Oversized System

[Slim R410a]

I've got a customer that wants to move an existing wall mount, cooling only, fixed speed, with a capacity of 3.6kw (2.7kw sensible) into a room 2.8m x 3.8m. (2.4m h)

For a new installation for this room size we would install a 2.0-2.5kw system. So, apart from the unit short-cycling, what other disadvantages would there be from installing the 3.6kw? Or would it just be better in the long run to install a new 2.0kw?

[Argus]

You seem to have answered your own question, Slim.
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Mercury Topaz specifications

[Slim R410a]

I see what you mean but as always its the cost that may stop a new install.
From a customers point of view they have a 2yr old panasonic split in perfect working order and just want to move it, spending over a grand on a new install was not on there mind.
My question is has anybody had experience in fitting units this size into small rooms (normal office conditions) and whats the worst that could happen??
Iv'e seen on another site a 10kw LG cassette that was moved to a new reception area which needed 5-6kw and the coil froze up because they had set it on 18deg, an engineer had to put it on heating to defrost. (or was that just because it was an LG!)

[Argus]

.

You've done it again. Slim - answered your own question, that is!

An oversized system is prone to short cycling and poor evaporation which will make a bad situation worse.

You will probably rue the day that you proceeded against your better judgement if you reinstall the thing, because when it breaks down repetitively it is you who will be picking up the tab.

Look at the intended the new position, do the sums, size the kit and if it is seriously over the top quote a new system. If the customer won't have it, walk away. What's the point in installing something that will cost you your profit?

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Ford D3 platform history

[Slim R410a]

Thanks for the advice, Argus, I've made my mind up and Im sure you can guess what it is!

[Brian_UK]

Another possible aspect Slim would be excessive airflow from the oversized unit.

If it is an office environment then your customer could start getting complaints from the staff and quoting Health and Safety type regulations may be useful.

[Argus]

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Absolutely.

There's no point putting in something that just ain't right.

You'll end up buying a pup.

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vaporizer answer

[Makeit go Right]

You could be right.........but it's not so much the size of room but more the actual cooling load. That might be worth checking into to see if the 3.5kW system is not so over the top.

Could be there is a lot of solar and a couple of computers etc, which might just leave a dinky unit straining away.

[Dan]

Another important consideration is humidity control. An oversized unit will accomplish the sensible cooling in such quick order that you will not dehumidify the room. Long run times are required to wring the greatest amount of moisture and achieve comfort levels of RH. Thus you might have a dank muggy and moldy room.

[Abby Normal]

I get a little leary of anytime an AC is sized based on an area.

The warning of too much cooling and not enough dehumidification usually materializes when ambient dewpoints exceed say 17C.

I sold a lot of geothermal in Canada sized for heating and often 100% oversized for cooling and no humidity problems beacuse the ambient humidity was not that high.

Take that same approach in a more humid climate and you have a good envirnoment for hanging meat carcasses

[winfred.dela]

Abby Normal:
I get a little leary of anytime an AC is sized based on an area.

The warning of too much cooling and not enough dehumidification usually materializes when ambient dewpoints exceed say 17C.

Abby Normal:
Take that same approach in a more humid climate and you have a good envirnoment for hanging meat carcasses

Lately (since mid 90s) a lot of engineers just do an AREA measurement and calculation then divide it with a MAGIC NUMBER and immediately advise the customer the required equipment size.
Even with free software and cheap computers, most does not even run the software for cooling load calculations before suggesting an eqpt size.

I still remember the old days (me working 20+years ago as a newbie in maintenance dept. of US naval base in Subic), when we have to survey an office count the no. of men and women, check/interview the end user for latent heat loads, sensible loads, open a book to get CFM/person required by NavFac regulations.
And then with the help of manuals, books, a cooling load form (Trane or Carrier), scientific calculator, we will be able to come up with capacity in BTUHs, CFMs, Sensible heat load factor, fan static pressure, etc. etc. etc. . .

Our boss (Fil-Am) will even review it before submitting a budgetary costing which maybe delivered in a year (if urgent).

Past forward Now in my part of the tropics. . .
we always find oversized/undersized eqpt in a lot of fairly sized office. . .
with electronic thermostat that is adjusted by women and again re adjusted by men.
We then need to explain that the men and women have different cooling load requirement.

Maybe this is the price we have to pay:

Be a magician with numbers in times of easy access: mobile phone, internet connection, SMS. . .

[Abby Normal]

worst design scenario know to man is three women and one thermostat

One will be fine, one too hot and the other too cold. The next day the fine one is cold, the cold one is hot, and the hot one is in my bed

[Temprite]

You can do as many calculations as you like to find the required heat load.

But if you ore only quoting on a wall hung split unit you have to draw the line somewhere.

Whats the point of working out the exact R value of every single wall, area of every window and as many other particulars you can think of if you dont get the job.

Having a rule of thumb is OK in my book for small jobs as long as some common sense goes along with it.

[The Saint]

Maybe you dont get the point of Mr Slim. I would suggest a capacity control for his system.

[Abby Normal]

You can do as many calculations as you like to find the required heat load.

But if you ore only quoting on a wall hung split unit you have to draw the line somewhere.

Whats the point of working out the exact R value of every single wall, area of every window and as many other particulars you can think of if you dont get the job.

Having a rule of thumb is OK in my book for small jobs as long as some common sense goes along with it.

In heating, if you are quoting a replacement in a subdivision, or a home built during a known code cycle, you can ball park the HEATING pretty easily, and worry about which room was always too cold etc after you get the job.

In cooling you can ballpark it high if you do not want to put the time in when bidding. But with cooling, orientation with respect to the sun is everything. Identical houses orientated differently will have different cooling loads.

Here, it does not really matter how hot is outside, what matters is whether it is sunny or not and what the humidity level is.

Your avatar looks 'arid' and as I have said, the woes of oversizing are not going to bite you in the ass when the ambient dewpoints are low.

[Temprite]

In cooling you can ballpark it high if you do not want to put the time in when bidding. But with cooling, orientation with respect to the sun is everything. Identical houses orientated differently will have different cooling loads.

Here, it does not really matter how hot is outside, what matters is whether it is sunny or not and what the humidity level is.

G'day Abby.
I Agree with everything that you have said here. When I said that a rule of thumb is OK for wallmounts I also said that common sense has to be used as well. Any system larger than this and I would spend more time to find the required load. Some people think rule of thumb will work for every install this is obviously not true.

Your avatar looks 'arid' and as I have said, the woes of oversizing are not going to bite you in the ass when the ambient dewpoints are low.

This was a painting done by my great grandfather. He lived in England and painted landscapes so I would say this is an English landscape. I live on the coast there is not a huge amount of really humid days here.

[Slim R410a]

Some interesting points made here,

The customer has decided he wants the existing system installing. I've made aware of the short-cyling and draughts so that they cannot complain at a later date in the future.

I am still thinking along the lines of tyring to reduce the systems capacity. The outdoor unit will be sited on a mezzenaine (sp) inside a building, well ventilated, no real heat gains, roof height is something like 10m.

[frank]

mezzenaine (sp)

Hi Slim

Mezzanine

[Samarjit Sen]

Hi Fredy,

When i used to calculate the heat load about 40 years ago, we used to refer to Carrier Handbook and Trane Handbook and use a slide rule, which I still have. In my experience a slightly oversized AC unit will not have any adverse effect. Further a detail load assessment has not been made, which I feel is very important in case of finding the heat loads wether it is for air conditioning or refrigeration. Thumb rules will always get you in trouble.

[winfred.dela]

Samarjit:
Thumb rules will always get you in trouble.

That's usually what i preach to the young guns. Always reminding them that rule of thumb are for checking only.
Also, it can be use judiciously by anyone near your length of experience (in A/C & Refrigeration) but not for those that are just starting in the trade.

But there will always be in our midst those that are trying to look and sound smart. . .
These are the one's that we should be LEERY (just have to borrow this word, does it sound nice Abby?).
I really enjoy reading posts by westerners, it always sound nice and sometimes poetic.

[Abby Normal]

Have you ever been as leary as timothy?

[winfred.dela]

Abby Normal:
Have you ever been as leary as timothy?

Timothy? Do not know any Timothy.
Leary? or Leery?

Maybe, i need to study English Literature in my free time so i can answer appropriately.

But i doubt if i can find extra time, I kind of need more than the 24 hours of each day coz around 1/4 of the day is spent here in RE forum. . .

[Abby Normal]

Search Timothy Leary

[winfred.dela]

Thanks Abby.
Have found the guy. Now, i know something about Timothy Leary.

Need to read some of his works before i can answer your question.

Have you ever been as leary as timothy?

[Abby Normal]

lol, would be consuming something and not reading. He had a fondness for LSD

[Makeit go Right]

The customer has decided he wants the existing system installing. I've made aware of the short-cycling and draughts so that they cannot complain at a later date in the future. I am still thinking along the lines of trying to reduce the systems capacity.

Suggest you note the potential problems in your quote, also. {You do write quotes, yes?}

As for reducing capacity, whatever you plan on that, offer it as a phase-2, which you can go ahead with, if needed.

That way you have done all you can, and customer takes the responsibility for any oversizing problems that might show up.



QUESTION… If one exceeds the run length, the duty of the system will start to reduce. That is well known, but can any damage / excessive wear occur in that scenario?

Thinking sideways on this OP, one way of reducing output would be to increase the run-length outside correct limit. But, I guess it’s not quite a easy as that, eh?