Maybe less dramatic but there is a risk to undersize on newer build properties, especially where max output during defrost conditions doesn't match datasheet and if build quality is lower than design..

I just wrote some more thoughts here on the new MCS heat loss calculator and the ventilation calculation, you may find interesting community.openenergymonitor.org/t/new-mcs-he...

Makes sense in building physics terms but if this just adds to over-inflated heat loss in pre-2000 properties then it's not helping. Key is to have good agreement between calculated and measured heat loss.

We will do a detailed comparison

Performance on space heating has been great so far, COP 4. While a buffer was installed as per viessman spec, we also have the option to bypass the buffer and we've been running it with the buffer bypassed all on the primary circulation pumps just fine over recent days 👍

That's a great way of approaching it, always good to come at it from both angles, that should be part of standard heat loss calc methodology rather than relying on calculation alone

Thanks Keith, all about a useful amount but not too much , thanks for picking these points up!

When I was getting our heat pump here, because of the twitter heatpump discourse, I was worried that our model was oversized, but found in practice that it actually performs very well! (It's well within @openenergymon.bsky.social's range of useful oversizing).

Yes that's correct, weather comp for boilers has not been widely adopted or mandated as is the case elsewhere.

That's a great result! it would be great to see more of an industry push to include weather compensation with boilers , it would help get people more familiar with heatpump controls as well

Thanks James!

Impressive!

4. Our analysis of heat pump max outputs during defrost conditions is available here docs.openenergymonitor.org/heatpumpmoni... and discussion of how much margin is good to add and how to avoid oversizing here docs.openenergymonitor.org/heatpumpmoni...

3. There are risks to accurate heat loss, if your using a design temp of 50C and improving heat loss calc process, consider lowering design flow temps to nearer 40 and add a margin to cover lower than datasheet outputs during defrost conditions docs.openenergymonitor.org/heatpumpmoni...

2. The secret to making design temperatures of 40C (or lower in practice) possible is accurate heat loss calculation and tuned weather comp docs.openenergymonitor.org/heatpumpmoni...

1. Low flow temperatures = high performance (heatpump 101) but did you know high performance heatpump often run at around 34C on the coldest days? docs.openenergymonitor.org/heatpumpmoni...