ASHP with back boiler - how would you do it?

9 Mar 2022
Reaction score
United Kingdom
Interested on thoughts on this;

I'm at a stage in house renovation where it would be least disruptive to do this, I'm interested on thoughts on the following as its gone around in my head too many times.

I've got a 14kw (4kw elec) ecodan (monobloc) ASHP, I've reduced the energy use from 15Mwh down to 12MWh per year (there was a fault) and then further down to just under 9MWh per year by splitting into 3 zones -one per floor and a couple of other improvements).
Additionally, I've got a 6KW log burner in the lounge (no back boiler)

In the back of my head I'm wondering if I can make it yet more efficient to run by utilising a back boiler on the log burner in the lounge.

There isn't a buffer tank on the system and it has a single coil 210L tank for DHW.

My thoughts are, 28mm run to log burner with back boiler in lounge (on a pumped system) and install a buffer tank (size a bit of an unknown 100-150L?) on the return feed to ashp after the 210L tank and situated next to it.

Originally I thought of a twin coil tank, but this wouldn't allow the heat from back boiler to be used on the CH circuits.

After a bit more thought, this is where I'm trying to figure things out in my head.
Preferably the back boiler circuit would be unvented but I'm aware of the issues with uncontrolled heat source etc here.
In my head, the ASHP return feed would run through the coil in the buffer tank (reducing the extra glycol volume).

Back boiler circuit would use towel rail in the bathroom (which it passes en route) as a heat dump. Ideally the towel rail would also be then free to be used during summer months without circulating water back thru the now cold back boiler.

This has all come about (apart from rising elec costs) as I have been speccing a generator that can run the heat pump if/when the rolling blackouts start.

I could get away with a much smaller generator to just run the pumps and controls, if there was an alternative source of heat.

Log burner gets used for approx. 5-6 hours a day from September through to March, there would be something quite satisfying about lighting at 4pm and knowing that the energy is being used in other rooms of the house in the meantime / amount the ASHP comes on for is being reduced. The "new" log burner/back boiler I've considered is a Coalbrookdale Severn. Bonus would be getting over the dilemma of making towel rad warm in summer without an elec element.

Not asking for a design as such here, aware of G3, Hetas etc and concerns you guys may have. I'm intrigued to know how it would best be done / if it could be done with a unvented buffer with additional safety measures, and more importantly, if it would be worth it or has anyone done something similar?

If it was a vented "buffer", would I be correct with ASHP return indirectly heating the buffer tank, which stores the water from the pumped back boiler/towel rad circuit, and then has a F/E tank in loft above it?

Laddomat 21 has come up in my research as being a good idea to prevent the stove getting too cold when initially lit.

There's probably something huge that I've missed here (towel rad running at crazy temps, actual energy calcs give a 30 year pay back etc)


(can attempt to draw a pic/diagram if it helps)
Sponsored Links
Have a look at thermal stores. The one I have is 300ltrs (bit small, 500 would be better). You can have whatever coils and taps you like in the thing- so for your setup I'd suggest you run the ashp through a coil, run the woodburner direct (this is vented btw), have low taps with a blending valve to run the ufh, higher flow tap for any rads, dhw either an external plate hex or another large surface area coil.
Laddomat (loading valve) on the woodburner is a v good idea- prevents backflow and prevents condensation in the woodburner firebox.
You'd need a good sized (60 litre or so) metal or non-thermosetting plastic f & e tank, a heatloss rad on the lined from the woodburner running on gravity only if needed.
You can do unvented stores with woodburners but there's a load of engineering needed to prevent explosive steam buildup. Vented is inherently safe, long as you've used metal everywhere if it gets too hot it boils- which is noisy and wasteful of energy but won't bring your house down.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links