Replacement for Exhaust Heat Recovery System

[chrisru]

I'm currently looking at a house purchase and a bit puzzled by the heating system. It's a 15 year old Nibe Fighter 360P (https://www.manualslib.com/manual/1240673/Nibe-Fighter-360p.html) which the original builder admits is a little undersized, the property is well insulated but 200m2 rather than the 160m2 suggested limit, they suggest it outputs around 5kW. We are looking to extend and would end up with 250m2, I also understand that the 360P is end of life and spares getting harder to come by so a replacement makes sense on all fronts. 

The house has the vents to capture warm air from the lounge and bathrooms and an exhaust chimney, with vents in the rooms for fresh air to come in (see diagram), UFH throughout and TRV to bedrooms. The house is very well insulated and fairly airtight.

The options I see are
1. Replace the 360P with the updated model - the Nibe F370 this will be supported for longer, is a more efficient model and MCS registered so would qualify for RHI. However it still have the existing limitation in terms of output. With the Nibe systems in Winter mode then it will try to augment the heating using an electric coil and this proves expensive. The current owners supplement the heating using a wood stove, the heat from which is then captured and recirculated which is an options and I could go for a pellet stove which could be automated to some extent.

2. Replace the 360P with an ASHP, this will be more efficient and capable of heating the house without supplementary log stoves etc. This would leave the existing vent system so I had though that I could add an exhaust heat pump like the SunSystem TDA S 200 Sunsystem Exhaust Air Source Heat Pumps - Green Phoenix Ltd which would improve ventilation in the house and recover heat to compensate for the vents in the rooms, this would then provide DHW and as this unit has a coil it can be heated by the ASHP as well. I'm not quite sure how I would control these two together and whether this is overkill, all the houses I've had before have been sufficiently leaky to not require a ventilation system.

Any input and thoughts welcome. Attached is a diagram of the 360P and vents, and the TDA S 200 manual.

Thermopump_Exhaust_Heat_Pump-TDA_v0.2_EN.pdf

[PeterW]

29 minutes ago, chrisru said:

The house has the vents to capture warm air from the lounge and bathrooms and an exhaust chimney, with vents in the rooms for fresh air to come in (see diagram), UFH throughout and TRV to bedrooms.

I’m slightly confused. What is providing the heat to the UFH and rads ..?? It can’t be the Nibe as it wouldn’t be able to cope (or is that the issue..??!) as its output is far lower than you would need. 
 

I would go for standard ASHP and then put a centralised duct fan unit on the existing extracts to reuse them. Putting proper MVHR isn’t going to work unless you put new fresh air ducts into the house via the unit and remove a lot of the fresh air vents.   
 

 

[ProDave]

I am very skeptical of the principle of an EAHP.  Yes it removes heat from the exhausted air, but that heat then has to be made up by the heating system as the incoming replacement air is at outside temperature.  It kind of gets into the perpetual motion problem, unless in winter there is so much heat extracted from the exhaust air that it is leaving the building significantly colder than the outside temperature.

 

It must work to some extent as some on here are using it and happy with it, but the amount of heat it can deliver is small.

 

A conventional ASHP and mvhr is science that I understand and makes sense.  Though retro fitting mvhr properly might be a lot of work.

[Gone West]

1 hour ago, ProDave said:

I am very skeptical of the principle of an EAHP.  Yes it removes heat from the exhausted air, but that heat then has to be made up by the heating system as the incoming replacement air is at outside temperature.  It kind of gets into the perpetual motion problem, unless in winter there is so much heat extracted from the exhaust air that it is leaving the building significantly colder than the outside temperature.

 

It must work to some extent as some on here are using it and happy with it, but the amount of heat it can deliver is small.

 

A conventional ASHP and mvhr is science that I understand and makes sense.  Though retro fitting mvhr properly might be a lot of work.

In my system the EASHP removes heat from the exhaust air after the heat exchanger has recovered the heat from the extracted air and as a result the air leaving the vent is very cold. Our EASHP is rated at 585W and with a good CoP can deliver a reasonable amount of heat but not enough to heat the house in the winter. Used in conjunction with extracted air from warm bathrooms it is sufficient to maintain 23C in our small house (130m2).

[ProDave]

7 minutes ago, PeterStarck said:

In my system the EASHP removes heat from the exhaust air after the heat exchanger has recovered the heat from the extracted air and as a result the air leaving the vent is very cold. Our EASHP is rated at 585W and with a good CoP can deliver a reasonable amount of heat but not enough to heat the house in the winter. Used in conjunction with extracted air from warm bathrooms it is sufficient to maintain 23C in our small house (130m2).

So yours combines the function of an MVHR as well passing the incoming air through a heat exchanger first?  That makes a lot more sense and I can well imagine the exhaust air after the heat pump is indeed very cold indeed.

 

the system described by the OP did not include an MVHR element and the incoming air to the rooms appears to be raw outside cold air.

[chrisru]

Thanks all, and yes as you all suggest the system is not sufficient to warm the house in winter but they run a very large log stove and leave doors open, and the EAHP will recycle that heat as well. I think the ASHP as a first step is the obvious one, and it's just whether to go with something like the Sunsystem as a EAHP DHW solution alongside it - they are pretty good value for what you get.