Differences between modern AC and air source heat pump compressors

[Jon_Doe]

Hi all.

I'm wondering, what are the major differences between an AC compressor and an air source heat pump compressor.
Wen i say AC, i mean small, domestic, split units, not the large industrial ones.

There are a few differences i noticed from studying datasheets but couldn't find more in depth data.
Let's say that both compressor are scroll inverter, to reduce number of variables.

1. AC usually sits in the lower part of the power spectrum while heat pumps in the upper part
2. Newest inverter AC units use R32 and vapor injection compressors (to help when outside air is way below freezing) while air source heat pumps seem to be stuck with the deprecated R410a (with or without vapor injection)
3. AC usually is designed for cooling only (no reversing valve) but there are bidirectional units on the market. Even with the A+++ energy rating, cooling mode has more plusses than heating due to less temperature delta. Even so, there are reversible AC units with A+++ efficiency for both cooling and heating even if most units on the market are A+++/A+ or A+++/A++ for cooling/heating.

If i were to consider the same external circuitry in both cases (TXV, reversing valve, moisture filter, oil system, condenser, evaporator, etc), having normalized both heat pump and AC to the same power, would they perform the same or different (available power and COP at the same inside/outside temperatures?

 

[HVAC-Novice]

There is no inherent difference. AC is a heatpump.
Manufacturer just selects based on the size and operating conditions.

what is called a heapump often has a reversing valve to both heat and cool. but principally, a heat pump also could heat only.

In heating application, you have to design the unit to deal with other temperatures than a typical AC application. Like an AC evaporator doesn't need to operate in below freezing temperatures, a heat pump does.

In theory, the compressor could be the very same one.

 

[Alistair_Heaton]

the is it an inverter compressor system or old school AC stop start is the biggest choice these days in europe with both.

And A2A heatpumps are R32 inverter where I am mostly these days in Europe. A2W is more of the old R410A. It gets down to -20 here in winter but most installers only fit -15 rated systems which causes issues.

There was something I saw about patents and the reason why some areas prefer R410A over R32 still.

 

[dbill74]

ACs and Heat Pumps both use the vapor compression refrigeration cycle to move heat from one space to another with direction of flow being the main difference. ACs move heat from the inside of a building to the outside (thus cooling the inside), whereas HPs move heat from outside to inside a space.

Selection of AC vs HP comes down more to geographic location and cooling/heating load requirements. Where I'm at in south east USA, heat pumps are very popular and are very efficient. But on those really cold nights in mid winter, the heat pump function is often supplemented with electric heat strips. A little further south and the heat strips are not needed. Go north a ways and it is more efficient not use a heat pump and go full on furnace.

For reasons I cannot fully explain except to say it has to do with the direction of heat flow, the efficiencies of heat pumps and ACs are different. ACs use a SEER or EER rating whereas heat pumps use Coefficient of performance (COP).

Electrically speaking, the energy draw will be similar in heating or cooling mode; that is until the electric heat strip kicks in.

 

[HVAC-Novice]

Efficiency depends on the temperatures on both sides (evaporator/condenser) and will change daily/seasonally.

Heating will be inherently more efficient IF the temps on both sides are the same since you can use the compressor heat (=electricity you put in) whereas this is wasted in cooling.

COP is unitless, EER is a mixture of Btu/h cooling capacity and W (really stupid to use IP units, even more stupid to mix IP and metric units, but this is the US....). So it is unitless but based on a metric and an IP power unit, you can convert EER to COP.
SEER takes varying EER over the seasons (S is for seasonal) and can't be directly compared to COP or EER.

 

[Jon_Doe]

Thank for all the feedback.

Cooling is inherently more efficient than heating since the summertime cooling Delta T's are much smaller than wintertime heating delta T's.
By design, i would only use the unit with outside air above freezing (A2W when outside is above freezing or geothermal which is always above freezing when outside starts freezing).
So my COP should be somewhat higher just by doing this.

R410a is predominant over here for A2W HP but 2023 is last year for it as it's being phased out.
R32 is predominant in AC which is always bidirectional (both heating and cooling).

I would only consider inverter driven compressors which leaves Vapor Injection as the only major difference.
Vapor Injection (VI) or Enhanced Vapor INjection (EVI) - the same thing - allows the unit to work in heating mode down to -20C ... -25C outside air.
It also help a little bit when cooling with very high outside temps.

Efficiency is improved by 10%-15% but is more expensive due to adding additional HX and TXV.
My hunch is that VI units cost about twice the non-VI units but i don't have official feedback on this - people who know this info are surprisingly few.

May or may not be worth the cost difference, it all depends on what the ambient temps and delta T's are expected.

 

[HVAC-Novice]

It should be easy to find out the cost difference by just getting a quote for each option. Some residential units are even sold on websites where you just see the price without getting a specific quote. So I don't understand why you say there are only few people who know the cost.

I think you are overthinking this. Just use the manufacturer efficiency data and as long as you compare the correct data, you know how they compare efficiency-wise. Two similarly efficient units will use similar technology. A lot of device design is balancing cost with size of HX and sophistication of compressor and controls. It isn't a big secret how to make refrigeration more efficient, it is all about building it to a price point.

From my (limited) experience I always found Mitsubishi has more efficient options besides their less efficient ones. Other manufacturers could easily build equally efficient units, they just choose not to (market not there etc.). The only real decision is what refrigerant to bet on that will be available in 20 years.

 

[Jon_Doe]

Thanks.

I think i was a little unclear.
I know the costs, i don't know the exact technologies inside each unit.
For example. I found 2 Mitsubishi units with what seems to be like identical specs but one is twice the price.
Now, i don't know if it's because of bling functions like wifi and noise etc. but it's twice the price.
There's not much data in the datasheet except the A++ / A+++ ratings.

MSZ-AP35VGK / MUZ-AP35VG is twice as much as MSZ-HR35VF / MUZ-HR35VF
Specifications are not in the same format so comparing is difficult.

Apparently they're almost the same.

 

[HVAC-Novice]

I'm too lazy to find each respective spec sheet. If you post them, we can have a look. But I bet there is a difference and if you contact Mitsubishi rep, they can tell you. first few applications I had to talk to them to narrow it down and now I can select my won and just check in with them every once a while to see what is new.

I don't want to speak for Mitsubishi, or show a favor, but Mitsu publishes tons of very detailed data and they have an (initially) overwhelming number of models. This is really good, but initially you may need help from them to narrow down the choices. They also usually show EER/COP etc. So you should get objective numbers to compare. note that some numbers only are on the condenser or the evaporator spec sheet. So for a split system, you need to look at all spec sheets. They also have units that look very similar that are for residential, commercial and true 24/7 operation. Plus all control options. Some also have options for very low ambient operation (which a server room may need). I bet there is a good reason if one unit is twice the price.

Don't ask some random stranger son the internet about Mitsubishi products. Ask Mitsubishi.

 

[Jon_Doe]

MSZ-HR (cheap unit)
2 datasheets with different kind of info:

https://aerconditionatmitsubishi.ro/index.php?controller=attachment&id_attachment=334

https://www.google.com/url?sa=t&rct...ad_full/4097&usg=AOvVaw0VU6Bt5ojefKm4iQc3p532

MSZ-HP (expensive unit)

https://aerconditionatmitsubishi.ro/index.php?controller=attachment&id_attachment=360

I need to find locally a knowledgeable Mitsubishi person as most online distributors / retail stores don't know a lot.

 

[HVAC-Novice]

The more expensive has higher COP/SCOP. they also have different capacities. So that is one reason for price differences. The one has WiFi standard, the other has it optional. there likely are more differences and there also may be a difference for what duty they are built. The one says it is multi-split, which may be some sort of VRF option, the other doesn't. You need to look at all documents for each series to understand the difference.

I don't know how Mitsu UK distribution works. But they likely have an actual rep or a UK branch office that can give technical advice and sell to you. And they likely have just warehouses and home improvement stores that sell them and those won't have much knowledge.

 

[Alistair_Heaton]

multi splits are different beasts loads more valves and control electronics and quite often come with two or more indoor units.

Its why I ended up doing a twin system. It worked out 2/3 the price of a multi split with an extra 2.5kw of output.