Oversizing Split A/C Inverters

[back2space]

Theres an argument i'm having at the moment that you can oversize split systems as long as they are inverter controlled. For example a 3.5kw system. Manufacturers quote the capacity of 3.5kw based on the unit being on high fan speed (which can be noisy) you may run the system in low fan speed most of the time so this may actually be providing the required "sizing" of cooling perhaps 2.1kw or less. With the unit ramping up and down as required.

On days where it gets extra hot you bump up the fan speed and the extra capacity then comes in handy!

Theres a post on here somewhere saying its reccomended practice to oversize heating system to allow the extra capacity to come in usefull on cold mornings and monday morning warmups.

Surely if its wrong to do this the system would be cycling in and out, but due to inverters they will ramp down.

My system at home is oversized by about 1.5kw, we run the system in low fan speed all the time and the system still cycles off occasionally but it means on hot days I just put the fan upto medium and we have the extra capacity.

Correct me if im wrong.

[airefresco]

I would say you are 50% correct. Heating systems are normally oversized as you say, the problem comes with cooling. Air conditioning is designed for cooling with heating as a bonus in my opinion. Air conditioning systems should be sized correctly, oversizing something is worse than undersizing something in my opinion.

I don´t think it being an inverter makes any difference, you should size on their nominal rating.

[Brian_UK]

I hope that this isn't going to develop into another anti manufacturer thread again.

If you are not convinced by the numbers quoted to you by a supplier I would suggest that you engage a consultant with professional indemnity insurance to design and specify the system for you.

[back2space]

I hope that this isn't going to develop into another anti manufacturer thread again.

If you are not convinced by the numbers quoted to you by a supplier I would suggest that you engage a consultant with professional indemnity insurance to design and specify the system for you.

Not sure what your getting at Brian as there is no mention of manufacturer, just want other peoples opinions on oversizing of inverters.

Why do you think I want a system designing?

[beagle]

Frank had a great description of why this is a bad idea in another thread sometime ago, it's a little too late for me to go hunting for the quote though but imo inverter or not oversizing just doesn't make sense, sure you'll be laughing on the one day in the next 5 years that we hit the big 100 again but other than that it just doesn't make economic sense. The right kit for the right job, don't over complicate things for yourself if it needs 3.5kW throw a 3.5kW system in.

[Brian_UK]

Not sure what your getting at Brian as there is no mention of manufacturer, just want other peoples opinions on oversizing of inverters.

Why do you think I want a system designing?Your previous problems and it appears that you are starting afresh with a new system.

I wish you well with it.

[back2space]

Thank you Brian.

I have taken on boards peoples advice around the forums and elsewhere all reccomending my chosen manufacturer for the new system.

My installer has been brilliant and is at the moment pricing up the new equipment. Part of the reasons the installer was not on side at the beginning is down to LG refusing to admit there is a problem, it has taken about 3 engineers plus the installer to all witness the weak and terrible control methods of the LG before deciding to get rid of the cause of the problems.

For instance if both units were running on the LG multisplit and one was thermostat off I understand that the EEV stays open 10% for oil return in heating mode. With a room temp of 20C the unit that was thermostat off the fan would stay on to clear heat from the coil occasionaly however this got worse and worse and the fan would stay on longer as it seemed too much refrigerant was being sent down the pipes. We had air off temps of 36C on the unit that was thermostat off and also 36C on the unit that was thermostat on!!!

I am assured on the mitsi multisplit this wont occur but the EEV still stays open slightly but I shouldnt expect air offs of 36C on a unit that is thermostat off.

[stefs_cruiser]

Hope your new system is sized on the nominal ratings as per manufacturers specifications.
So that the unit operates within the range it is designed to.

[back2space]

The systems are slightly oversized by the nominal ratings by about 1.5kw.

Reasons for this have been explained above.

The perfectly matched system struggles on the coldest of mornings to regain the temp.

We have therefore added an extra kw for the unit to "play with" for these instances.

This also allows for internal doors of the house to be left open for heat/cool to get through to non air conditioned rooms such as kitchen/bathrooms/hallways.

Being an inverter 1.5kw is not a problem as it will just ramp down when it needs to. Issues such of cycling off will be in summer if the units are requried to heat on chilly evenings with mild outdoor temps.

maybe not such an issue on a good control strategy such as mitsi.

[stefs_cruiser]

If a unit is rated at 3.5Kw, then most are capable of 4Kw or even higher if required, and can go as low as .8Kw.
But they are designed to spend most of their time around the 3.5Kw area.

But forcing a unit to run lower than its rated capacity, is just asking for trouble.
The unit will be forced into other control strategies to combat various expected/predicted issues.
Excuse my terms, they are mostly Panasonic based, as I know them quite well.

Ideally the compressor runs all the time, but when oversized the unit will try and maintain temp at a lower speed, but if it still over performs, it will exceed the temp setting, before switching off, this causes unnecessary temperature fluctuations.
Due to cold draft prevention, minimum off times, maximum off times, minimum run times etc.

Other modes it may drop into, and then start behaving unexpectedly are.
Low frequency protection control 1 (step up freq. to return oil)
Overload protection control (indoor coil gets too hot)

This is all on heating; but if it is oversized on heating, than more than likely it will be oversized on cooling as well, where it will have other similar issues as well.
But people keep thinking they know better than the designers…….
Good luck with it…

[back2space]

If a unit is rated at 3.5Kw, then most are capable of 4Kw or even higher if required, and can go as low as .8Kw.
But they are designed to spend most of their time around the 3.5Kw area.

But forcing a unit to run lower than its rated capacity, is just asking for trouble.
The unit will be forced into other control strategies to combat various expected/predicted issues.
Excuse my terms, they are mostly Panasonic based, as I know them quite well.

Ideally the compressor runs all the time, but when oversized the unit will try and maintain temp at a lower speed, but if it still over performs, it will exceed the temp setting, before switching off, this causes unnecessary temperature fluctuations.
Due to cold draft prevention, minimum off times, maximum off times, minimum run times etc.

Other modes it may drop into, and then start behaving unexpectedly are.
Low frequency protection control 1 (step up freq. to return oil)
Overload protection control (indoor coil gets too hot)

This is all on heating; but if it is oversized on heating, than more than likely it will be oversized on cooling as well, where it will have other similar issues as well.
But people keep thinking they know better than the designers…….
Good luck with it…

Thank you for your input on this, you say that the unit is designed to spend most of its time around the 3.5kw mark. But what is the point in designing them so they can go as low as 0.8kw if this is bad for them. For instance as someone else pointed out many months of the year even perfectly sized systems may be oversized. The capacitys are based on outside ambients of 27C so if its 21C and you require inside cooling to 22C then your unit may be over sized.

Surely adjusting the fan speeds to low speeds or even ultra low speed reduces the capacity and keeps the system running avoiding the cycling off problems.

My room requires 2kw of heating and cooling on normal conditions so if the 3.5kw unit is hovering around 2-2.5kw mark this is ok.

What im saying is there must be a certain limit to how accurate it has to be.

Rule of thumb im told is as close to size as you can get.

On cold winters days or early mornings you may require 3.5kw output to raise the temperature otherwise its going to struggle.

Being oversized by 1kw or so shouldnt cause me any problems I am being told by the installers.