JamesPa

Its a good article thanks. However the key, is surely that, if you don't require mixing between flow and return (to balance flow rates or for some other concrete reason), then make it impossible by using a 2 port tank, thus eliminating the mixing loss. If you do require mixing then you will have to suffer the inevitable (so far as I can see) reduction in system efficiency. It is (unless I have missed something) just lazy design to put in a 4 port tank when no mixing is required.

I see lots of pictures of buffer tanks in unheated plant rooms, so not so unrealistic. My preferred plumber wants to put mine in the garage, ie almost at outdoor temp. He wont be allowed to because he wont be allowed to install a buffer tank at all, but realistically there would be nowhere else to put one if I were to cave in. Others will of course diffe,r but the scenario is not unrealistic. Without some sort of feedback loop from the demand, how can the house side pump not run all, or at least most, of the time? The only way the system can determine if more heat needs to be supplied to the house is either to sample the return temperature or get feedback from a in-room thermocouple/thermostat. The latter is built into the HP control panel and may well turn the pump on or off in the experiment (I don't think we actually know), but of course this measures the temp only in one room. This all depends on the exact config (Grant, for example, who do more or less everything else wrong from a purely thermodynamic point of view, have a sort of intelligent 'sampling' mode whereby the pump is switched off when there is no demand but switches on periodically to check the return current temp, potentially quite a good strategy). A classic mechanical bi-metalic thermostat, which many houses will have, surely has (at least) a 2C hysteresis. I agree its too much to be comfortable, but again not a totally unrealistic scenario. There really are so many variants that its impossible to cover all in an experiment. I think a certain amount of informed interpolation, combined with a bit of theory (ideally confirmed by experiment) is needed to work out where any given actual scenario lies.

See my comment above. In summary, even if the buffer is lossless, there is still an explanation for poorer efficiency.

So far as I understand it, a 4 port buffer tank exists only because there is some mixing envisaged between flow and return. If no mixing is required you wouldn't use a 4 port tank, you would use either one or two 2 port tanks. As soon as you mix flow and return you will decrease the efficiency (even if there is zero loss) because the HP will need to deliver a higher flow temperature to get any given temp at the emitters.

I think the challenge here is that there are lots of possible scenarios and the author has chosen 3 fairly common ones, rather than necessarily trying to identify the contribution of each individual change (if indeed they are strictly separable, which I doubt). I'm not actually clear that scenario 2 does not also employ load compensation.

Hence Vb = (Pmin-Dmin)*Tmin/(Hmax*4.2)-Vs (or zero if this yields a negative number) ! where Vs is the system volume when anything that can shut off (eg due to zone or thermostatic valves) has done so (and everything else is as described at the start of this thread). RobLee is (I think) saying that Tmin can arguably be set to zero with modern inverter HPs. The formula doesn't consider the energy reserve needed for defrost however.

Basically a thermal store to load shift into cheap rate periods... Lets do some crude math. 1cum (1000l) of water heated by say 20C would store 4.2*1000*20kJ = 23kWh, a decent amount of energy at least in the shoulder season, or for a well insulated house, so in principle yes. You would have to think about the flow temp/capacity of the HP and how the low temperature water (which will cool as the day goes on) gets distributed, also I suspect it would be best to plumb it specifically for a storage function not as a 'buffer tank. It would be a lot cheaper than 23kWh of battery storage, albeit that it wouldn't store excess PV in the summer. Now where is that IBC container I had kicking around?

I probably wouldn't by choice, but the panels I have are 12 years old and no longer available

Thats not how I read it Scenario 1 uses a buffer tank and no Weather Compensation and a third party thermostat which, in fairness, appears to be a fairly common setup (why it would be, other than ignorance, completely escapes me) Scenario 2 uses a buffer tank with Weather Compensation and no third party thermostat, another common setup Scenario 3 uses no buffer tank and does use Weather Compensation, also a common setup It could be argued that there are at least two more scenarios (and several others as well). Scenario 1a with a buffer tank, no Weather Compensation and a third party thermostat to control the room temperature but using the HP controller to maintain the buffer tank temperature Scenario 1b with a buffer tank, no Weather Compensation and the HP controller maintaining the room temp The three chosen are not unreasonable examples of what is done, based on what we hear on this forum. I think the key is Scenario 3 vs Scenario 2. I presume that the lower flow temp (and thus higher COP) in scenario 3 is because there are no buffer tank losses. It is inconceivable that there is not some loss in a 4 port buffer tank due to conduction and possibly also due mixing if the flow rates don't precisely balance, and loss equals lower efficiency unless there is a compensating factor. To be honest I have never understood the argument for a (4 port) buffer tank in a sufficiently simple system. I can see the argument in a system where you need to have different flow rates/temperatures in different zones (eg when you have both radiators and UFH), and I can also see the argument for a simple volumiser (ie 2 port tank plumbed in the return) where the system volume is otherwise insufficient, but in a simple system with sufficient system volume what does a buffer tank do - other than take up space and loose heat?

Interesting article here about why buffer tanks and hydronic separation etc are (according to the argument) usually a bad thing. https://renewableheatinghub.co.uk/how-to-correctly-install-heat-pumps-so-that-they-work-properly-and-efficiently Basically the author is advocating a direct connection from heat pump to distribution system, applying occams razor, and claims to have evidence (which he cites) that this is best for efficiency. I have to say that, intuitively, Im not surprised provided that the system volume is sufficiently large and the modulation depth of the pump sufficiently deep to avoid short cycling.

True I suppose, but in fairness power = W and the formula assumes kW. Should stick to SI consistently I suppose.

In this formula Tmin is expressed in seconds not minutes. Sorry for any confusion!

Tmin needs to be expressed in seconds not minutes. Sorry I didn't make that clear in the original post, its kind of obvious if you have a physics background, but not otherwise.

I don't think there are actually any regulations relating to ASHPs either, merely planning constraints and the general requirement not to cause a statutory nuisance. The problem is that the installation requires planning consent (unless it fits the MCS rules and is installed by an MCS registered installer to MCS standards) and my LPA planners currently (I think) view this as in the same category as air-conditioning, and so impose noise constraints as part of a planning consent. An external oil boiler would, I think, also require planning consent but not a flue or vent pipe nor does a small outbuilding. Thus an oil boiler is an outbuilding, even if it has a noisy vent, will not require planning consent (but could still constitute a statutory nuisance). One totally in the open probably would though!

Well quite! I think this is a weakness of the MCS assessment procedure, but having said that its still miles better IMHO for this application than something based on BS 4142:2014+A1:2019. I'm all in favour of nuance and flexibility when its proportionate to the circumstance, but when the 'development ' amounts to replacing a domestic central heating boiler with a piece of equipment worth £3K in order to do the same job, a full noise assessment according to a 76 page document is totally disproportionate. The exception would be in very noisy neighbourhoods, where the background drowns out the ASHP at any likely separation. I guess that would suggest a slight modification to MCS020 to take this possible circumstance into account and then it could simply be adopted as a standard condition. Based on a simple methodology like this it could then be made a requirement that manufacturers publish and certify the range of acceptable distance/barrier combinations for their unit so only manufacturers have to do the calculations. If we could get that agreed across a sufficiently large market (say, for example, the EU) then manufacturers would even design to/certify against it, it in the same way as they design to/certify against eg electrical and other specs. If ASHP is to become the norm, as it currently it must, then it has to be as easy as gas in all respects.

I will need to read up both on Tigo and on my current inverter! The latter supports two strings, but has two inputs for each string, im not sure how these relate (probably just wired directly together internally). The current array is anyway split 1.5kW/2.5kW so I think its might well work simply to add to the new panels (with optimisers) to the shorter string. First step it to check that the SunnyBoy is happy if the maximum available power exceeds the spec, even though the voltage and current do not. I'm struggling to imagine it would do anything other than operate slightly sub optimally under these conditions to throttle back, but Id better check before getting too excited.

The plan with the roof mount is to take the weight on the walls, thus circumventing the question and avoiding any possibility that the roof deck vibrates and acts as a megaphone. The unit will sit on a unistruct framework which will transfer the weight to the flank wall of the house at one end, and the outside wall of the garage (to which this is the flat roof) at the other. The feet at the garage end will sit on a part of the deck supported directly by a joist, to avoid deck sag; at the house end the plan is to attach the unistruct directly to the house wall with anchor bolts. Given the span (2.6m) it needs the back to back (P1001) channel to take the weight without so much deflection that it feels uncomfortable, but actually its well within the maximum permissible load. If the Unistruct framework itself starts vibrating, a pair of mid-span feet will be added and jacked up just enough to transfer a little weight to a part of the deck directly above a rafter, in order to damp it down. A sideways extension of the same framework will serve as the mount for two solar panels to be added at around the same time (see separate thread in the PV Forum), the ashp provding the weight to hold them down. Unfortunately the location of the solar panels (determined by the position of the sun) provides only minimal sound screening. It took me a while to work the arrangement out, and of course may run into some practical problems when its implemented, but that's the plan. Really neat would be if an acoustic screen also provided the personal protection against roof falls when maintaining the unit. The location is just about right and it could be mounted on the 'end' of the unistruct framework. It would look a bit odd though (but perhaps the fact it also acts as personal protection would help 'resolve' it visually) The MCS calculation says that, if the assessment point is invisible from the unit (and 250mm either side of the unit) then it gives a 10dB attenuation. I find this implausibly high unless its very solid, but presumably the MCS figures do have some basis in fact. Having said that you implied a post or two back that a window ajar is worth 10dB-15dB, so perhaps 10dB for a visual barrier is not so unreasonable.

Im using both inputs so will still need some form of optimiser I suspect. I crunched the numbers using the 2005 PVGIS data (which is the download it gives by default) capping the o/p at 3.84kWh in each hourly time period. The calculated aggregate 'sacrifice' because of the output power cap is just 47kWh! I dont know whether 2005 is an average year or not, but 47kWh is small enough to be negligible. There are some periods where the peak input exceeds 4.2kW but neither the voltage nor the current will be outside the limits, so I wonder if the inverter will care? Im guessing perhaps not, it may just operate slightly sub optimum. I wonder how to find out - perhaps ask SMA..

That's rather a useful idea. I have an SMA SunnyBoy 4000 with a total of 4kWp panels, not all at optimum angle so in practice I suspect the peak input is a bit less (I could check the logs I guess). I think that in principle I could add 4*400W without breaching the input limits. I imagine that I would need to put at least some of the panels on individual optimisers as they would be much newer and thus a different spec (and in some cases a different orientation) to existing. Tigo seem to do optimisers which work with any inverter and (according to Midusmmer wholesale) aren't needed on every panel, only the 'out of spec' (ie new) ones. This is beginning to get interesting; the saving on inverters might pay for an MCS installer so I can still get the export payments on the whole combined array, as well as claiming the FIT payments on the original using the proportionating approach that is in the updated FIT document you referred to earlier. There would, I suspect, be a period during the summer where the inverter (which is limited to 3.84kW o/p) clamps the output, but probably not for much of the day or for many months. It needs a bit of back of envelope work to see if this limitation matters much. I feel a spreadsheet and some downloads from PVGIS coming on.

Interesting thanks. I think I will first try to persuade the LPA to change the condition to the one in MCS020 (which, IMHO, is the only defensible one (other than in very loud neighbourhoods) given that it is the one applied to permitted development. I doubt they will agree, if only because it means admitting they were wrong. MCS020 is achievable, I designed the installation to meet this and submitted the calculations with my planning application, but it appears that they were ignored. If, as I fear, they do not agree, I remain minded to appeal the condition, and request as part of the appeal that it be modified to specify the MCS technical condition (but not that the condition include the requirement for an MCS installer). Appeals are FoC, its just the time factor which puts me off. The NPPF says 'planning conditions should not be used to restrict national permitted development rights unless there is clear justification to do so.' So clearly what the LPA are doing is against the spirit of the NPPF if not the actual letter. In parallel with the appeal I'm tempted to do broadly as you suggest. Depending on how hard the LPA appear to be fighting, I will take a view on the risk. Or I might just decide 'sod it' and have the louder and more ugly unit (of the two I was considering) installed by an MCS registered contractor if I can get a reasonable quote. The problem with a backup attenuation plan is that it involves a large box around the unit which, because its sited on a flat roof, will look rather odd. Its also not clear how much reduction is achievable. Daikin, for example, do a low sound cover with 3dB reduction (see link below) which, depending on how much the background noise assessment can be manipulated, may (or may not) be enough. I'm not ruling it out, but it would in principle need separate planning consent (in practice I wouldn't bother, its removable after all), there don't seem to be too many pre-fabricated examples and its not entirely predictable! Incidentally the MCS calculation says that a visual barrier reduces the level by 10dB. I wonder if that's true in reality. All in all a bit of a mess, I was hoping that, given the lack of objection to the application from the neighbours (who have their own ASHP for a swimming pool) and the largely positive comments from the LPAs Environmental health officer, this would have been straightforward. Its a real disincentive to being green (as if there weren't enough disincentives already).

Because my actual measured export is pretty much spot on 50% (no teenage children so sane hot water usage!) and Octopus pays much more on measured export than on deemed export.

Pretty much all done already with the exception of triple glazing (but I have progressively upgraded the glass only in my double glazing as panels have failed, wow does it make a difference!) Working also on a heat pump to replace aging boiler, if my LPA will allow it (see separate thread about the crazy noise constraint they have applied) Over the year I currently use the same as I produce, however of course the generation and the use times of day don't coincide so, yes, I am reliant still on grid import. With a HP the balance will of course change in favour of import. The two initiatives are sort of linked, together they reduce my scope 1 and 2 emissions from my house (using the government reporting methodology) by 78%. All my calculations to date conclude that batteries don't work financially, unless electricity prices go up again significantly and stay up (which I don't rule out). If the alternative is grid export then they make absolutely no environmental sense at all because they consume scarce resources for no environmental gain.

Only, I think, because of regulation. If they can be installed for a reasonable price and I can get paid for the extra export then I think it benefits both me and the planet. The exported energy will get used elsewhere down the road, with efficiency, presumably, very close to that of self use, because not many of my neighbours have solar panels. Unless of course you are arguing that local solar is not good for the planet even if you self-use the energy harvested.

I haven't yet bought the micro inverters, my existing inverter is a string inverter. The export limit was suggested as a way to circumvent the requirement that, to get paid for export, you need an MCS install. So by setting the new inverters to zero export Im not compromising the payments on the existing install We haven't got teenage children so use only a sane amount of DHW! On summer days the DHW is fully heated by 11am or even earlier and I export for the rest of the day unless we are doing something 'power'. The panels are facing 25 degrees west of South to, so the rest of the day is actually most of it.

I've often wondered why they bother building this 'assumption' into the calculation. They could more simply say that the calculated sound pressure from the heat pump at the assessment point should not exceed 37dbA, which would lead to the same pass/fail outcome in all cases so far as I can see. As you say the background noise level may in practice be lower than 40dBA, but there is a level below which the background noise is irrelevant for this application, because its too low to be heard indoors (where it matters). Below this level only the sound from the heat pump matters because that's all you are going to hear (or not). MCS seems to take this into account rather well (much better than a background noise- relative condition in fact). So far as I can see MCS only fails (to deliver a sensible conclusion) where the background noise is so high as to drown out a heat pump. In my house (and therefore presumably also my neighbours house) I can barely hear the background noise if I stand 1m from an open window. So its totally irrelevant in the practical case of being indoors when its bitterly cold, windows are shut and the ASHP is working at maximum. What matters is the absolute value of the noise from the new source, which is what MCS effectively relies on.