Posted by: @tobyg

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@allyfish and @iancalderbank, thanks again. I'm now concentrating on trying to do a heat calculation; I've tried again with heatpunk and am finding it a nightmare to navigate. I'll DM @derek-m and see if he can suggest anything more user friendly.

To save you the trouble of sending a DM, I don't know why Ian made that statement.

I have never actually carried out a heat loss calculation, though I have seen a number of spreadsheet versions on the forum, I can't remember exactly where they are located.

I do believe that MCS have one on their website.

ah, sorry, just seen this. I've messaged you but never mind.

ah, sorry @derek-m, just seen this. I've messaged you but never mind.

@allyfish, @iancalderbank,and @editor, I'm trying to take your advice and do a heat loss calculation. Can any of you point me to something more user friendly than Heatpunk please?

I've just remember that I did actually use this site a year ago, even before we applied to ECO4, is this the sort of thing? https://www.bestheating.com/btu-calculator?gclid=CjwKCAiAleOeBhBdEiwAfgmXf0xgUN1ee-hAQHlFlJBi8aa42DWe0ZrQTriXDiij9r5rH6K-EfOJXxoCpqgQAvD_BwE&gclsrc=aw.ds

Thanks.

Posted by: @allyfish

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Posted by: @iancalderbank

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just trying to assist the OP: are you saying that at -3C, its effectively a 7.5kw machine? what's the target WT?

Yes, at -3degC target LWT should be 45degC, but tends to never get above 40degC, leaving a shortfall of heating duty and requiring supplementary secondary heating to achieve desired indoor set point. Part of this is due to the sizing of my ASHP, 10kW, where it should really be a 13kW. But the larger part is due to the frequency of defrosts, which Grant units have a propensity for - sometimes up to every 45 minutes - twice as often as other OEMs, and the amount of energy that removes from the hydronic circuit each time vs the time the ASHP requires to replace that lost energy.

Interesting the Samsung 12kW being a de-rated 16kW, thanks. 4kW minimum turn down is quite chunky,  and the unit will cycle frequently especially if the actual heat loss in the OPs property is towards their estimate. The grant will chug along at 2-2.5kW minimum turn down, and 600W-700W power consumed, which is very affordable in the milder heating seasons.

With my refrigeration engineer's hat on, the way A2W ASHPs defrost isn't clever. It's a by-product of most domestic heat pumps being designed as reversible refrigeration units to provide hydronic heating and cooling function, but under UK MCS only used for heating. Losing heat energy from the hydronic circuit to use for defrost isn't ideal. It's contrary to current thinking on the most efficient way to design hydronic circuits using minimum system components, getting rid of buffer tanks, headers, etc and minimising the hydronic volume. The minimum system volume is usually determined by the defrost function, a poor approach. Passive or hot gas defrost methods could be used, or stored energy from condenser superheat. There's other ways to defrost an evaporator without removing energy from the hydronic circuit.

 

 An interesting thread. I have a Grant 13kW and am trying to get a better understanding of what it should be able to deliver in different conditions. The specification tells me it can deliver 13.6kW at 7deg outside temp and with water temp at 35deg. But what should it be able to do at say 0deg or -3deg air temps?  Does this sort of data exist anywhere?

I’d suggest using the freedom toolkit for doing your heat loss, it’s a relatively robust excel workbook put together by GH, quite a while ago now.

I used it for my system design, and ran the same data through heat engineer to double check. They were within 100w of each other.

https://github.com/HughF/HeatPumpCalculator

Thank you very much again for the comments above. There’s some of this that is stretching my understanding and again, if you’ll bear with me, I’ll replay what I’m taking from the last set of posts and ask that you correct anything wrong - thanks. Apologies if I repeat myself a bit here – just trying to get it straight in my head.

I am still trying to find my way with a heat loss calculator (thanks @hughf), and intimated by the MCS one. Just in case anyone here is able to give some guidance or answer questions at some point tomorrow that would be fantastic (I make no assumptions!).

Thanks @iancalderbank and @allyfish for your explanations about flow temperature. I assume the target flow temperature something one can adjust post-installation?

With radiators that have been sized for 50c, if I operate a lower flow temperature of say 40c, then space heating will obviously not achieve design temperatures; would that be just as true for the 12kW Samsung as the 10kW Grant?

@allyfish, you say: “There isn't a set back function on the Grant controller, you can't knock the LWT down 2 or 3 degrees overnight, which is a shame. If you could I would do that. If you turn a master room control thermostat down you start cycling the ASHP on and off, and that's not good or efficient control.”

I don’t know the difference between a set back function and a master thermostat – this seems to say to me that with the Grant (but not the Samsung, which does have a set back function) you can’t adjust the HP power, rather it’s a binary matter of turning it on, or off; otherwise it will cycle.

We’re saying that in very cold weather (i.e. design conditions, sub-zero) the Grant is unlikely to get close to supplying the radiators at the target flow temperature (the set point); this is even more the case if the HP isn’t being run 24 hours a day, and is getting going from a ‘cold start’. @allyfish, when you say: “When my CH system first starts in the morning … the leaving water temperature is a slow rise up to set point”, do you just mean when it’s very cold or generally?

I get the impression that most of the time, in milder weather, with the Grant as well as the Samsung you’d expect to turn it off for a few hours overnight, compared with which in very cold spells there is some extra help to be had from running the Grant 24 hour a day, as you suggest @peterrsaltings.

I hadn’t appreciated the difference between efficiency and cost. The thrust of what your main point, @peterrsaltings, is that with the 12 kW Samsung, if it’s oversized, there are ways to finesse it depending on how much you have it running, and when, and your tariff. This is a different take on it, because I thought the point with it not being able to modulate down to the lower input that’s required most of the time meant that when it’s on it’s chewing through more power than you need, and it’s costing more (your model almost sounds half-way to a conventional gas heating approach!).

Posted by: @tobyg

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“There isn't a set back function on the Grant controller, you can't knock the LWT down 2 or 3 degrees overnight, which is a shame. If you could I would do that. If you turn a master room control thermostat down you start cycling the ASHP on and off, and that's not good or efficient control.”

The Grant does in fact have a couple of potentially interesting features which could offer set back and boost functionality:

Night Mode - which will reduce the compressor frequency (essentially dial down the power a bit) to reduce noise and power consumption.

Low Tariff Mode - which will increase the target temperature by a set amount over whatever the current value is.

Both of these modes are configurable (you can set the max compressor frequency for night mode and the target temperature adjustment amounts) but unfortunately these modes can only be enabled by closing an electrical contact which therefore needs to be physically wired up to some sort of time-switch or smart switch.  So you either need to get the installer to do it or you need to open up the wiring panel on the heat pump and start tinkering around in there yourself.

I don't have these connected up on my system so I've no idea how effective or useful they might be in practice. 

I could potentially see some benefit though - I have the Octopus half hourly tariff which has expensive electricity between 4pm and 7pm every day - that's when I would trigger the night mode - at present (if it's not too cold) I just programme lower thermostat temperatures for that period which tends to switch off the heat pump completely.  It might be better to keep the heat pump running at a lower compressor frequency rather than have it switch off and then on again - but who knows.  

Posted by: @tobyg

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I assume the target flow temperature something one can adjust post-installation?

you can of course adjust the flow temperature that it runs at post install. But, if you adjust it to lower than what your radiators have been sized for, you'll be cold. but its also possible that if the heat loss has been over-estimated, then so has the design flow temperature.

Posted by: @tobyg

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With radiators that have been sized for 50c, if I operate a lower flow temperature of say 40c, then space heating will obviously not achieve design temperatures; would that be just as true for the 12kW Samsung as the 10kW Grant?

yes. if the radiators are sized for 50C flow at -3C outside to achieve 21C inside, then if you run them at 40C flow they will achieve a lower internal temperature, regardless of HP size. same applies if you try to run them at 50C but because of very frequent defrosts and the temp going up and down, the actual average temp is only about 40C.

Posted by: @tobyg

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@allyfish, you say: “There isn't a set back function on the Grant controller, you can't knock the LWT down 2 or 3 degrees overnight, which is a shame. If you could I would do that. If you turn a master room control thermostat down you start cycling the ASHP on and off, and that's not good or efficient control.”

I don’t know the difference between a set back function and a master thermostat – this seems to say to me that with the Grant (but not the Samsung, which does have a set back function) you can’t adjust the HP power, rather it’s a binary matter of turning it on, or off; otherwise it will cycle.

there's no set back function for the LWT on samsung. you can set a timer to change the target room temp but not the target water temp. So this is basically the same as reducing the temp on a master thermostat. the target water temp is entirely driven by the weather compensation settings. Note changing the target room temp has no effect whatsoever on the WT of the HP on a samsung, meaning the heat output does not increase so it doesn't actually "do anything" to try to achieve that higher setpoint, other than keep running . Some brands DO have this functionality, its sometimes called autoadaptation or room compensation.

Posted by: @tobyg

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We’re saying that in very cold weather (i.e. design conditions, sub-zero) the Grant is unlikely to get close to supplying the radiators at the target flow temperature (the set point); this is even more the case if the HP isn’t being run 24 hours a day, and is getting going from a ‘cold start’. @allyfish, when you say: “When my CH system first starts in the morning … the leaving water temperature is a slow rise up to set point”, do you just mean when it’s very cold or generally?

sounds like allyfish has a slightly undersized unit so it may not be the best example to look at for warm up time.  it is best however to not get your HP to have to do a large cold start in the morning - if doing a setback, only let the temp drop by 1C or so overnight, and set it back up to comfort temperature well before you get up. this is especially the case if your HP doesn't have a means of increasing its WT in response to the house being "in need of higher heat power".

Posted by: @tobyg

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I get the impression that most of the time, in milder weather, with the Grant as well as the Samsung you’d expect to turn it off for a few hours overnight, compared with which in very cold spells there is some extra help to be had from running the Grant 24 hour a day, as you suggest @peterrsaltings.

on pretty much any system it will run on cycles in milder weather and more or less constantly in cold. Some people set their units to an overnight off, some do the opposite as electricity is cheaper. the key issue is that in mild weather you need to avoid short (5-10min) duration cycles as this is really bad for efficiency (and if persistent, bad for the unit itself). short cycling is usually because the water heats up too fast (not the house) so the heat pump shuts off. there is still a heat demand so once the water's cooled a bit, the heat pump runs again. keys to avoiding short cycling are not being vastly oversized, and having enough radiator area.

long cycling is fine (see below)

Posted by: @tobyg

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I hadn’t appreciated the difference between efficiency and cost. The thrust of what your main point, @peterrsaltings, is that with the 12 kW Samsung, if it’s oversized, there are ways to finesse it depending on how much you have it running, and when, and your tariff. This is a different take on it, because I thought the point with it not being able to modulate down to the lower input that’s required most of the time meant that when it’s on it’s chewing through more power than you need, and it’s costing more (your model almost sounds half-way to a conventional gas heating approach!).

if your house needs 2kw, and your HP modulates to 2kw, you can leave it running constantly and it'll almost certainly be very efficient. if the HP low modulation point is 4kw, then your house overheats by 2kw. how fast that makes your house reach the point that the depends on the maths for your house, key point being how much temperature swing you tolerate. the wider the swing, the longer you can make the cycle. samsung has a swing of 1C (you can't change it). a typical "smart boiler" thermostat might have a swing of 0.2C.  for example today, 9C outside, on a 1C internal swing, my HP (samsung, 4kw low point) has been in a 4 hrs on 2 hrs off cycle . which is fairly efficient. If had been able to modulate down to 2.6kw, it'd have run constantly and been more efficient. If using a 0.2C swing stat with a gas boiler it would have cycled a lot more quickly but if this approach used with a HP it  would have been a lot less efficient, not recommended at all.

Posted by: @tobyg

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I don’t know the difference between a set back function and a master thermostat – this seems to say to me that with the Grant (but not the Samsung, which does have a set back function) you can’t adjust the HP power, rather it’s a binary matter of turning it on, or off; otherwise it will cycle.

Just to clarify @allyfish comments to avoid confusion (I hope) - they were talking about the need for an automatic setback function at nighttime which would reduce the WT calculation e.g. during the daytime if outside temp is 10, set WT to 40, but during nighttime/setback, if outside temp is 10, set WT to a lower 35. This would have the effect of lowering moment energy consumption during setback, which should mean less cycling.

The Grant unit (I have a 10kW Grant) modulates energy consumption down very well, as low as 600 or 700 Watts, and it will happily run at a 'steady state' for many hours. I rarely see my Grant "turn off" because it can't reduce its energy into the house sufficiently. Any cycling comes from when the room thermostat is interfering.

Hi,
@iancalderbank has covered all of the discussion points accurately. It seems Ian’s system is set up for best efficiency, mine for lowest cost. My Samsung 12KW yesterday was only on 4am for a few hours, then again at 1pm for a few hours, and then again at 7pm for a few hours, with flow temps around 38C. Generated 45Kw (spot on to compensate for heat losses at 10-12C), used 11KW, COP 4.0. So it cost about £2.25 to heat the house yesterday.
So, yes, it is long cycling, but should be okay. There was no ‘short’ cycling of the heat pump whilst it was on. I use the Samsung controller Timers to set the house target  temperature to align with the variable tariffs, between 18.5C set back after 7am and 10pm) and 20C when I want the house heated. 

As you say Toby, there is quite a difference between your estimate from gas usage, and the estimates provided (including all your improvements?)

I had two companies carry out an MCS heat loss assessment, and I paid for one of those. Both said 7.5KW per hour across various room temperatures and -2C outside.

Both were wrong, by 50%! That seems a common theme.

One supplier suggested a 10KW (single fan) Midea, the other the 12KW Samsung (as the 8KW Samsung was too small and nothing in between ).

They assessed the existing radiators based on a 50C flow temp at -2C ambient were adequate and this definitely worked, just not the most efficient setup! I resized some of the rads and adjusted the weather compensation to get 45C flow at -2C, improving the COP significantly. What I then found was that there were a few rooms that did not quite get up to the target temp and the only way to fix this is to change the radiator, or live with it.

Temperature compensation takes care of the increasing heat losses as the ambient temps outside falls, and yes, this can be tweaked post installation. If you don’t have underfloor heating (which is a match made in heaven), then it’s really important to get the size of radiators in each room large enough and balanced, so that with a single flow temperature, all rooms get to and are maintained at the desired temperature. But in reality this is really tricky and you have to accept a “range” that is comfortable.

Below is an interesting article ( yes, I read articles from other forums) suggesting the MCS method may overestimate heat losses.

https://forum.ovoenergy.com/my-smart-home-138/correctly-sizing-air-source-heat-pumps-ashp-bigger-is-not-always-better-14428

I used ThermoPro wireless thermometers, combined with actual heat pump energy generation numbers (the Samsung controller provides input and output power data on a daily/weekly/monthly and annual basis), to record starting and finishing temperatures over a given 24 hour period and I turned on the heat pump in the evening to get the house back to the starting temperature. I also used data from a local weather station to monitor the outdoor temperatures on an hourly basis. I did this over several days with different ambients 10C, 0C and when it got to -5C overnight. See the graph in the Ovo link who did a similar thing with a lot more data points.

The measured HTC (heat transfer coefficient) was 220W per degree C per hour. This results in a Heat Loss of 4.84KW at -2C. That is nowhere near the rated output of the 12KW Samsung, and it can barely modulate down to this level.

But, to minimise cost, and running the heat pump on the Octopus Energy Cosy off peak tariff for 6 hours a day, and topping up a few more hours mid morning and in the evening, if required, the heat pump needs to deliver nearly 10KW over 12 hours to keep the house warm each day at -2C average outside.

The alternative would be to use a 5KW heat pump running 24 hours a day with no margin to keep the house warm below -2C. That would be more efficient and use less energy, but probably cost more to run on my variable tariff. And the house would get cold if the average temps dropped below -2C.

Additionally, the 12KW Samsung is rated at around  9.5 KW at ambient -7C when the flow temperatures will be close to 50C. The house then needs 6KWHr, or 140KW in total, so if such conditions occurred, it would have to run for 15 hours of that day. Still very capable. The lowest overnight ambient temperature last winter at my property was -10C and the fence opposite the heat pump got to -20C!

As I said, the drawback is that on days like today, when the ambient is around 10-12C, day and night, the heat pump was only on for 3 or 4 hours and most of that time was modulating down to 1KW input (the lowest it can go), with 4-5KW being generated. The compressor is a bit noisy when doing so.

Having said all that, it’s a fantastic product, reliable and so efficient. A 3 bedroom, 3 reception detached property with only electricity, costing less than £1200 per annum for heating and hot water. What’s not to like.

Good luck with the Heat loss calculations and below is a handy link to a Stelrad conversion table to work out sizing factors to correct for lower flow temperatures.

https://www.stelrad.eu/stelrad_eng/wp-content/uploads/Temperature-correction-table.pdf

PS The other nice advantage of a slightly oversized heat pump is that it can heat the DHW tank more quickly….and then catch up on heating the house whilst it’s being doing that. I have heard of people with heat pumps matched to the heat losses that it can take ages to heat the hot water and then the house gets cold. But again, only a problem in really cold weather.