are probably more compact and all find and dandy for aircon purposes where they're sucking air from the coolest pool at ground level and chucking it upwards. You're fighting the rain and snow if doing that for heating though.

 

 

[Beelbeebub]

2 hours ago, SteamyTea said:

Have you not just described a split A2WHP system, but with a smaller external fan unit. 

 

Yes I have. 😁

2 hours ago, SteamyTea said:

I think the problem is physics, you would need a large airflow externally, which may generate more noise, and not be very efficient.

The main point is the airflow/coil size for an indoor unit must be similar to the outdoor unit (given they are both transferring similar amounts of heat with similar DTs) and the indoor units are much quieter and more compact than the outdoor units. 

 

Inside units use crossflow fans which are very quiet and would lend themselves to a flatter device.

 

If a HP could be fairly slender (say 200mm thick) and could be set against a wall because it is sucking top to bottom or side to side or bottom to top rather then front to back then the size in other directions isn't so much of an issue. 

 

You could probably stick a 2000x1000x250 box on a wall fairly easily but a 1400x800x400 unit that needs to be 300 off the wall is a bit trickier. 

 

 

[Beelbeebub]

1 hour ago, DanDee said:

The internal 3kW a/a runs the refrigerant at 35-40C vs 20C indoor air, so dT of 15-20C, the outdoor 3kW unit has a dT of 3-7C, thus the air flow difference between the two would make the indoor unable to harvest the heat. So we a need a split with everything inside with just the evaporator coil+fan outside, but the same coil and fan size like now just in a more compact case(without the compressor and HEX+bits).

The GSHP already sits everything inside.

 

https://www.ochsner.com/en/ochsner-products/air-source-heat-pump/

Pretty much, but with an outdoor unit that was more like a wall mounted air con internal unit than some sorry of picnic table/korean BBQ 😁

[Beelbeebub]

1 hour ago, markocosic said:

 

People buying heat pumps, for the most part, have plenty of space. It's only if you have a stupidly small house in a stupidly small plot with a stupidly high heat loss (UK...) that size is an issue.

 

And big fans are efficient and quiet...so...that's what you get.

 

Split units with JUST the fan/evaporator outside is the direction you'd go if you wanted to minimise the bulk of the outdoor gubbins.

 

But that puts the moderately noisy (unless you go to some effort) compressor indoors and it's not *that* much bigger to plonk it outdoors so outdoors is what you get.

 

The yanks do top-exist units with the fan on top and the motor/compressor gubbins inside the coil. I suspect these are probably more compact and all find and dandy for aircon purposes where they're sucking air from the coolest pool at ground level and chucking it upwards. You're fighting the rain and snow if doing that for heating though.

 

 

stupidly small houses on stupidly small plots with poor insulation pretty much describes the UK housing stock!

 

I get the arguments that it's just as easy to stick the gubbins outside and in the case of monoblock units you get the massive advantage of no messing with refrigerant lines. 

 

It was more a thought about how the usual format for outside units of horizontal axial fans sucking from the back and blowing out the front whilst the same thermal flow inside is serviced by a crossflow fan sucking and blowing planar to the wall. 

 

What advantages could we get by utilitising the indoor format for the outdoor unit (even with a compressor and phx) 

[JohnMo]

I think if manufacturers weren't constrained by costs things could be different. For example my ASHP is the same external casing for a 4, 6, and 8kW units. Will be a lot of part sharing going on, instead of dedicated parts for the large and small unit. 

21 minutes ago, Beelbeebub said:

You could probably stick a 2000x1000x250 box on a wall fairly easily but a 1400x800x400 unit that needs to be 300 off the wall is a bit trickier. 

My unit isn't small, but it's no where near the size you are referring to, the 14 to 16kW versions are that size, but 4 to 8kW are 828mm tall and 924mm wide. The 4 to 8kW being more typical of the size being installed.

 

Pretty sure the 4kW, in a dedicated casing could be made a lot smaller, even as a monobloc.

 

 

[markocosic]

The outdoor units are small BUT they need a fair old clearance around them.

 

I would guess that new Octopus unit will probably have a rear-entry semi-top-exit arrangement with the official figures stating tighter than standard clearances to the rear, near zero clearances to the sides, and a lot less clearance for the exit than contemporary units where the OEM instructions (recommendations only; but effectively a hard limit if you're wanting the OEM warranties / not wanting to expose yourself to any potential claim / wanting to be an MCS union accredited installation)

 

Some units for colder climates have side-entry and front-exit; with those side entries looking like a "short sleeved shirt" so that they don't suck up snow/leaves/rain.

[Beelbeebub]

3 hours ago, JohnMo said:

I think if manufacturers weren't constrained by costs things could be different. For example my ASHP is the same external casing for a 4, 6, and 8kW units. Will be a lot of part sharing going on, instead of dedicated parts for the large and small unit. 

My unit isn't small, but it's no where near the size you are referring to, the 14 to 16kW versions are that size, but 4 to 8kW are 828mm tall and 924mm wide. The 4 to 8kW being more typical of the size being installed.

 

Pretty sure the 4kW, in a dedicated casing could be made a lot smaller, even as a monobloc.

 

 

yes, the samsung 5kw units were about 800x800x400 but i'm pretty sure they didnt have any pumps etc inside.

 

But it's the thickness that's the problem.  none seem to be much below 400 and many are towards 500.  Coupled with the 300mm standoff means you effectively have a 700-800mm block sticking out from the wall, this is pretty big at ground level and can preclude alot of side mounts between houses

 

something with side inlet and top outlet that could be fitted to a wall would be much easier to site (think overgrown meter box) and possibly planning (again, think meter box)

 

If the units were 4kw units and pretty small, wall mounted things (again think meter box) that could be twinned up to provide 8kw, that would cover alot of cases and provide better modulation.

 

 

[DanDee]

 

[markocosic]

How about the original Octopus heat pump with it's ice sticks and a kg or two of direct-expansion-piped-into-the-house R290...

 

https://www.femaxholding.com/octopusenergy/octopus-hoszivattyu?lang=en

https://www.femaxholding.com/octopusenergy/assets/doc/technical_data_en.pdf

http://octopus.lt/wp-content/uploads/2012/08/user_manual_eng.pdf

 

I quite like the one that they did for the church with the frozen cross 😄

 

 

[SteamyTea]

28 minutes ago, markocosic said:

quite like the one that they did for the church with the frozen cross

Can you stick people to it.

[Beelbeebub]

2 hours ago, DanDee said:

 

Yeah, though even slimmer by using a crossflow fan. 

[Beelbeebub]

37 minutes ago, markocosic said:

How about the original Octopus heat pump with it's ice sticks and a kg or two of direct-expansion-piped-into-the-house R290...

 

https://www.femaxholding.com/octopusenergy/octopus-hoszivattyu?lang=en

https://www.femaxholding.com/octopusenergy/assets/doc/technical_data_en.pdf

http://octopus.lt/wp-content/uploads/2012/08/user_manual_eng.pdf

 

I quite like the one that they did for the church with the frozen cross 😄

 

 

I like it! 😁

 

they basically looked at the frost and deicing problem and went " fu*k it!"

[SteamyTea]

31 minutes ago, Beelbeebub said:

fu*k it

I just looked up the thermal conductivity of ice at -30°C.

2.5 W/m.K.

So can still get a fair bit of energy though it as the face frosting would not be too thick most of the time.

[Beelbeebub]

1 hour ago, SteamyTea said:

I just looked up the thermal conductivity of ice at -30°C.

2.5 W/m.K.

So can still get a fair bit of energy though it as the face frosting would not be too thick most of the time.

But about 100th that of aluminium and the ice will be thicker.  Maybe if you had some sort of sonic de-icer, vibrate the ice off the fins? 

[SteamyTea]

23 minutes ago, Beelbeebub said:

But about 100th that of aluminium and the ice will be thicker.  Maybe if you had some sort of sonic denicer, vibrate the ice off the fins? 

As ice gets colder, it conducts better, and absorbs less energy to.

A garden ornament at -100°C should start to freeze out atmospheric CO₂.

Imaging collecting that is garden bags and taking it to the dump on a Sunday morning.

[markocosic]

Hang it the other way around, de-ive just the corre, and let the giant ice pipes come crashing down on unsuspecting visitors. Saves 90% of de-icing by volume lol.

[markocosic]

Official instructions are for manual defrosts btw. Hit it with a wooden stick if the ice gets so big that it bridges the pillars.

 

Works for countries where the heat demand in the "mild" weather (say 0-5C when humidity is an issue) is low; and most demand is when it's actually cold and the air is bone dry by definition.

[DanDee]

 

https://www.alfalaval.com/globalassets/documents/products/heat-transfer/finned-tube-air-heat-exchangers/product_leaflet_ace_model_a_en.pdf

https://www.alfalaval.co.uk/products/heat-transfer/finned-tube-air-heat-exchangers/finned-tube-air-heat-exchangers/finned-tube-air-heat-exchangers/

 

Edited September 18, 2023 by DanDee

[HughF]

Are you thinking like this:

 

 

Ground source, with the brine running through an outdoor radiator/fan unit?

[Beelbeebub]

49 minutes ago, HughF said:

Are you thinking like this:

 

 

Ground source, with the brine running through an outdoor radiator/fan unit?

Not quite, that just makes for 3 working fluid loops! 

 

It was more a thought that the outside units could be made smaller, sleeker and modular for much easier fitment. 

 

Imagine, instead of a big box stood off the wall, there were 1 or 2 of these type of things (suitably designed for outside use etc) stuck to the wall a bit above head height. 

 

They'd be coated black or maybe even matched to your exterior. 

 

Inside would be a bix much like your boiler in size with the compressor and a plate heat exchanger, possibly a central heating circulator pump as well. 

 

It would hum a bit like a fridge when running but then so do boilers.