I would try hard to put it inside the thermal envelope of the house.  They're not normally noisy, but there's always lots of pipes, valves, an immersion etc that aren't insulated, so they end up leaking heat.  Better that heat ends up in the house if possible.  The pipe from ashp to cylinder leaks heat too, fine into the house in winter, but a nuisance in summer when you'd like the house cool.

There are (expensive) phase change small rectangular tanks these days by Sunamp, which you might like as it can be squeezed into smaller places.  They offer two temp options; for an ashp you would use the lower temp option.  Probably less flexible than a water tank as it only works at a certain temp, but interesting.

Thanks Robl. Both the utility and the loft are inside the house's thermal envelope. Interesting what you say about the pipes leaking heat. That probably argues against putting the cylinder in the loft as it might be our main bedroom and we wouldn't want it hot...

If installing your cylinder in the ground floor, be careful about thermosiphoning that will drain the heat from the cylinder at a rapid rate.

I've got a Mixergy cylinder where this is exacerbated because of how they pump internally in the cylinder to keep a constant temperature throughout the tank.

Having the tank on the ground floor (below the level of most outlets) causes the 300l tank to cool inside of a day.

As an alternative location (assuming you can overcome thermosiphoning), is to take a leaf from Mick Wall's book and build a plant room outside the house and locate all the kit there.

I have done the same and come in to the house at the first floor level to join in to the existing pipework. The plant room is insulated to the hilt and often does a good 10°C higher than outside temperature and the same if not hotter than inside the house.

@aceshigh I’ve heard a ‘london loop’ can prevent thermosiphoning - it’s a 30cm or so pipe down then looping back up.  The down segment prevents thermosiphoning, and you’d put it on the ‘out’ connection of the tank if it’s low down in the house.

I had not heard about thermosiphoning. So placing the storage cylinder on the ground floor is less efficient than in a higher floor? And the Mixergy tank you have makes it worse, not better, @aceshigh? 

I feel very ignorant right now... 🙁 

@kk_777 don't worry, I hadn't heard about thermosiphoning either until I started looking in to why the tank heat loss was so great.

As for the tank being on the ground floor being less efficient, I would say yes, unless you (or your plumber) understands the risk and mitigates with the pipe work being adjusted to prevent thermosiphoning. Mine didn't and has since suggested fitting a motorised valve to overcome it (at my cost).

The Mixergy tank makes the problem worse due to its design with the inbuilt pump to circulate water within the tank to maintain temperature. Because I've got slightly cooler water coming back in through the DHW return circuit the tank is then compensating and the entire tank cools down quicker.

This isn't a fault of Mixergy exactly, although they could have highlighted the risk in their documentation in my opinion. This probably happens with a lot of cylinders placed in garages and utility rooms across the land. It is just that with the monitoring available on the Mixergy the problem stands out.

To AN Other person with ABC tank they wouldn't see that the tank is cooling considerably and likely wouldn't notice that they're using extra gas / electricity to reheat the tank if they've got it set on a timer each morning or night.

I've not got any timer set on mine at the moment, I heat the tank once (with ashp or extra solar PV) and expect the tank to remain hot for a minimum of two days, so I notice it when it cools 300 litres within a day and the heat capacity on the Mixergy app drops like a stone.