Welcome to the forums! My initial thought is that placing an ASHP in a cellar won’t work.

ASHPs require good airflow to function efficiently, and placing them in confined spaces like a cellar can lead to poor performance. Ducting the exhaust air through a wall could work in theory, but you'd need to make sure the system isn't starved of air. The duct size would need to match the fan's aperture, but you may also run into issues like condensation buildup, noise, increased defrost cycles and airflow restrictions. You’d also need a way to introduce fresh, "warm" air from outside.

It’s generally recommended that ASHPs be installed in open areas where they can freely exchange air with the environment. Placing one in a cellar could lead to efficiency problems and potentially void warranties.

Is an air to air heat pump a more viable solution?

That said, I'll be interested to hear what others think.

Thank you for the quick reply.

It's a bit hard to explain it. But if I drill enough holes/add a grill or replace the door with a mesh/net then I'd have a way to bring in a lot of fresh air from the outside. The entrance is protected against the elements so I am not worried about rain. Etc... it's facing a large tunnel that goes under our house with wide opening on both ends.

Wouldn't insulating the exhaust duct sort out any condensation issue?

We don't like air to air heating to be honest. We have it in our main property that is located in a hot country. It's wonderful for cooling but not so much for heating IMHO. Also. We don't like the aesthetics of an indoor unit (our main property is a new built so the internal units are hidden and ducted)

However: why would there be any difference for the heat pump itself? It would need to change air the same way as air to water. No?

Thanks again.

@gild

A Ground Source Heat Pump could be placed in a cellar I do believe, but of course you would probably require a borehole to provide the thermal energy source.

@derek-m Thanks.

I thought about it too. But I don't see how is that feasible for many reasons:

1. There is no one in Shetland to bore it.

2. The basement is very close to the sea (a few meters) and almost at sea level. Additionally the house is built on a rock. I wonder how easy would it be to bore through it...

Lastly - is it even possible to bore inside an existing structure? I thought that boring machines are pretty large (I saw them boring holes for our main house foundation)

Thanks!

As you have now mentioned that you are close to the sea, would it be possible to use the sea as your energy source? You could still have a GSHP, so no air flow, but with the collector in the sea if that is possible.

@gild, I see where you’re coming from with the idea of adding grills or mesh to bring in fresh air. 

For an ASHP to function optimally, it needs a constant supply of fresh air. The volume of air required depends on the size and capacity of the ASHP. Let’s say, for example, you have a 10kW ASHP, which can require moving around 2,000 to 3,000 cubic metres of air per hour (depending on efficiency). This means your setup would need to supply fresh air at a rate of roughly 30 to 50 cubic metres per minute.

Thank you both!

@derek-m  Is that even legal? I thought that the crown owns all the shoreline. How do they work? Is it a standard GSHP buried in the sea? There must be technical implications due to the corrosive nature of sea water?

@editor I understand this principal. But I was also thinking that the fan is responsible for pushing all this air volume. So wouldn't that work as long as the room is ventilated well enough? Or is there some process of heat transfer with ambient air that doesn't go through the fan?

I'm just providing possible solutions, whether they are legal or feasible is for you to explore.

Water has a specific heat capacity that is approximately 3500 times greater than air, so instead of the 30 to 50 cubic metres per minute of air stated by Mars, for the same thermal energy you would require 8.6 lpm to 14.3 lpm of sea water.

Would it be possible to draw water from the sea into your basement, pass it through a heat exchanger connected to a GSHP, and then send the cooler water back to the sea?

Why are you making something easy so difficult, put the ashp outside.

It a get a dedicated indoor unit, where you put 2 x 150mm holes through the wall. But you will be limited to 3kW.

@gild, there are alternatives like water source and ground source heat pumps, as Derek suggested. However, these systems can be incredibly expensive to install, and in many cases, the cost can be prohibitive, even if you’re located near the sea. Water source heat pumps, particularly those drawing from rivers or the sea, are extremely efficient and a fantastic option for those living near reliable water sources.

Is it legal? That’s a good question. Using sea water for a heat pump would likely require permits, as the shoreline in the UK is indeed owned by the Crown Estate, which manages the seabed and foreshore. If you’re considering using sea water as a heat source, you’d need to check with local authorities about permissions and regulations.

As for how they work, sea water source heat pumps are different from ground source heat pumps. Instead of burying pipes in the ground or down a borehole shaft, the heat exchange pipes are submerged in the sea (or other water bodies). There are technical challenges, especially related to the corrosive nature of salt water, which can severely damage the pipes and other components. Because of this, specialised materials, such as corrosion-resistant coatings and marine-grade metals, are used to avoid degradation over time.

That said, as johnmo mentioned, an external ASHP is likely your best solution. Installing one in your cellar, unfortunately, may not yield the best results due to the challenges in airflow and space.

Thank you all.
I am not trying to make things complicated. I have no place to put a heat pump as our property doesn't have any land beyond the house itself and the only wall I can in theory use - is facing the street in a conservation area and this will never get planning permission (not to mention that it will be detrimental to the street so I don't even want to try).