Look like I have a weekend sat in the loft again. the wife will be chuffed. Anyhow, installer coming left week. Might need to leave this for them to all run through and sort.
If you have a mitsubishi preplumbed cylinder then its unlikely to be a low loss header, although that's what they call it, it has no volume its just a distribution pipe so separates the primary and secondary flows but its just a pipe.
Look like I have a weekend sat in the loft again. the wife will be chuffed. Anyhow, installer coming left week. Might need to leave this for them to all run through and sort.
Well in fairness it's his job! No need to answer the questions btw, you could as you say just leave it to the installer.
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
@alsocold123456 Only the primary circuit runs during the defrost cycle so shouldn't be taking heat out of the water sitting in the secondary heating circuit. I thought there was a minimum volume required for the primary circuit but I can't see any reference to that, only a minimum volume for the heating circuit by which I presume they mean the combined primary and secondary. Are you running in WC mode?
I would be careful with that presumption. Minimum volume is at least in significant part about defrost, and what matters is the minimum volume available to the heat pump (the fundamental is that the water needs to contain enough energy to melt the ice!). If both primary and secondary water pumps run during defrost, that is indeed the sum of the primary and secondary volume, but if the secondary water pump switches off then only the primary volume plus the volume of the LLH is available to the heat pump for defrost which may not be enough.
For clarity: The only reference to system volume I can find in the documentation I have relating to my EcoDan refers to the space heating circuit. There are other references to the primary circuit but not in this instance.
Of course in a single loop they are effectively the same, but I shouldn’t have been so definitive to say that the secondary heating circuit isn’t used as though my EcoDan seems to only use the primary circuit volume to defrost (as the power draw remains the same as when it is running in anti-freeze mode) if that were to not provide enough heat to complete the defrost cycle then the secondary pump would have to come on to make the entire space heating volume available – hence the volume in that circuit being specified in the documentation.
I doubt that using the whole heating circuit would get to the point of actually cooling the house as surely the FTC would define the defrost cycle as failed before that point.
no buffer, no volumiser, piped 35mm Low loss header with one feed off to 2 UFH manifolds, and 1 rad run. Total system volume I have worked out as about 150-175 litres.
I've attached a fairly typical graph for fully open system at temps close to zero. I need something more consistently closer to 40 degrees.
In this case I would say the secondary pump is running during defrost and using the UFH circuit volume as the return temperature does not dip that much below the flow temperature, suggesting the cooled flow is going through the slab and being warmed up before passing over the return thermistor. I posted an example of my defrost cycles on p1 of this thread, in which it uses just the primary circuit to complete defrost and the return temperature tracks the flow temperature right down very quickly. You can also see that post-defrost the heat output spikes to its maximum output, which is higher than the nominal output of the 8.5kW unit, to bring the flow temp back up to the required level before backing off. In the case above the flow temp returns quickly so the unit is at maximum output as soon as defrost is over.
I see that on this, and on Gary’s example, the flow/return dT is around 8° which is about as much difference as the HP can add. The compressor working at maximum is why you are getting such frequent defrosts - paradoxically asking it to lower the flow temp might allow it to run for longer and produce more heat overall and drag you up by your bootstraps. Is the pump set to max speed in the settings? Increasing the flow rate is the way to narrow the dT (possibly) or transfer more heat (possibly).
Mitsubishi EcoDan 8.5 kW ASHP - radiators on a single loop 210l Mitsubishi solar tank Solar thermal 3.94kW of PV
I've had the pump speed on max and it's seems a bit worse, can't be sure yet, as the compressor ramps up even harder and defrosts even more. i've tried increasing the flow round the secondary circuits, and this does narrow DT, but it drags the LWT down not the return temp up. the only way i can get LWT to raise is to restrict flow on secondary circuits as i think it goes lower than the primary flow and more water returns straight back to pump.
i think there a few minor adjustments i can make to this now by twiddling the odd nob here and there and might have to leave it to installer to overlook everything.
in still can't get past the fact it might be a bit underpowered. i've looked on my heat pump monitor and i can't seen another house that's 263m2 that's a retro fit, not new, that has and heat pump my size. the tend to be all 14-16 Kw units.
i know the hest loss wins, but just an observation. let's see.
in still can't get past the fact it might be a bit underpowered. i've looked on my heat pump monitor and i can't seen another house that's 263m2 that's a retro fit, not new, that has and heat pump my size. the tend to be all 14-16 Kw units.
Perhaps. My 200 sw m 1930s retrofit with fabric upgrades of various quality at various times has a measured loss (from gas consumption) of 7-7.5kW. Early indications from the ASHP are that it might be at the bottom end or a bit less, perhaps 6.5-7kW. Scaling that its a bit less than yours.
Re "For clarity: The only reference to system volume I can find in the documentation I have relating to my EcoDan refers to the space heating circuit. There are other references to the primary circuit but not in this instance."
I get that but bear in mind that Mitsubishi are Japanese so you cant expect every nuance to be perfectly conveyed (Im not sure you could even if they were British). The fundamental is that there needs to be sufficient system volume available to the heat pump for defrost, wherever it comes from.
I doubt that using the whole heating circuit would get to the point of actually cooling the house as surely the FTC would define the defrost cycle as failed before that point.
I tend to agree, but nevertheless the FT cools so that might, over a period of time (several cycles), cool the house (thats certainly what I see with mine if I have it in noise reduction active).
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
Hi, just picking this up again. My installer has been out and the flows are going through the UFH slab and rad circuit on defrost when they are not meant to on the Ecodan. So that's getting put right and various other things. So hopefully will be able to get my LWT a bit higher when its colder.
I have reduced the temp I'm calling for now to 32 degrees LWT when its cold outside. This reduces the defrost to only one an hour now and keep the compressor frequency to 80hz or so, which actually heats the house a bit better than asking for 40 degrees that maxes the compressor out that gave 2 defrosts an hour, which was cooling the slab for 6 minutes when it did so that it stuggled to catch up with prior to the next defrost.
I have never checked but I assumed all my secondary pumps are running during a defrost as there is only 10M of 28mm pipe in the primary not sure what volume that holds but a defrost lasts 5 mins.
I do feel the rads cool during a defrost but not long enough to drop the house temp
This post was modified 1 month ago 2 times by Gary
i'm not sure what it will do, but Mitshubishi engineer said the 11.2 ecodan needs to have the secondary flows closed during defrost. I was / am getting stuck in a loop of maxed out compressor defrosting -- which cools the screed a bit, then back to maxed out compressor which then heats the screed that was just cooled by the defrost, then a few minutes more to try to get some room temp, then back to defrost ....... repeat. I still think at the end of the day, a 14kw machine would be better for my house, but happy to be proved wrong
That's exactly what mine does only with radiators and not underfloor. I measured it recently during defrost cycles, every radiator in the house went cold to the touch and then took 10-15 mins to reheat. When it's around freezing outside we can get up to 2 defrost an hour so heat at target temp 42C ish at radiators for only around 15 mins at a time twice an hour. Thus house cools down. I'm thinking to bite the bullet and try installing a 100L+ buffer tank but not sure where it would be best placed in the flow circuits. The heat pump circuit on my system is very short (low volume) as outside unit is immediately on other side of the wall to the plant room. Will explore whether I can turn off radiator circuit pump during defrosted, that might help keep the water in the radiator pipework warm during the defrost cycle.
This post was modified 1 month ago 2 times by AlsoCold123456
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