Absolutely nothing at all to stop any ashp being used with a vented tank except that I'm not sure anyone advertises a vented tank with a large enough coil.  Most tank manufacturers will do specials or you can use a plate heat exchanger instead of a coil (arguably better anyway), or just plumb  an 'unvented' tank with a vent (although this is unnecessarily expensive).  Really this shouldn't be an issue for any half competent plumber.

 Doubtless some will give a spiel about legionella, you should run a legionella cycle in accordance with recommendations (although others may argue that this is unnecessary).

Sadly you are getting the usual nonsense of installers who can't think outside their very small box and insist on using the ashp manufacturers pre plumbed cylinder so that they can put complete rookies on the job whilst still charging a premium.

Permitted development applies only to one heat pump so you will need express consent if you already have one.  Once you have express consent MCS isn't necessary unless you want the grant.  Sadly (and disgracefully imho) MCS is necessary for installation under pd. 

You probably can't duck the loss calcs because emitters need to be sized but it might be possible to persuade an installer to use your actual consumption for the system sizing. Most will probably refuse however because as a customer you can't be trusted and likely over-size as a result. 

Gledhill do a range of OV HP tanks .. eg Gledhill SL Plus HP 300HP3SQM EE Open Vented - PLUHP300OV, though these are not, of course, part of any manufacturer's standard kit. Sounds like I figured right - I need planning approval for the second HP. Might be easier just to upgrade the existing one and go A2A for the whole bungalow. I used to despise hot air heating, back when it was gas furnaces, but since I want aircon anyway ..

The fact BUS doesn't apply to A2A is another stupid government decision. Much of the continent (well, Scandinavia) runs on A2A as far as I know, thus avoiding a whole ton of freezup issues.

Legionella and even DHW in total are not really a consideration.. I have enough PV and battery to just run an immersion heater on cheap (thanks Octopus) overnight electrons, and leave the heatpump to do the space heating.

As you say though, finding a flexible installer, out here in the wild especially, is a challenge .. much easier to shift BUS-funded boxes en mass. Even Octopus (as Which recommended) won't play with me, whether because they just don't have anyone out here, or whether because of non standard needs, is hard to tell. Maybe they just don't like anyone who has actually done some research. 🙂 

Posted by: @dave1234

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As you say though, finding a flexible installer, out here in the wild especially, is a challenge .. much easier to shift BUS-funded boxes en mass. Even Octopus (as Which recommended) won't play with me, whether because they just don't have anyone out here, or whether because of non standard needs, is hard to tell. Maybe they just don't like anyone who has actually done some research. 🙂 

Not sure you are any worse off than those like me who live close to London when it comes to flexible installers.  Many including me report similar problems.

Octopus seem to have a fairly good reputation for being tolerably flexible but will only engage if your house fits their current target market.

Few installers, in my experience,  like people who have done any research, oresumably because it makes it more difficultf for them to spout bs to get the sale they want.  Those that do like such people deserve our custom.

Unfortunately the bus and pd rules, both of which require mcs, have created an industry which some argue is not healthy.

Posted by: @dave1234

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I am struggling to find an installer, or manufacturer, who can fit me an ASHP in conjunction with a vented DHW tank. Out here in the wilds of Shropshire, 1000 ft up, the water pressure (or even availability) of mains water is iffy so there are two large cold water tanks in the loft. This also avoids the need for pressure relief to the outside world, in the event the immersion heater decides to boil the tank. Am I chasing unicorn poo, or do such systems exist (even on the Welsh marches)?

I guess I need an MCS installer to avoid planning issues, though I may have those anyway since I have a 20 YO A2A aircon system, and I believe I am only allowed one HP under permitted developments? I do have 10 years of energy consumption data from my oil burner (10 kwhr per day per degree day of heating needs) so maybe I can duck the heat loss calculations?