UncleQ

Definitely!

Well, we’re very pleased to report that our full planning permission was granted this afternoon after 13 months, 5 completely different designs and 2 full planning permission applications. It was particularly difficult as our plot is outside the development boundary, i.e. classed as open countryside, so became an NPPF paragraph 131 (para 63 in old money) application.

We'd also look to power the DHW cylinder or Sunamp from the same external ASHP, so I guess something around the 8kW would be more than sufficient with a bit of headroom. On the MVHR front, I guess that it's preferable to slightly oversize the air volume it can shift to keep the noise to a minimum?

Much appreciated! I also double checked my figures and spotted a mistake I made. Our total house air volume is actually 650m³ which ups the heating load required to 3.1Kw.

How do you size an MVHR flow rate? Is it as simple as the total air volume of the house divided by the ACH figure you need to achieve? e.g. in my case about 350m³ / 0.43 ACH = a flow rate of 150m³ per hour is required?

I used your calculator spreadsheet with met office data for my area and came up with a maximum heat requirement of 2,765W in February. I'll ask my architect to remodel his calculations. Thank you for your tool @JSHarris!

Wow, you guys have given us a lot to think about and we're very grateful for your real world experience, which is exactly what we were after. For a few hundred pounds it really is a no-brainer to at least plan to put the UFH loops in while the slab is being laid, even if we end up not using it, at least it keeps our options open. Our plot has a fair amount of trees which as a fuel source was one of the reasons we were considering a stove, but having just had the discussion with my wife we'd be quite happy to ditch if it just adds complexity we don't really need. So, plenty to think about but after this thread we're leaning more towards UFH for our ground floor heating from an external ASHP that can also feed DHW (tank or SunAMp), and perhaps a Genvex type MVHR with ASHP to supplement heat or be a backup in case of failure, but mainly to help cool if needed. Which external ASHP brands do people recommend? Friends of ours have a Mitsubishi unit powering their UFH/DHW and are very happy with it.

Yes, it's our architect's biggest concern. We've cut it down significantly to get to 90m2 but have had to add overhangs and bris soleil along with window panels from Bruag to compensate.

Thanks for you reply. Wow, those figures are seriously low, quite amazing and something we could only hope to emulate. I guess that if it is a possibility that our actual heating demand could be considerably less, then perhaps we should start with a Genvex Premium 3 or equivalent, and see how it works? We could always then add an additional heat source later on, particularly if we build in a water duct reheater into the MVHR fabric but not connected to start with?

Thanks for your reply. It's all theory at this stage of course, but 7Kw is what our architects thermal modelling software suggests. Our raft foundation of 141m2 is specced for between 0.09 to 0.13W/m2K U value, the 450mm ICF for the walls is 0.11 and the roof to 0.13. That said, we also have a significant amount of triple glazing at nearly 90m2 which will push the overall figure up a bit. It would be great if the total heating demand ends up being less, that would be a bonus!

We're about to build a 5 bedroom near passiv haus home of 240m2 over 2 floors using 450mm ICF and a raft foundation giving us 0.10Wm2 U values with triple glazed windows in an airtight configuration. I've been trying to narrow down our central heating and DHW solution and have narrowed it down somewhat. Our thermal modeling suggest we need a peak central heating requirement of 7Kw and history with 4 kids suggests 250+ litres of DHW. I appreciate lots of you have UFH but we're trying to avoid complications with a wet system with embedded pipes, so were thinking of a purely air-to-air solution for central heating. I can't seem to find the obvious 'nirvana' solution combining an external large volume ASHP with an MVHR system to distribute the warm air. The closest I can find are the Genvex type MVHR units with a built-in ASHP, which whilst they appear to be a nice and neat integrated package, suffer from their inherent restriction on the volume of air they can ingest, thus topping out at the 4-5Kw mark and risking being noisy at full blast trying to reach that. My current thinking was to use a Genvex Premium 3 for the majority of our space heating and cooling needs supplemented by a wood burning stove for the colder winter days, but am concerned that this still might not be enough or could be excessively noisy so may need to be supplemented with pre or post heating or another solution. For DHW I really like the idea of the SunAmp heat batteries but am a little wary after reading about the 50% re-charge issue with their latest models discussed on this forum at length. So my questions are as follows: are there better options we should consider for our space heating needs using an air-to-air solution? e.g. a standard MVHR connected to an ASHP using a duct reheater? Can this cool? is it a practical idea to supplement using a more conventional external ASHP to 'top up' the Genvex unit through the use of a water duct pre or reheater? I understand the theory of both pre and post duct heating using for example a VEAB CWW unit, but would be concerned whether or not the Genvex controller could incorporate them seamlessly without requiring a custom solution? I can't imagine that this question hasn't been asked/debated before, apologies if I haven't spotted all posts and many thanks for any help you can give.