ASHP with back boiler on wood stove, thoughts?

[Dannss]

Interested on thoughts on this (I've also posted on another forum but thought this might be a better place to ask);

Long time lurker, have gleamed some great knowledge from this site, finally signed up as I cant find an exact answer to the below. I'm not a heating engineer, but an electrical engineer that has mostly sussed how the current system works (feel free to correct any understanding/terminology below).

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. CH flow temp is weather compensated, HW kept at 55 deg c.

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 without interfering with weather compensation/cooling the HW.

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 now aware of the issues with uncontrolled heat source etc here 2 tiers of safety required.
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 (and maybe rad) in the bathroom (which it passes en route) as a heat leak.
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 using the heat in buffer tank which at that point would be heated by ASHP.

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 (directly heated) water from the pumped back boiler/towel rad circuit. This would then have a F/E tank in loft above it/ sufficiently high up in airing cupboard (prefered).

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)

Cheers!

(can attempt to draw a pic/diagram if it helps)

In summary -
Existing ASHP and dhw tank.
Add back boiler circuit in which is indirectly connected to ASHP flow/return circuit.
Try and get all of it into airing cupboard.
Enjoy added benefits of buffer for ASHP.
Get around the towel rad in summer issue,
Sit back and enjoy the feeling of utilising all heat instead of wasting a lot of the log burner output.
Rough level of work required so that I can weigh up if its worth it/amount of kit required/space it might take up.

 

[ShaunCorbs]

No sorry as log burner high temperature system

Heat pump low temperature system

 

[jamie_bennett]

Hi,

It is possible to link an ASHP along side a log burner.
The two most common ways of doing this is by using a neutral manifold or a thermal store / buffer tank.
1) The neutral manifold will have the heat pump and stove plumbed into either side, inside this manifold will be a series of baffles which prevent high temp heat from the stove circulating around the ASHP. There are then a number of zones coming off it, EG rad zone, h/w zone etc.
2) The thermal store option is more expensive but benificial in the long run to link the two heat sources together and you can add additional heat sources in the future too. Depending on what type of thermal store you go for will depend on the stove and hot water usage but the set up is quite simple. The ASHP should have its own tappings for the flow and return and so should the stove, again this is to prevent any circulation around ASHP and stove when the opposite is running.

The majority of boiler stoves are rated to be fitted on pressurised systems. They have a quench coil fitted inside. If you fit a stove which is on a pressurised system then you must fit the over heat valve and pressure relief valve as mandatory. Personally. I still recommend a heat leak radiator being fitted.

Cheers

 

[Dannss]

Hi Jamie,

Many thanks for the reply, I was definitely under the impression that it is doable, perhaps not the popular, but not impossible to achieve.

One of my goals at this stage is to figure how much the existing system would have to be changed to fit this in. Interesting to note about the quench coil, as I've only looked at older wood burners so far, it isn't something I've come across.

I've attached a crude schematic of current setup showing the main features at present.

Its an interface between the two systems on the return to ASHP (in the black box) I'm trying to figure out. I'm probably wrong, but replacing existing HW tank with thermal store in my head would reduce efficiency of the system as ASHP would have to always run at 55 deg in order to replace heat taken out by CH circuit which would reduce the COP? - unless this overcome by the height of the coils in the tank?

If a separate buffer only needed to be kept at weather compensation temp for CH (32-47) in CH mode, and could be assisted by the back boiler, in my head that is where there can be gains/ would reduce the amount that the ASHP 'runs' to heat the circulated water.

When HW demand comes on, the second (buffer/neutraliser) tank would of course heat to 55, but again could be assisted by back boiler/ when it changes back to CH mode... the 55 deg water in the HW tank is left alone.... and the buffer tank, well ...buff's and heat is dissipated into house through rads.

As I said, not a heating engineer, just trying to get my head around it. The guys who installed the ASHP are supposed to be giving me a call about this, however, I'd rather have my head around things if/when they call back. I've only chatted with one of the more senior / non designer chaps there so far and wouldn't want to waste their time if its a no go.

Edited to add, I can see the issue where the second tank gets heated to a higher temp than the ASHP is designed to handle. I've had a flick through the installers guide but cant find a quoted figure for max flow/return temps to the external unit.

 

[ShaunCorbs]

Hasn’t any of your system been upgraded for the ashp install eg bigger rads to match the lower flow temp approx 40dc ?

 

[Dannss]

Yes it has, I haven't been able to install wet underfloor heating, but all rads are specced at at least 2 x the "normal" size required working at 30dt.

Its a 100 year old house, solid wall, no cavity, designed to breathe. Probably wasn't the best contender for ashp, but there is no mains gas and the ashp had been installed before we purchased, improving insulation and unintentional draughts where we can (as well as log burner also have 3 x open fires in the house)

 

[ShaunCorbs]

Working at a flow temp of 30 ? As a dt of 30 won’t work for heat pumps has to be around 5dt

Eg flow 35 return 30c