Air Source Heat Pumps

richardc1983

This is bugging me now

Due to constant refrigerant circulation on units that are thermostat off (eev valve remains open 10% to allow oil return) this is still enough refrigerant to raise the coil temp above 40C so that the fan comes on to clear heat from the coil, this is raising the temp in rooms that dont need the heat, meaning the back room which is the coldest room of them all could do with that heat is taking longer to warm up!

In theory yes the outdoor unit is over capacity if all three units are running demanding heat but reality of it is that 2 units are pysically switched on, only one is demanding heat so it should have the full capacity of the outdoor unit to itself.

It doesnt seem to be getting this as the air off temp is not as warm as if only one unit was pysically on.

If one unit is pysically switched on then air off temp is about 42C if 2 units are pysically switched on but only one is thermo on then the air off temp on the one that is thermo on is about 32C.

I think those EEVs are staying open too far as the coils still get warm if they are pysically switched off but not as much as when the are thermo off and switched on.

Any ideas? I know that if all three are running thermo on I can expect slower warm up but not if only one is running and the others are at standby thermo off.

thechippy

Rich,

Has the refrigerant charge been checked properly, this could be an issue.

The eev's staying slightly open does not sound right to me either.

richardc1983

Its correct due to it being a multi split the eev's stay open 10% to allow oil return back to teh compressor.

It only does this in the heating mode.

If it wasnt like that you would end up with liquid or gas refrigerant going back to the compressor on units that were off.

thechippy

richardc1983 wrote: »

Its correct due to it being a multi split the eev's stay open 10% to allow oil return back to teh compressor.

It only does this in the heating mode.

If it wasnt like that you would end up with liquid or gas refrigerant going back to the compressor on units that were off.

Sorry mate, still sounds wrong to me. Where did that info come from??
If the eev's are closed, there can be no flow of liquid or vapour back to the unit.

I still think there is an issue with the refrigerant charge - it's critical on these systems. When it was originally installed, do you know if the engineer weighed in the correct additional charge over and above the base charge? and the appropriate charge on installing the additional ahu? It can't be guessed (which some bad engineers do) it has to be calculated according to the diameter of the liquid line and their respective lengths.

richardc1983

When it was originally installed the engineer did not have to add additional refrigerant and worked this out as per page 3

http://mylg.co.uk/data%20for%20site/Multi%20Splits%20commissioning.pdf

The pipe runs were not exceeding the pre charge ammount.

This time round when the new unit was installed he added 24 metres of pipe run to the system but no additonal refrigerant was added.

LG stated that the system was pre charged to 70metres, hence why he didnt add any.

This has turned out not to be the case, we have had to add in 640grams tonight, when the formula said we only needed 310grams because there wasnt enough in the system.

Anyway will see how it runs now.

Regards
RIchard.

thechippy

How was the 640 calculated when the system said 310?

richardc1983

Not sure how he calculated it but whats the consequences if its over charged now with the extra 300 or so?

For example if the system is charged with 450grams and your only using 250g of that, where is that other 250g stored?

Does the system store a certain ammount somewhere?

Before in cooling the outdoor unit fan used to run really slow but cooling was fine, now with the top up in cooling mode the outdoor unit fan is running fast now.

Is this correct?

thechippy

richardc1983 wrote: »

Not sure how he calculated it but whats the consequences if its over charged now with the extra 300 or so?

For example if the system is charged with 450grams and your only using 250g of that, where is that other 250g stored?

Does the system store a certain ammount somewhere?

Before in cooling the outdoor unit fan used to run really slow but cooling was fine, now with the top up in cooling mode the outdoor unit fan is running fast now.

Is this correct?

A "slight" overcharge is ok, the system can cope with that, what they don't work well with, is an undercharge.

If the cu fan is now running faster in cooling mode I would not worry, it's a result of an increase in discharge temp/press as the system now has more refrigerant.

What is your total combined pipe run measurement at the moment??

richardc1983

The fan in cooling though would run slow even if it was about 25C outside and then even slower in cold weather. Now its cold outside its running very fast? Would you say an extra 300g is a slight overcharge?

Total pipe run on the liquid line is 39metres.

The unit can have a total of 70 metres:

I have just posted on another forum this is what I have put:

Ok the indoors are

MV12AH x2
MS12AH N40 x1

Outdoor is FM25ah.

Installer has been out tonight, he has put a total of 640g in.

The total pipe run is 39metres and the outdoor unit should have had 450g of charge from the factory.

Dont ask me how he calculated it but he used scales and then added more in as he said it was still taking the refrigerant in.

Afterwards he tested the system, in cooling previously with only 2 indoor units running the outdoor fan would be spinning really slow, the cooling effect seemed fine indoors and we could always reach set points. This was with an outdoor ambient of perhaps 24C.

Now with an outdoor ambient of 9c the fan in cooling mode is rotating a lot faster? Maybe it was undercharged last time or the original installers did something?

Heating performance seems ok but will this additional ammount of approx 310g over the 330 that should have been added cause problems.

For example if the system is pre charged with 450grams and your only using 250g of that, where is that other 250g stored?

Given that the system is pre charged with 450g, this will be in the pipework somewhere presumably.

So if another 450g was put into the system and it was too much would it just store that internally in the outdoor unit?

thechippy

If the outdoor fan was running slow in warm weather, this would indicate an undercharge from day it was installed.
Your system has a base charge of 2150grm which will take into account a combined pipe run of 7.5m
You are 31.5m over that so...

Allow 20g per m = 630g. Take off correction factor of 450 = add 180g

Your total system charge as we speak should be 2330g

The only way to tell properly is to weigh out the entire charge into a clean new recovery cylinder. That way the 410a can be put back in with any additional charge required.

As for the extra refrigerant if overcharged, some systems have a reciever, so would be stored in there. If not, it will increase the liquid in the bottom of the condensor. Too much additional liquid in the bottom of the condensor and the efficiency will drop off.