JohnMo

A drawing of what you're doing and where you propose to locate the manifold and boiler/ASHP would actually help. We're all guessing otherwise.

Some info here also

This is the first topic I found on it - on another forum https://community.openenergymonitor.org/t/vaillant-arotherm-plus-cooling/26658/15

You have to buy a plug to make it cool - Valiant UK want to charge many hundreds of pounds for the part, but the same widget is also used on boilers (not for cooling) and it basically a resistor. There is thread on here that has all the parts numbers. The gas version is about £10. You plug it in and the heat pump that has all the cooling options show up on the controller - another UK ASHP con - premium ASHP you have to buy extras for, to make do a basic ASHP function.

Grant doesn't state anything or care, MCS don't care either. In the past under a different schem there was a requirement for cooling not to be possible - but that went away about 10 years ago. Most heat pumps will cool, its just a matter of finding what settings are hidden away. The new R290 Grant heat pumps, Grant state they will not cool, but there is a dip switch in the casing that can be set to allow automatic change over of heating or cooling, heating only or cooling only. Grant default is heating only. So exploration you may find it can cool. All ASHP have the ability to cool and they must move to that function (refrigerant 4 way valve) to do defrosts. You've contradicted yourself.

So obvious question, can you not locate the switch away from the steelwork? Wrap steel in 25mm PIR (or more if you have space) and seal all joints with aluminium tape - cold bridge gone. Or use a dry lining pattress box, instead of the normal ones for skimmed walls?

We started with One Insurance (think that's they are called) now with direct line. I found I answered the questions that were asked only. We are ICF, so our wall construction is concrete etc. Is it an open timber frame - or just a timber frame?

I wouldn't do that. Use a coupler in an emergency, but wouldn't plan to do it. Couldn't you decide where the manifold is going and make the connections to the manifold. Then pressurise the loop and add mechanical protection as needed.

I would be more concerned that you actually discovered the steel and you can put your hand on it. It should be fully insulated not hidden behind the plasterboard uninsulated.

How big are you planning - floor area and number if storeys, on how big a plot?

But do you really get that many days at -3, compared to days at 7 to 10 degs.

Maybe, but most houses on here are outside their comfort zone, ventilation heat loss just too small, to allow s tick the normal tick box. Hence plenty are specified way over sized.

Use a normal domestic one, your just heating a buffer as far as it's concerned. Depending on hot tub size a big one. To keep the ASHP inhibited and pressurised you may need to plate load the hot tub, but size the Plate Heat Exchanger (PHE) correctly, you have a 2 Deg approach temp. Then you have no issues with DHW heating, it just switches the diverter valve as normal. Add a second zone for cooling, so it bypasses the hot tub or have the hot tub cooler in summer and as cooling buffer. Heat pump doesn't care if it doing heating or cooling it just moving the refrigerant 4 port valve to a different position (as long manufacturer hasn't disabled the cooling function). Most heat pumps have the functionality to have different zones as cooling only, heat only or both. And the functions can be independently set for each zone.

I just looked and came to same conclusion you did, even NE Scotland I can have in two days.

Have you considered using a single heat pump? Size the spa heat pump big enough for the house, use the volume of the hot tub as a heat source for the house via a plate heat exchanger and mixer valve. Heat pump does two jobs, keeps hot tub temp stable, and or DHW via a diverter valve.