I thought it would be good to give an update with the weather compensation turned on since 21st Dec.  I can see the consumption has dropped significantly but this could also be a result of the warmer conditions if you look at the chart below.  Are there any other suggestions to bring down the consumption further as I still think there may be other areas to consider?

I was thinking UFH might help but had some questions :

- What size pipe would be advisable for a ASHP?

- If the maximum temperature is ~27c for the UFH will this be sufficient to warm the downstairs particularly in the colder conditions?

- I was planning to keep radiators upstairs.  I'm assuming it's not possible for the heat pump to have different heating temperatures for upstairs and downstairs and the higher temperature would still be required and mixed into the UFH circuit as required. 

You're consuming significantly less kWh per day after the cold snap with the WC enabled than you were before at comparable outdoor temperatures. So the WC looks to have made a big difference. Hooray!

With one ASHP, the flow temperature requirement of the wet radiators upstairs, and their sizing, will be the limiting factor to how low you can set the WC curve. UFH controls can mix or divert to avoid overheating the floor from too high a supply temperature. Any options to increase emitter size upstairs? doubles to triples, extra rads, etc?

If you know what design temperature your current UFH was based on you can estimate the % performance in relation to that for lower flow temperatures. 27degC sounds low, and for a moderately insulated EPC C or D property, it might not be sufficient for a prolonged cold spell like we've had.

We went away and it was interesting to see what kWh our ASHP system uses when we're not around consuming electricity. The house used 25kWh over 24hr based on seasonal average temperature about 6 or 7degC, heating on 18hrs a day. We probably consumed 3-4kWh of power for fridges, freezers, stuff on standby, etc., and 1-2kWh for hot water, just to offset standing losses. So the ASHP is consuming about 20kWh a day to keep the house 18-19degC.

@allyfish Yep it certainly seems better and will check next time we get below 0c temperatures.  I don't have UFH yet but was thinking of installing downstairs given some of the flow issues I mentioned before.  27-29c is generally the maximum suggested depending on final floor finish from what I've read - https://www.warmup.co.uk/blog/guide-to-underfloor-heating-heat-output .  I too thought it was low but seems to be the configuration suggested and the last thing I want is to install it and then find out that it doesn't heat the house up well enough.   

The graph I sent previously only covers electricity consumption for the heat pump i,e HW and Central heating.  It's from the metering and monitoring installed.

If anyone has installed UFH with a heat pump I'd be curious to know how well it works, whether there are savings expected, what size piping was installed 

I've been following this thread and have noticed a drop in consumption since applying WC, but still getting spikes although reducing the length of dhw settings.

I've also just checked the parameters in the settings for the 2 HP, heat pump 1  was not doing anything at the time.

<HP1
CWRT 29
CWFT 29
FREQUENCY 0
DISCHARGE TEMP 19
POWER CONSUMPTION 100
FAN CONTROL ROTATION 100
DEFROST TEMP 8
OUTDOOR TEMP 8
WATER PUMP CNTRL RTATIN 2700
SUCTION TEMP 17

HP2
CWRT 43
CWFT 50
FREQUENCY 84
DISCHARGE TEMP 82
POWER CONSUMPTION 3000
FAN CONTROL ROTATION 730
DEFROST TEMP -1
OUTDOOR TEMP 6
WATER PUMP CNTRL RTATIN 2700
SUCTION TEMP -1

The flow regulators are reading 28l each.

I'll be adjusting the defrost setting on hp1 as looks to be set too high? 

@bretix have you managed to get good efficiency with2 ASHP?

Out of interest do you have a picture of the flow regulator and are you finding your setup to working efficiently

I've been checking my usage again given the recent colder weather and I thought the previous change to using WC resolved the cycling I was seeing previously which it did seem to fix.  However I can see with the -0c conditions the unit appears to be cycling every 30mins as seen on the graph.  Any ideas why this might be happening?  I think I remember someone mentioning defrost cycles might be a reason.  Any other thoughts and what could be done to stop this behaviour?  

My smart meter looked to be running a 5-6kwh when this pump was running during these cycles and even then the temperature in house would not go above 19c.  I used 80wh today and was wondering if anyone else is experiencing similar issues?

@soniks I feel your pain, I'm seeing consumption of that magnitude, quite often peaking at over 120kWh on really cold days. I don't have the same system as you (I have 2 x 12kW Samsung units) but I'm seeing very similar. In this weather my defrost cycles are kicking in every 45 mins or so. They're only brief, perhaps 5 mins, but their impact in reducing buffer tank temp and then a slow build up in ASHP outlet temp seems more marked. The main ASHP never gets near its set point for outlet temp and I suspect this is part of the reason. I've largely switched off the slave unit as it seems to make no perceptible difference to the house but just adds to my bills. One unit can just about keep the house at 18° in this weather.

Posted by: @soniks

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I think when it's on fixed set point the HP tries to get to temperature as fast as possible which is obviously not efficient. 

Definitely my experience when I had a brief test manually controlling the LWT I quickly went back to weather comp.  I think best to do at @TomC is, use weather comp and shift it up and down (this is possible on the Samsung Gen6, may not be possible on other heat pumps)

Posted by: @soniks

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If anyone has installed UFH with a heat pump I'd be curious to know how well it works, whether there are savings expected, what size piping was installed 

I am in the process of doing it.  Completed the bathroom (150mm spacing with 16mm pex al pex pipes) and the kitchen/dining (100mm spacing, topped with 100mm concrete that I will polish at some point same UFH pipes as bathroom).  I am going for all loops at max 50m downstairs, which together with the 100mm spacing enables low temperatures. 

It definitely works, but from a pure return on investment point of view, probably does not add up, you will need to do the maths for your specific situation.  I am focusing on efficiency (future run cost savings), excluding capital investment [ROI], hence the strategy.

Clearly the concrete has a massive environmental impact, but I hope to offset that with being able to use it as a thermal store, by moving heat from rooms with a very high solar gain to some of the north facing rooms with solid walls, as well as put some excess solar thermal into the concrete in spring and autumn (very carefully of course so as not to damage the heat pump - still working on how to do this reliably)

Given we experience radiant heat differently to warm air, I can say that the UFH rooms are much nicer to be in.

I put in 28mm pipe (this for a heat load of 16kW @-2) but for a different delta T and LWT to what I am planning now, with hindsight and knowledge, (so the pipes will be a bit small in my case for any load above 12kW) which hopefully won't be too many days in the year. My research suggests targeting a flow speed of around 0.9 m/s.  Mine will be higher than that for some of the time.  I have a lot of pipe between heat pump and first manifold.

@william1066 Thanks for the response. I think I can stomach the investment if it means the house will warm up to temperature ie 21c when it's cold.  What's annoying is my bills are high and the house still isn't warm enough.  So with your setup you were able to heat up to temperature with UFH when it was -5c?  Do you mind me asking what area you are heating and insulation levels

@william1066 remind me what LWT stands for?

Posted by: @soniks

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remind me what LWT stands for?

......

So with your setup you were able to heat up to temperature with UFH when it was -5c? 

"leaving water temperature" aka flow.

Regarding your second question

TLDR response: Where the heat load @ -2 degC outside is < 100W [concrete/tiles] per sqm of area no issues with maintaining temperature. I expect the rooms with wood that are < 70w per sqm will be the same.  When I locate the original heat load cal docs will send you the exact numbers.

I think more important than my specific house details (which is varied in insulation levels from no cavity 350mm solid walls, to a new extension with latest building regs specs is the heat loss. You need to decide how many days of the year you can cope with something lower than your normal comfort level.  -2 is the normal standard where I am which is around 3-4 days of the year where the heating can't maintain the required temp. 

If you don't already have one you need to get (or do it yourself - there are various spreadsheets and online apps to do it) a heat loss per room survey.  Then assume from a UFH emitter perspective max 100w out per sq/m for concrete floors, going down from there, say 70w per sqm meter for wood covered floors and continuing down for other coverings.  If you have enough "square meters" in your room for the heat loss then UFH will "work".  But that just means it will be able to emit enough heat.

Now you can start to "play with" the design.  In my case, efficiency was the main goal, so high flow rate, low delta T and low flow temp.  This means no low loss headers or any mixing of loops.  Definitely not sending a high flow temp, to then mix down.  I did get some stuff wrong, all part of the learning journey.

Because I have only done about 60sqm of UFH now (all working great) I have 100sqm to go, hard to know.  I am not too worried about the odd cold day, this is about the overall efficiency for all the months that the heating will be one, and so far when it is 5-10 degrees, even my existing radiators in the part not converted, are able to get the rooms to about 18 so I am confident the UFH will work without issue.  Except for one room which has a heat load at -2 of 160w per sqm - so it needs some insulation to get it to 100w per sqm or I need to have a heated ceiling and heated floor.

My UFH is 150mm spacing up stairs and 100mm spacing downstairs so I can run a reduced flow temp and delta T. 

I am not an expert by any means, this is all DIY - for the learning journey, and to get exactly what I want at reasonable cost, there are some really good experts out there to teach you the fundamentals, e.g heat geek, but also John Cantor and others.  And of course, this great site.

It is currently about 3degC my flow temp is 27.2 and delta T flow to return is 3 degC and the room looks like this right now.  I have probes in the floor, just not yet connected to home assistant yet, but will add those soon.  I am interested in the floor temp, vs room temp, vs flow temp, flow rate etc.  I do get cycling issues when I drop the flow temp (by dropping the weather comp curve) too quickly, need to model that better and automate.  A wip.

Hope that helps a bit, if not let me know.