Welcome to the forum @tommyt11

I don't think that the Mixergy tank has been described here before. I'm aware of it because of discussions on another forum a couple of years ago.

I have a 280-litre thermal store, which delivers Underfloor Heating and Domestic Hot Water. I'll post a generic diagram here, although no one would be likely to have all possible options installed:

It will assist others if you could describe how the Mixergy system builds on the thermal-store principles, and what you like about it. 🙂 

In our last house I put in solar thermal tubes from Navitron into a low-mounted coil on a 180l vented heat bank (thermal store but with external plate heat exchanger for DHW). The boiler fed in hot at the top of the tank, and the return to the boiler was tapped off a TMV that mixed in colder water from the bottom to keep the return at 50 or so and allow fast recovery whilst keeping boiler condensing. It was all DIY (that's why it was vented) and worked well.

Back to Mixergy. We have an unvented system now, much preferred. We have good mains pressure but poor flow so I have a 300l CW accumulator after the water softener.  Therefore the Mixergy can deliver oodles of DHW at a constant temp due to its design. Basically it floats a column of hot atop the cold, and a little external pump pumps cold from bottom to top as it is heating (and modulates as appropriate depending on the heating power at that time, be it immersion or indirect coil). Currently I only have the immersion connected via a PV diverter and Eddi. That has delivered a full 210l via dribbled excess PV every day since May (the tank has not been exhausted at day end most days though). I probably won't connect the indirect coil at all as we'll be connecting ASHP via plate heat exchanger when we get it (granted, when heating via ASHP it does the whole tank in the traditional style rather than top down). However, considering how its going and our excess solar PV stats from last winter, I only anticipate using the ASHP to heat it for maybe 3-4 months of the year. 

There will be UFH going into ground and lower ground, and rads on first floor. At 150mm UFH spacing and with the amount of insulation we are adding, it will be quite a low temp system I hope. Weather comp. Am looking at the usual suspects Vaillant, Midea, Samsung, etc.

Posted by: @tommyt11

It was all DIY (that's why it was vented)...

I'm not sure that is a statutory requirement.

When I installed my thermal store in a renovation project in 2005, Part L1b of the Building Regulations did not require a qualified installer for an unvented cylinder if it was used primarily for space-heating. I am certain of that because it was the Building Control Inspector who advised me to buy that particular thermal store!

I installed everything myself, except the gas boiler. At the time that was covered by Corgi registration for all installers.

I've just checked Part L1 of Building Regulations (last updated 2021) online. The requirements have changed considerably and the emphasis is now on meeting specific thermal targets for each element of the build.

Even so, I can't find any stipulation that the installer as an individual must meet certain requirements. The calculations and evidence of complying with the specified parameters must simply be presented to the building control surveyor.

It's possible that you are still correct in saying a DIY person may not now 'sign off' a pressurised heating system. But that must be due to another piece of legislation rather than the Building Act.

Can anyone else shed light on this?

Nice setup. I forgot to add before, my heat bank had the rad circuit flow tap off about half way down. I can't remember who the tank was made by but they went bust a few years ago. I'll have to dig out some photos...

@tommyt11, welcome to the forums. We look forward to getting updates on your reno. Have you identified an ASHP that's of interest to you?

Vaillant Arotherm is the front runner at the moment due to superior after sales/service, friendly refrigerant and good controls. I had my HP man over this morning, I'll see what he comes back with cost-wise.