Heatpump output vs efficiency dilemma

[SteamyTea]

10 minutes ago, Beelbeebub said:

Does that make sense?

Does to me.

[Archer]

43 minutes ago, SteamyTea said:

Worth checking how the HP modulates, some just open or close the expansion valve, that just reduces output without saving any input.

How would you check this in advance, I thought it was all about inverter power? Is it stated on the technical spec somewhere normally?

 

18 minutes ago, Beelbeebub said:

The one thing I would caution about modulation range, which I noticed from A2W units and may (or may not) apply to A2A.... 

 

The modulation range is generally quoted as the max and min outputs at a given temp. 

 

For the ducted A2A units that we are looking at (Mitsubishi FDUM series) this doesn't appear to be the case. The min/max output is listed together as 1.1kw / 7kw with no temperature range. Interesting that there's so much variation between the type of unit though. 

[Beelbeebub]

4 minutes ago, Archer said:

How would you check this in advance, I thought it was all about inverter power? Is it stated on the technical spec somewhere normally?

 

 

For the ducted A2A units that we are looking at (Mitsubishi FDUM series) this doesn't appear to be the case. The min/max output is listed together as 1.1kw / 7kw with no temperature range. Interesting that there's so much variation between the type of unit though. 

The only way would be to get the data table showing output at min/nom/max for each outside/inside temp. 

 

The min/max output must be related to temperatures even if they don't mention it in the brochure. For example is that 7kw at 15C or - 15C? Given manufacturers always like to put a good spin on specs I'd bet it was the maximum in close to ideal conditions. 

 

Similarly of I wanted to show the lowest output I could (to make the modulations seem better) I would absolutely quite the output in fairly cold conditions. 

[Beelbeebub]

I don't have the figures on my phone, but I'll look up what the "Advertising Executive" min/max figures are for the heatpump i have data for and then compare it with the max @-5 and min @12C figures which I would argue are the more representative figures. 

 

[TonyT]

2 hours ago, Archer said:

I love the fan strapped to the ceiling, that's absolutely brilliant!!

 

Thanks @DanDee for the detailed post as well, that's really helpful. I think from what you've said, and other comments above, oversizing shouldn't be a huge issue hopefully on our system (because of the high modulation range) but could be on others. Seems like there is a key distinction between A2A systems (which generally have a higher ratio) and A2W; even more so with non-inverter systems. In both cases it may be preferable to undersize if anything and rely on a backup heat source for extreme conditions.

 

For some A2A systems that modulate right down it's worth considering that larger output units may be more efficient than smaller, "correctly sized" ones in some circumstances. The Mitsubishi range for example have a number of units in the same series with the same minimum output but increasing max output's. The larger units have significantly higher COP values. I guess this is very situation specific though. 

 

In our system design we will have two wall thermostats, one for each floor and they'll be located away from ducts to hopefully avoid the distortion effect that you referenced. Because we're re-using the 1970's gas warm air ducts on the ground floor it's a single return duct in the hallway. Upstairs they'll be a supply and return duct for each room. 

 

The other thing I was wrestling with was optimizing the ceiling ducts for both heating and cooling which seem to be at odd's with each other. As I understand it, heated air needs to be forced directly down to reduce stratification and a warm layer at ceiling height (ie. what your ceiling fan does)... In cooling mode you want the opposite - a horizontal spread so the cool air naturally sinks and mixes. 

 

Found these Halton diffusers with a wax actuator that automatically changes the air throw and direction based on flow temperature. Seems like a neat solution, will post back once we've installed to say if they've worked out. 

 

 

So it’s a destrat fan??

[DanDee]

11 hours ago, SteamyTea said:

Worth checking how the HP modulates, some just open or close the expansion valve, that just reduces output without saving any input.

 

Any details/data/document on this?

[DanDee]

11 hours ago, Beelbeebub said:

The one thing I would caution about modulation range, which I noticed from A2W units and may (or may not) apply to A2A.... 

 

The modulation range is generally quoted as the max and min outputs at a given temp. 

 

So it may be a max of 12kw at - 5C and 3kw min at - 5C

 

So 4:1

 

Except you don't use the minimum modulation at - 5C. Thatvs when you need max power! 

 

You need min modulation when it's warmer, say 10C or 15C.

 

Thr minimum a HP (at least the ones I looked at) can modulate down to at the higher temps is higher than at the lower temps - which is obvious really. 

 

But that means the minimum you HP might be able to turn down to, when you actually need it to turn down, is higher than you might think. Say 5kw 

 

When I looked at the "max at minimum vs minimum at maximum" values I was seeing a modulation ratio closer to 3:1

 

Does that make sense? 

 

In all data sheets the rated output be that min/normal/max is always stated at the same 7outdoor/20indoor for A2A and 7out/35water or 55water. With other temp ratings specifically pointed out.

 

I don't know where your -5 comes from.

 

The lowest minimum output at lower temp also does not necessarily holds true, due to the safety minimum modulation of the compressor at freezing temps which is higher than at  +7...+12.

But spinning the compressor at the same speed will obviously put out more heat with every degree of outdoor increase.

[DanDee]

 

[SteamyTea]

11 hours ago, Archer said:

How would you check this in advance

 

48 minutes ago, DanDee said:

Any details/data/document on this?

@markocosic mentioned about it on here somewhere.

[DanDee]

30 minutes ago, SteamyTea said:

 

@markocosic mentioned about it on here somewhere.

To a refrigeration engineer that sounds like saying that a car in order to keep the same speed(or reduce it's input) down hill, instead of reducing the gasoline feed, it just goes into neutral or lower gear.

[SteamyTea]

13 minutes ago, DanDee said:

To a refrigeration engineer that sounds like saying that a car in order to keep the same speed(or reduce it's input) down hill, instead of reducing the gasoline feed, it just goes into neutral or lower gear.

Or putting the brake on.

 

Not quite the same, more like pulling a plug lead while keeping the throttle position the same (PI (expletive deleted)s up the analogies a bit, but you get the idea).

[DanDee]

13 hours ago, SteamyTea said:

Worth checking how the HP modulates, some just open or close the expansion valve, that just reduces output without saving any input.

My question was rhetorical,

 

Why say something so skewed just because someone else said it?

 

No fact check?

[SteamyTea]

19 minutes ago, DanDee said:

Why say something so skewed just because someone else said it

Because I respect what he says.

If we all marked each others homework, not much else would get done.

[Beelbeebub]

2 hours ago, DanDee said:

 

In all data sheets the rated output be that min/normal/max is always stated at the same 7outdoor/20indoor for A2A and 7out/35water or 55water. With other temp ratings specifically pointed out.

 

I don't know where your -5 comes from.

 

The lowest minimum output at lower temp also does not necessarily holds true, due to the safety minimum modulation of the compressor at freezing temps which is higher than at  +7...+12.

But spinning the compressor at the same speed will obviously put out more heat with every degree of outdoor increase.

I picked -5C as the coldest you are likely to see in the UK therefore also the highest load you are likely to see.

 

15C was picked (again slightly arbitrarily) as the temp at which most houses don't need much extra heating to stay warm, some will be lower, some higher.  But this is the point at which you want you HP to be running at it's lowest.

 

So from the table you posted the 7C range (red) would be 2.6-11.2kw or 1:4.3

 

But in reality you are more interested in the max power at -5C (or maybe -2C or whatever you pick as the coldest day) which is 8kw (green)

 

And your minimum requirement will be at around 15C which is 3.4kw

 

So your practical modulation is more like 1:2.3

 

 

I did a rough graph....

 

 

The kink in min from 2 to 5C is interesting, my guess is the % modulation isn't true compressor speed, but % of available range, ie the compressor speed at 10% modulation isn't the same at every temperature.

 

For reference this is the graph I got for an A2W (nominal 9kw) HP, which is worse than the other graph

 

[Beelbeebub]

The important bit is (i guess) what the performance of the HP is cycling around the +10C range.

 

If it can cycle with no major efficiency hit then no problem.  The COP should be pretty high then anyway.

 

But you do spend a awful lot of time in that region in the UK, alot more than the -5 point.

 

So I guess that's where the undersize argument comes from, gaining more efficiency in the right hand side of the graph at the expense of a few days of poor efficiency on the left.

[SteamyTea]

4 minutes ago, Beelbeebub said:

But you do spend a awful lot of time in that region in the UK, alot more than the -5 point.

[Beelbeebub]

where were they for, the top graph seems to be from somewhere pretty chilly!

 

but in both cases time below -5 is much less than time above 10C

[SteamyTea]

Just now, Beelbeebub said:

where were they for, the top graph seems to be from somewhere pretty chilly!

Met Office Central England Temperatures.

Top chart is min temp, bottom max temp.

Here is the mean temps (well actually the mean of the daily median temps)

[ProDave]

24 minutes ago, Beelbeebub said:

I picked -5C as the coldest you are likely to see in the UK therefore also the highest load you are likely to see.

Please sir, can I mention we often see -10 for a week or more in winter.

 

Granted I am not your average UK location.

[Beelbeebub]

7 minutes ago, ProDave said:

Please sir, can I mention we often see -10 for a week or more in winter.

 

Granted I am not your average UK location.

Crikey! 

 

In which case you would need to look at the max output at - 10C 😁 and possibly some extra woolly jumpers!

[ProDave]

43 minutes ago, Beelbeebub said:

Crikey! 

 

In which case you would need to look at the max output at - 10C 😁 and possibly some extra woolly jumpers!

It has not let us down yet and we have a WBS as backup

[Archer]

4 hours ago, DanDee said:

 

In all data sheets the rated output be that min/normal/max is always stated at the same 7outdoor/20indoor for A2A 

That was my understanding of it as well

[markocosic]

4 hours ago, Beelbeebub said:

But in reality you are more interested in the max power at -5C (or maybe -2C or whatever you pick as the coldest day) which is 8kw (green)

 

And your minimum requirement will be at around 15C which is 3.4kw

 

Not so.

 

If the heating goes off at 15C (because solar gain and internal gains equal the losses) then your number is 0 at 15C and 8kW at design condition (excluding internal gains)

 

This extends the modulation range required and means it's more likely that you spend longer operating at/near the minimum outputs.

[Beelbeebub]

1 minute ago, markocosic said:

 

Not so.

 

If the heating goes off at 15C (because solar gain and internal gains equal the losses) then your number is 0 at 15C and 8kW at design condition (excluding internal gains)

 

This extends the modulation range required and means it's more likely that you spend longer operating at/near the minimum outputs.

Sorry, I meant the minimum heat requirement for the house (aka 0kw heat demand) will be around 15C and at that point the minimum the HL can output is around 3.4kw so the HP will have to be cycling. 

 

I don't know how much of an issue that will be from an efficiency, longevity and comfort point of view.  If there are huge issues there, then oversizing might not be bad idea. 

[markocosic]

IME pretty uncomfortable to have 3 kW zero kW 3 kW Xero kW blowing at you.