Reality says otherwise though.
It doesn’t matter what they `calculate’
What counts is how much heat you actually need.
All you need to be able to convince them of is what it actually takes to heat your house and keep it at the design temperatures over a 24 hour period.
Then there can be no argument as real life facts trump paper guessimations.
when you have the survey ask them if they have taken in to account that the rooms next to other rooms are heated and are not at outside temps , and if your a semi or terrace ask them what temp they assume next doors internal temps are ,
i went through he same process and got my 9 kw lowered to a 6 kw i did try and get them to lower it to a 5 kw but they wouldent buy it ,
my heat loss this year has not been over 4 kw
https://emoncms.org/app/view?name=My%20HeatPump&readkey=1da9e37af3d5ac9669792233ea85675c
and now that the heat pump is more efficient than the 9 kw it proves i can go even smaller , but at least mine is now continually running now at ok cps /scop and usage
Thanks Chris.
We have already had the talk about next door as it is a semi. They assume the next door property is empty and not heated, at least that was what the engineer thought was the case, so the engineer while on the phone to the surveyor, suggested to the surveyor to try semi, terrace and detached to see if we can find something that explains the difference from survey(s) to reality.
They also discussed the composition of our original walls, which is solid stone with no cavity. Apparently, they have been caught out by Cotswold stone (limestone) where heat loss was more than expected.
Not entirely sure what stone we have, might be sandstone… Or what impact it would have. It could even be a form of limestone given our location
I do have a good idea of what the walls look like as the whole back wall of the house was knocked through for our new extension.
Though only the front at one side of the house has this stone now.
I’ve seen a research paper but can’t find it, maybe related to this Our energy efficiency research explained | spab.org.uk
Essentially a lot of solid walls outperform the rated values depending on composition.
Edit: more details Understanding the performance of solid walls - Designing Buildings
Thanks for this, very interesting links.
Interesting read, thanks.
Did you notice at what point in the hysteresis range this modulation begins?
Is “one step size” equivalent to an offset drop of 1c?
I think this is is it stepping down off the curve twice. With the Octopus engineer set Madoka modulation set to 10.
The first step it drops the target from the curve at 34.1 to 31.6C
The room continues to over heat so its drops another step size from 31.6 to 29.1C.
So the modulation seems to drop off the curve by a quarter of the modulation setting for each of the two steps.
I am assuming its now blows past the overshoot value and can then only cycle to control the room heat. I suspect if I lowered the overshoot I would see much more cycling.
and another example
The octopus engineer set my curve back to their default and upped the Madoka modulation to max (10). ironically with the modulation now at max its dropping the target temp to at or below what I had tweaked the curve down to.
So it seems with max Madoka modulation its dropping below the curve by a max of 5C over two steps. One good thing about setting the Madoka modulation to 10 is it becomes very clear its not happy with the curve.
Looking at it, I think the Madoka modulation steps are occurring at 0.5C over the room thermostat target for each step. So after the first step, if it’s still 0.5C over the room target some time after the first step then it applies the second step.
Am not 100% sure but it looks like this is what’s happening.
I have modulation switched off.
I do a form of forced cycling. I program the MMI target room temp to be 1 degree higher in the cosy off-peak time slots. The HP is then likely to be on at cosy times and, depending on the weather, less likely to be on at other times. The heat pump rarely has enough time to need to start cycling itself.
This works for me both comfort-wise and economically. The emonTH room temperature
has sat in a range of 1.6 degrees for the last week, which is perfectly acceptable to me.
I have the 11kW Daikin installed by Octopus in September, on a heat loss calculation of 9.5kW. Heat Geek recommeded a 7kW Altherma. My house is 1950s cavity wall with a newer extension. I’ve looked at the electrical consumption against flow temp and noticed the COP drops when the consumption drops below around 850W. My assumption is the compressor starts to run less efficiently. Here are a few data points:
I’ve set up the WD curve and max flow temperature to keep the electrical consumption above 900W. It’s still early days but I’m getting reasonable COPs and the cycling isn’t too bad.
Happy new year!
I have an oversized 16kW ERLA16DW17 bibloc unit. How oversized? Well, I do not know exactly:
In average, during this past month it generated a whooping 4.77kW…, if we assume continuous operation. The day of highest generation (coldest day), it has produced about 160kWh, or around 6.7kW over 24h.
The unit is rated for 16kW (35/7ºC), but only about 11.5kW around freezing.
The installers did not do any heatloss calculation that I could see, but these past days temperatures have been:
And if 6.7kW of heating plus hot water has been enough, then we are at about 200% oversize factor.
As you can see, reported energy figures deliver great COP, which is contrary to what you guys have observed in the metered Daikin units on the site. I can think of a few reasons, to explain this:
Some days I wish I had the OEM hardware!
I saw another thread somewhere which confirms this: once the room temperature falls below target by 0.5 degrees, it applies 1/4 of the modulation value (2.5 for you). Then if it falls by another 0.5 degrees, it applies another 1/4.
I use modulation of 4, so 1 degree change to LWT for each 0.5 degree error. For the most part, this then gives just enough negative feedback to keep the room at 0.5 degrees below the target temperature (19.5 degrees with a target of 20 - when it drops below, LWT gets increased, and when it reaches 19.5, LWT reduces again).
I’ve switched to RT mode and set it to 19c. Modulation 10. What I observed is:
Not sure if I’ll stick with RT mode in the long run or use LWT and HA to modulate the flow temp. There’s pros/cons to both.
Pro to RT mode
Cons to RT mode
Pros to LWT mode
Cons to LWT mode
Sampling seems to save about 50% energy by running 3 minutes on, 3 minutes off. A saving of about 50% or 50wh - probably not worth worrying about.
*in theory, if the WD curve is set right, for very little other heating (solar gain, cooking etc), then the modulation shouldn’t be needed anyway. If we’re going over setpoint by 1.5c, either the sun is out or my wife is baking and we probably want the heat pump off anyway.
I run fixed lwt all the time.
All of the time the heat pump is on the compressor is running as is the pump.
You can’t `sample’ in lwt mode?
If the compressor stops for any reason, such as return too high, it will just start again as soon as it can, within a few minutes, it is not waiting for anything and there’s nothing to sample.
What do you mean when you say the pump is running for three minutes on and three minutes off, what is happening here? The compressor isn’t running?
Yes, what I’ve observed in WD LWT mode is when the flow temp satisfies, the compressor stops and the pump stops. I assume the same would happen in fixed LWT mode as well. Then periodically, the pump restarts, samples the return temperature and decides whether to restart the compressor or not. It looks like sampling mode is the default when selecting LWT mode - maybe it isn’t when using fixed output, I’m not sure.
And in more detail:
What is that?
Just electricity consumption?
That is not how lwt works, whether fixed or wdc controlled.
The compressor and pump do not stop because the requested flow temperature has been reached, otherwise heating cycles would last only a matter of seconds with Daikin heat pumps!
My 8kW Daikin runs continuously only stopping to do DHW, defrost, or when I turn it off.
My 9kW Daikin was the same except these always stop after 6 hours of continuous running and restart immediately.
I wonder what told your heat pump to stop between 00.40 and 01.10?
This is lwt mode, it doesn’t stop just because I reached the set flow temperature
The flow temp exceeding the overshoot (4c) above target is what stops the compressor.
I believe my unit is nearly 3x oversized, so it regularly hits its target flow temp when its 5c or warmer outside. It’s been running more constantly recently during the colder weather we’ve been having (hovering around freezing). I’m gathering data to present to Octopus but it looks like even when we’re around 0c, the heat output required to keep the house at 20c is only 4kw but we have an 11kw unit. I’m waiting to see how the figures look after this week when we’re expecting sub zero temperatures.
The screenshots I showed you were that weird warm period we got yesterday during the early hours where the outside temp jumped from 4c to 13c in about 5 minutes.
OK.
I am familiar with the problem 
It still didn’t stop because the set flow temperature had been reached though.
It stopped because the return temperature got too hot.
The heat pump keeps increasing the flow temperature within the allowed overshoot as the return rises too much. Eventually it runs out of permitted overshoot and has to stop.
Nitpicking maybe, but understanding that it was a rising return temperature that stopped the heat pump may help you in the future
My mental model was that as the return temperature creeps up, it will raise the leaving temperature above the configured value, until it has reached the overshoot. Then as the return temperature continues to rise, it will eventually shut the compressor off. I think in this case the pump remains running until the water temperature has cooled to (configured LWT - delta-T), when it can restart the compressor.
Something like that.
The important part is that lwt mode will keep the heat pump running forever as long as the heat created isn’t too much for the emitters to lose. It doesn’t stop because the set flow temperature has been reached.
It’s actually shut down by 1c over the set flow temperature plus any overshoot.
So, @chrisg was correct, it is the maximum flow temperature being breached that actually causes the heat pump to stop although the cause of the breach is a rising return temperature as a result of emitters being overwhelmed by the heat produced.
