New member and first time poster....

I have some information to share about my underfloor hydronic system and how it performs - perhaps of some help to people who are considering their own builds - I hope to get some comment from the assembled expertise here on what I might do to maximise efficiency.

The major components I have are:
Emmeti Mirai EH 1611 DC - 3.57kW max power consumption - heating mode only
300l Sunflow hot water cylinder - heated by the Emetti to 55C and a 3kW back up element to get it to 60C for legionella protection
230m2 of 130mm thick post stressed slab insulated underneath and at vertical edges with 40mm high density polystyrene.
Pipework is Pex 12/16 laid about 50mm below the surface, arranged in 12 circuits (but all are 100% open all the time)
Nest thermostat

I also have a Vera home automation system which has sensor inputs for outside temp, inside temp, underfloor inlet manifold, underfloor outlet manifold, and set in the slab. It can control the Nest with logic around any of the sensor inputs and outside current/forecast weather conditions (that's a topic by itself probably).

I am in New Zealand - north of Wellington and more moderate winter temperatures than most of you would experience. Temperatures in June/July tend to be around 10-14 daily max and overnight minimums range from around 8 (overcast/northerly wind) down to 0.

House insulation is very good by local standards but I suspect only average for a home in UK/Europe - a long way short of passive anyway. We have a lot of north facing glass so it can heat up very quickly on a sunny day but that heat is gone by 3am if there isn't also heat in the slab.

This winter is our second in the house and my first where I've had all the sensor information and have used the underfloor as the exclusive heating source (we also have a logburner). Observations and questions that are on my mind currently are these:

Observations
- Comfortable for us seems to mean an internal slab temperature at between 23.5 and 24.5 degrees.
- Running the ASHP during the day to charge the slab with heat while it's warm outside for optimal COP seems to work best. I am still learning but depending on outside temp it seems to need between 2 and 5 hours of running per day to raise the slab temperature enough to get through to the next day - i.e. daily cycles.
- A cold night (near zero outside) will see the slab drop by a degree in 12 hours, when it's around 8 degrees it will lose heat at half that rate.

My questions
- I am wondering if I should run multiple shorter cycles rather than just the one long one per day in case the return temperature is getting too high and reducing COP.
- What would be the simplest/best logic calculation to use to program when and for how long to run the heating each day. I suspect some combination of current slab temp, current outdoor temp (for COP consideration) and forecast overnight low.
- Our hot water use is 90% in the morning and evening so this is when the heatpump runs to heat hot water - would it be more (or less) efficient to run the underfloor heating at the same time as it is doing the hot water cylinder?

Thanks gg for your comments - some good stuff for me to think about here, and to answer your questions:

You have 230 m2 of slab – how much N-facing glass do you have ?
At a rough estimate of purely north facing glass I'd say 35m2. There's a short flyaround vid of the design I did in sketchup before we built here: https://vimeo.com/42255960
There is another 22m2 or so of west and east facing glass.

and what percentage is this, in relation to the floor?
North facing 35m2 / 230m2 = 15%

Do you have a sunspace ?
I had to google this to find out what a sunspace is and I'd say no - although the design with polished concrete and north facing windows is intended to capture and hold some heat from the sun in the slab. In the west bedroom for example there is a strip of polished concrete below the glass door.

The hydronic runs on 12 circuits, and they are all on, all the time: do any of them run in slab that is painted by northern sunlight ? If so, I would try shutting them off, PDQ…
Yes in the living area, although the 'painting' is in stripes and my experience is the sun isn't enough by itself to take the chill off the slab 24x7 I take your point - we could try turning these cuircuits down at least.

What is your fuel price for wood, vs electricity for the heat pump ?
Electricity is $0.30 per kWH
Wood is free when I have time to chop a trailer load of it from what we have on the property.

Any reason why you do not use the log-burner ?
We used it pretty much every night for the early part of this winter and most of last year's. I stopped using it about 6 weeks ago because I wanted to see how the house and underfloor performed without that as an extra variable. It is convenient not to have to fire it up when getting home from work and it doesn't give you a warm floor under your feet like the underfloor does; but we miss the ambience and of course it costs more in electricity. We will use it again when I feel I understand how the other heating variables work without it.


How much thermal mass does the home have ?
Walls and roof are timber framed so not much there, floor is 130mm x 230m2 so that's where most of it is.

What does it consist of, what color is it ?
The house construction is 140mm thick timber framing, clad in cedar weatherboards and plaster board interior lining. Corrugated iron roofing. Roof is near white, weatherboards stained to near black. The thermal mass in the floor is polished concrete with a light colour additive that makes it a little darker than normal aggregate concrete (probably needs a pic to explain).

Could you add any more; if not, would you be prepared to have drums of water standing around in various places?
I can't immediately think of anywhere we could add large mass or hide large drums without it being an inconvenience.

I think you have put your finger on some of it it anyhow – a big part of the secret is determining the time base for powering the slab – and the inertia of the latter - you are going to have to work this one out !

I think it would be better to run the heat pump in the hottest part of the day, and have the air intake in the hottest part of the house (where is the exhaust ?)
The heatpump is mounted outside the house with the intake and return pipes (insulated) running under the slab into a centrally located hot water cupboard / linen cupboard that has the manifold and hot water cylinder.

Re Legionella protection, check out whether there are several regimes applying in your local authority area - such as 60°C, once a week, or continuous 55°C etc.
Great question... a quick google doesn't turn up any specific guidance but I should be able to find out. The regular brief spikes that show up in the graph (inlet heat) outside of the underfloor cycle time are either transferred heat from the HWC element spiking to 60C, or from the ASHP reheating the HWC. Not sure which...

Hi again,

I should explain a bit about the chart because the legend is probably too small to tell what's going on.

The grey shading is night-time (sunset to sunrise)

Green blocks: shows periods where heating is on
Red: Outdoor temperature
Black: Thermostat setpoint
Blue: Indoors air temperature
Orange: slab temperature
Turquoise: manifold outlet to ASHP (sensor taped PEX connector nut)
Purple: manifold inlet from ASHP (sensor taped to PEX connector nut)

Notes:
Heating is on when thermostat setpoint (black) is above indoor air temp (blue). At the moment I am manipulating on/off cycles by turning it right up and right down.

Inlet and outlet manfold temps spike outside of heating cycle times (green blocks) when hot water cylinder is being heated.

The graph shows the ASHP is run in the afternoon hours to maximise COP, so you see the slab temp rise through the afternoon and into early evening, then fall during the night and early morning.

To answer some of your comments:
The floor is never too warm. Mostly its just nice underfoot- and only feels slightly less than ideal early in a cold morning sometimes - but then we do walk around in bare feet and it's never what you''d call chilly while we are running the underfloor, as we are now for a few hours each afternoon.

We don't have an offpeak tariff available. You are right I think that the afternoon is the best time to run - that's what I've been doing for the last month or so. This makes solar PVs something to think about perhaps....

We've never run out of hot water in the two years we've been here (but our girls aren't teenagers yet!)

Yep we have a lot of glass and you're right that our design is a long way from what folks here would call a green house I guess - this was intended in the sense that we wanted to incorporate some green building principles, but aesthetics and cost were often bigger drivers for our design decisions. Our insulated,heated post-stressed slab is considered a fairly major innovation in these parts though - NZ is well behind europe and north america in building for cold climates.

No we don't have a ceiling fan.... something to think about. The south parts of the house do get cooler than the north and I have wondered if I should install some sort of fan or ducting to even out the heat. This is even more of a factor when the wood burner is the heat source given it's located at the extreme north end (this was an aesthetic decision obviously!)

Thanks again for commenting - it would be a quiet thread without you!