ASHP Modulation ratio

[JamesPa]

Triggered by the discussion of standby power consumption (an important spec not often quoted) - what about modulation ratio (ratio of max to min output) which I have never seen quoted.  They all tell you that they have inverter driven compressors, but the point of this is so that they can be turned up or down depending on the load so the real question after getting over the marketing hype is how low will they go?

 

Older or less sophisticated boilers have a modulation ratio of about 3:1 (or don't modulate at all).  Modern/good boilers have a modulation ratio of up to 10 to 1.  Delving into the ecodan specs (which, to be fair, are very comprehensive indeed) the min output at 15C from the 11.2kW monoblock is about 4kW, so an effective modulation ratio of 3:1 (the interesting comparison is max o/p when the outside temp is low to min o/p when the outside temp is just below the point where you switch the heating off altogether.).  I cant find any comparable figures for other makes so I have no idea whether the Ecodan is good or poor in this respect.

 

We know that the ideal system just ticks along supplying constantly the same heat as the building loses.  To work like this from an outdoor temp of (say) +16C to (say) -2C with a target room temp of 20C, requires a factor of 5.5 modulation ratio at least, probably a bit more if the unit is 'oversized'.  So a 3:1 modulation ratio is insufficient without forcing the unit into on/off mode.   Do any others do better?  Does anyone else publish the data to work it out?  10:1 would be a good target.

 

 

[SteamyTea]

8 minutes ago, JamesPa said:

To work like this from an outdoor temp of (say) +16C to (say) -2C with a target room temp of 20C, requires a factor of 5.5 modulation ratio at least,

Does that come out the same on the kelvin scale?

[JohnMo]

Isn't that one of the reasons for buffers, to make up for the short fall in modulation. 

 

Why would you need your heating on at 16 degs outside?

 

Also MCS certification manual say to the heat pump is to meet the duty of an outside temp for 99.6% of the year.  So the worst 5 days can be ignored for sizing.  Which leads to a smaller unit being specified.  And helps with modulation range etc.

 

[dpmiller]

Pretty sure most quote min and max outputs?

 

Ours is quoted at between 3.38 and 9.51kW depending on temperatures

[SteamyTea]

6 minutes ago, dpmiller said:

Pretty sure most quote min and max outputs?

 

Ours is quoted at between 3.38 and 9.51kW depending on temperatures

That can be a bit misleading though as they often only state at 3 temperature differences. i.e. 3.38 kW at OAT -7°C W at 35°C. 6kW at OAT 7°C W at 45°C, 9.51kW at OAT 12°C W at 35°C, or whatever.  Makes comparison hard.

 

What is really needed is three curves at least with the water output temperature fixed at maximum i.e. 65°C

Edited April 26, 2022 by SteamyTea

[dpmiller]

6 minutes ago, SteamyTea said:

That can be a bit misleading though as they often only state at 3 temperature differences. i.e. 3.38 kW at OAT -7°C W at 35°C. 6kW at OAT 7°C W at 45°C, 9.51kW at OAT 12°C W at 35°C, or whatever.  Makes comparison hard.

 

What is really needed is three curves at least with the water output temperature fixed at maximum i.e. 65°C

Absolutely. Same with COP too as the required temp combos may result in the inverter throttling-back as it strives to achieve the most efficient pressure ratios. Speaking of, did you ever look at that TUV document I sent you for the CoolEnergy unit? 

[JohnMo]

MCS certificate manual suggests

 

"inverter ‘turn-down’ available (typically 25%)."

 

So likely you will not find much better, than you have seen already.

Edited April 26, 2022 by JohnMo
Spelling

[SteamyTea]

5 minutes ago, dpmiller said:

did you ever look at that TUV document I sent you for the CoolEnergy unit

No, got the light bulb manual.  Seem to have missed the CoolEnergy one.

was it by PM?

[dpmiller]

yep, but I'll stick it on here for completeness as it's relevant to the thread

CE-iVT9 EN14825 64.181.20.04362.01 Rev.00 TR Performance Pages.pdf

[SteamyTea]

24 minutes ago, dpmiller said:

yep, but I'll stick it on here for completeness as it's relevant to the thread

CE-iVT9 EN14825 64.181.20.04362.01 Rev.00 TR Performance Pages.pdf 142.77 kB · 0 downloads

Going to take me more than 2 minutes to look at it.

Wish they would put it all in a spreadsheet.

[ReedRichards]

LG Therma V monobloc 12 kW model.  Minimum power it maintains for any length of time is about 2.5 kW (according readings provided by my solar panel inverter).     

[JamesPa]

Responses to various comments above

 

7 hours ago, SteamyTea said:

Does that come out the same on the kelvin scale?

Yes, heat loss is dependent on temp diff not absolute temp

 

6 hours ago, JohnMo said:

Isn't that one of the reasons for buffers, to make up for the short fall in modulation. 

 

Why would you need your heating on at 16 degs outside?

 

Also MCS certification manual say to the heat pump is to meet the duty of an outside temp for 99.6% of the year.  So the worst 5 days can be ignored for sizing.  Which leads to a smaller unit being specified.  And helps with modulation range etc.

 

Yes its true that buffers are a kludge largely to make up for short fall in modulation albeit all they can do is extend the on/off times not change the on/off ratio.  But buffer tanks are lossy, take up lots of space and cost money thereby contributing to reasons not to have ASHP.  Most on here are probably in an 'early adopter' mindset, but if ASHP is to solve the massive retrofit problem we have then they need to be easy to use!  I am personally an early adopter but really don't want a buffer tank if I can possibly avoid it precisely for the reasons above and because it is a kludge.

 

Heating at 16C outside - maybe not, but a degree or so lower and definitely less.  16C is often suggested to be the 'switch off' temp for heating.

 

Its sounding from the above like 3:1/4:1 might be the norm, which is better than nothing, but not sufficient by some margin to work across the temp range without on/off modulation in addition.  I wonder if any are radically different or whether it depends eg on compressor type.

[dpmiller]

I think it'll depend more on the gas than the compressor, tbh. You need to make enough pressure to liquefy the gas so it can then be expanded again.  The compressor will only be able to do this above a certain speed, and then the expansion valve can only throttle the flow a certain amount too.

[JohnMo]

2x small heat pumps would give you turndown of 6 to 8:1.

 

Or if you have UFH, batch charge the floor, basically put X kW into floor over a period of hours and it will slowly heat the house for the rest of the day.

 

Or thermal store. Use an oversized PHE for DHW generation with a  2 degree approach temp.  So you are getting 46 deg DHW from a tank at 48.  Up the temps if you need. Take heating supply from thermal store.

[SteamyTea]

1 hour ago, JohnMo said:

2x small heat pumps would give you turndown of 6 to 8:1.

I have often thought HPs could have 2 or 3 compressors in them.

Compressors are not expensive. 

[JamesPa]

There is an interesting article here about compressor modulation technologies https://climate.emerson.com/documents/understanding-compressor-modulation-en-us-3844210.pdf  The best technology - 'variable speed' - which is more or less what it says on the tin - is claimed to be capable of a 7:1 turndown ratio ie probably good enough.  The article speaks quite a lot about the challenges this presents with oil management.  Another (which involves PWM of a solenoid valve) is simpler, but achieves only a 3:1 turndown ratio - definitely not good enough.  Both are described in terms of scroll compressors.  Multiple compressors is indeed a third option discussed.  I don't know which technolog(ies) are used in domestic ashps, 

 

Obviously having 2 small heatpumps to increase the turndown ratio is indeed an option, and also gives an element of system redundancy, but yet again its making the system significantly more complex and expensive to circumvent a technological/design limitation.  One off custom engineered solutions, or solutions with large buffer tanks, are fine for early adopters and enthusiasts like me and others on this forum, but for your average person in a 3 bed, moderately insulated, semi who is replacing a gas combi which delivers instant heating and hot water whilst taking up less than the space of a kitchen cabinet, they are, for the most part, a non starter. 

 

Unless gas prices increase significantly compared to electricity prices, so that a much lower CoP (say 2) than those of us on this forum aspire to still delivers savings or at least parity, ASHP will remain a niche market.  I'm getting concerned for the planet!  We need to build a shedload more wind farms to drive down electricity prices fast!

[HughF]

My ex-neighbour is the uk agent for m-tec (formerly Pico) heat pumps (Austrian), I’m looking into their specs to see if they have something suitable for our retrofit (yes, but eye watering prices). Their 12kW model can modulate down to 2kW which I thought was a pretty impressive range. Not sure if that is when it starts to cycle or not, I don’t have the details for that.

 

I’ll try and upload spec sheets and find a tuv certificate.

[ReedRichards]

10 minutes ago, HughF said:

... m-tec (formerly Pico) heat pumps .... Their 12kW model can modulate down to 2kW which I thought was a pretty impressive range.

 

So similar to my LG Therma V (which I mentioned earlier).  This has two fans, maybe it can behave as @JohnMo's 2 small heat pumps?  

[Marvin]

It maybe that for a typical residential property an ASHP needs to be able to run more efficiently without a buffer tank and the way forward may be a double compressor. Certinally my ASHP would have room in its current volume as the gubbins only takes up about half the height of the area to the side of the fan.

 

The government report (ask @SteamyTea) identified the problem for a typical home of finding room for a buffer tank.

 

But really,  far far lower energy usage results are achieved through AIM: Airtightness, Insulation and some form of MVHR, before going on to APE:

ASHP, PE and EV.

 

The only thing we're waiting for is a cost effective way to store electricity and then we won't care about the nuances of an ASHP COP. 

[SteamyTea]

2 minutes ago, Marvin said:

The government report (ask @SteamyTea) identified the problem for a typical home of finding room for a buffer tank.

There seems to be a reluctance to fit cylinders and buffers outside the house (in a suitable enclosure).

Flats are a different matter.  But then the energy usage is generally much lower, so A2AHPs, combined with larger MVHR could be the answer.

But it does seem odd that many people think that loosing 1m² of floor area to a cylinder is worth more than the same area as 'storage'.  The tat I keep in my house is just clutter, a heat pump may save me £300/year.

 

Considering how long HPs have been used globally, the reason they do not modulate much is probably because it is just not worth it.

The UK likes to think it has a unique climate, but it is not so different from many other coastal areas around the globe.

I was not quite old enough to remember the nuances of the transition from Town Gas to Natural Gas (though I remember the man coming around to change the jets in the cooker and boiler), but I bet many people thought it was not as good as after.

Probably the same when coal took over from timber.

[HughF]

2 hours ago, ReedRichards said:

 

So similar to my LG Therma V (which I mentioned earlier).  This has two fans, maybe it can behave as @JohnMo's 2 small heat pumps?  

That's very good for such a modestly priced unit. I'm impressed by the offering from LG to be honest - how do you find the controller? From YouTube research it seems to be the nicest/easiest to use.

[JamesPa]

For big houses with plenty of outside space losing space to buffer tank and dhw cylinder isn't much of an issue.  But most of the population don't live in such luxury, and we are building ever smaller boxes at a rate of knots.  

[SteamyTea]

2 minutes ago, JamesPa said:

For big houses with plenty of outside space losing space to buffer tank and dhw cylinder isn't much of an issue.  But most of the population don't live in such luxury, and we are building ever smaller boxes at a rate of knots.  

I agree about smaller newbuilds. But for retrofitting, which is about a 30 million homes, compared to 200k new builds a year, there is usually room.

The nation has to start to understand that there will not be the option of a small box, hanging on the kitchen wall, supplying all the house heating.

Those days are vanishing fast. So no point throwing the baby our with the bathwater, most existing houses could have an ASHP.

[JamesPa]

I really hope you are right.  But it is going to take some significant compulsion and/or incentives to convince people given the practical challenges, neither of which are vote winners.  Let's all hope our politicians have the staying power and conviction to lose votes by introducing unpopular policies for the long term benefit, or alternatively that technology improves to the point where the choice is easier.

[markc]

Just now, JamesPa said:

I really hope you are right.  But it is going to take some significant compulsion and/or incentives to convince people given the practical challenges, neither of which are vote winners.  Let's all hope our politicians have the staying power and conviction to lose votes by introducing unpopular policies for the long term benefit, or alternatively that technology improves to the point where the choice is easier.

A politician doing what’s right for the majority …. Good luck finding one of those