Evaluating different options for ASHP location at design stage

[Norbert]

So I am considering my options for my future ASHP, with a view to making it as efficient as possible. My project is outlined in this thread

However all that is necessary to understand for this topic is that this is a complete rebuild of a TF house on a 2 meter block wall to take the house above flood level.

 

The plant room will be on the first floor. Below will be a block built base, 2 meters high. This will be very well ventilated. I seem to have 2 options for my ASHP

1. On brackets fixed to the top of the block wall, so the ASHP is directly outside the plant room, say 400mm from the wall and at the same level.
2. In a bespoke hole near the top of the block wall. The front of the ASHP flush with the outside of the wall. Thus it would take air in from inside and blow the cold exhaust out

 

1) would be the traditional option, but 2) would have the advantage of normally warmer air intake. I am thinking that on a cold still frosty night when temp falls rapidly, the inside air would probably remain well above 0C due to the thermal capacity of the block walls and slab. Installation costs would be similar, but running costs lower. No doubt there will be times when outside air is warmer then internal, but at such times I do not think the ASHP will be required. See this SketchUp daubing to illustrate.

Any thoughts anyone.

Edited June 12, 2023 by Norbert

[Mr Punter]

The ventilation to the undercroft will be dependant on wind conditions.

 

You may need to change to a different size / shape of ASHP unit in future, so the build in option may no longer fit.

 

The external option looks easier to service and inspect.

 

Have you calculated the cost of the difference in efficiency?

 

Where is the flood line below which electrics are not advised?

[Marvin]

Hi @Norbert

 

Re your cunning plan option 2.

 

I don't know exact figures but the volume of air required to pass through the ASHP is massive.  In that case, if you were going to use the air from under the house it would soon become cold and you would need huge gaps for the air to enter the space under the house. This would also cause a cold wind tunnel under the house lowering the floor temperature.

 

I wouldn't do it.

 

If you want a win, add the exhaust air from the MVHR!

 

Marvin

Edited June 12, 2023 by Marvin

[RichardL]

#2 A couple of analogues:
Bespoke false chimney breasts with TV/Hifi built in...
All well and good until you need to change the component A2A for any reason & the new ones are a slightly different size.

At least kitchen units are for the most part standardised width, but even there fridge heights do what the manufacturer likes.

If you have space inside for the unit and grille in the wall (which meets the units spec) then perhaps - but a third party component with size you can't control built into the fabric - feels limiting.

[Norbert]

Thanks for the comments. I guess option 1, the normal approach, is the way to go for the practical reasons mentioned. Without some detailed data about how often the cellar, is how much warmer than the external air, and how much difference that would make to the running costs it is not possible to make a properly informed decision.

 

regards using MVHR exhaust, ... interesting; but how do you mix a low volume MVHR exhaust into a high volume ASHP input? I suppose you could just have the MVHR output cowl in the wall behind the ASHP. Is that normal practice? Would it make a real difference? I guess it would cost nothing extra so why not?

 

I already had it in mind that if the ASHP is external then I could certainly use the internal air for the MVHR input, without any of the potential problems. It would keep the MVHR input well away from the ASHP and HVMR output.

 

 

Edited June 12, 2023 by Norbert

[Mr Punter]

With the MVHR, the inlet and outlets like to have the same air pressure to keep the system balanced, otherwise the fans will be fighting to equalise.

 

The MVHR and the ASHP are performing 2 very different functions and it may be best just to install them as 2 separate systems unless you can quantify the benefit of mixing them.

[DanDee]

what about the humidity from the exhaust?

[Nickfromwales]

17 minutes ago, DanDee said:

what about the humidity from the exhaust?

Won't matter, as air is usually humid for a decent % of the year anyways  

The MVHR duct is a sound tunnel, so acoustically it may not be sensible, but also if there is even the slightest vacuum applied to it (the ASHP fan can be going at some serious pace for DHW) then you'll pull the MVHR out of balance, as stated above.

The energy in the stagnated air in the under croft would be gone in 10 mins or so, then you'd be solely reliant on sucking fresh cold / humid air in and through to get to the ASHP, so also pointless, sorry. 

No harm in thinking about these things, and defo worth asking  

[TonyT]

Use a gshp and put the coils in the river.

 

let the EPA argue about extraction (abstraction) as you aren’t pulling anything out the river!

Edited June 12, 2023 by Nickfromwales
Clarification

[Norbert]

44 minutes ago, TonyT said:

Use a gshp and put the coils in the river.

Yes, I have been pondering that one, but so far the view seems to be that it is not worth it. River temp in Jan/Feb was normally around 6C, fast moving water so good heat exchange

[JohnMo]

Would your option 2 ever work? How are you maintaining the side and top minimum clearances. How would you get access to wire it up and do maintenance on the heat pump? This option may gain nothing (most likely) or a degree on air temp if your lucky or if you don't have enough insulation in your floor. At a water flow temp of 30 at -2 your CoP should be around 3.6, if you managed to gain a degree, that may increase 3.75.

 

Why not place on the ground, easy to do, easy to install and maintain, little or no difference in CoP. You would be better off making sure your flow temps are a low as possible, ensuring you don't need a buffer etc. A poor buffer install and miss match flow rates either side of the buffer could cost 5 degrees in flow temp, that would knock you CoP down to 3.2 at -2.

[JohnMo]

47 minutes ago, Norbert said:

River temp in Jan/Feb was normally around 6C,

Our air temp at the same period was sometimes -9.  At 6 degs you should be getting a cop of 5, while -9 we would get just over 3. If you have a water source you have access too, it's a bit of a no brainer.

[markocosic]

1) Pop it on a stand to clear it of snow/leaves, said stand sat on the ground, just outside the masonry wall. Run the pipework up to your actual house. Avoids transmitting vibrations. Spend the savings on insulation and getting emitter temperatures as low as possible

 

2) Ground source in the river probably not worth the faff / cost for a newbuild where space heat demand is low and the reduced DHW performance in summer doesn't offset the increase space heat performance in winter.

[Nickfromwales]

4 hours ago, Norbert said:

River temp in Jan/Feb was normally around 6C

As stated by others, there's a fair amount of recoverable energy from air (or water) at 0^oC, so 6^oC is defo worth further exploration afaic. If the water is fast flowing then even more reason to go for that.

I have to tell you to make sure that you apply for an abstraction license, or at least ask if you need one, or i'll risk getting pulled over failing to do so.

[Norbert]

16 hours ago, markocosic said:

2) Ground source in the river probably not worth the faff / cost for a newbuild where space heat demand is low and the reduced DHW performance in summer doesn't offset the increase space heat performance in winter.

This is my current thinking too. If I was to put a heat exchanger in the river there would be no abstraction, however EA apparently likely to kick up about potential leaks of whatever fluid was going round in my system. Plus if it was a well insulated house what would be the pay back period for the extra cost of ground works and GSHP vs ASHP with lower efficiency. Given that I am past 70 not sure I will live long enough to benefit.

Edited June 13, 2023 by Norbert

[markocosic]

Meeting your maker aside... (somebody else will be using this even after you're gone)

 

I think in a UK climate the time that the air is COOLER than the river will be small; and you may well have LOWER efficiency on the ground than with an air source unit if your load is mostly DHW.

 

Different if it snows a lot or you're space heat dominated.

 

[sharpener]

1 hour ago, Norbert said:

there would be no abstraction

 

Nor with micro-hydro schemes but the EA make a fuss about them too.