Estimating maximum BTU size (not minimum)

[Silencer]

I have seen different calculators on the net. What is the largest size unit you would say can be properly used in a room 13mx5m with 4m high ceilings. Room gets little direct light but has 8m of sliding glass doors and 2 other windows. A kitchen, used only an hour or two a day occupies 1/2 of the space.

[Crossy]

Interesting question, not been asked that before, most people want the smallest (cheapest) unit that will do the job.

 

If you have an inverter type unit there's no real maximum as the unit will throttle to the required cooling level, that said many inverters do have a minimum cooling level below which they shut off (check the spec.).

 

If non-inverter type then i wouldn't go more than 50% over the standard Thai guesstimate of 600-700 BTU/m² or you will run into issues with the compressor not running enough to dehumidify the space leading to a nasty "clammy" feeling.

 

It might be worth splitting the required cooling between two smaller units, then in the cooler months just running one.

 

13x5 = 65m² so Thai guesstimate would be 39,000-45,000 BTU - That's one big cooler, definitely time to look at several smaller units.

[Silencer]

2 hours ago, Crossy said:

Interesting question, not been asked that before, most people want the smallest (cheapest) unit that will do the job.

 

If you have an inverter type unit there's no real maximum as the unit will throttle to the required cooling level, that said many inverters do have a minimum cooling level below which they shut off (check the spec.).

 

If non-inverter type then i wouldn't go more than 50% over the standard Thai guesstimate of 600-700 BTU/m² or you will run into issues with the compressor not running enough to dehumidify the space leading to a nasty "clammy" feeling.

 

It might be worth splitting the required cooling between two smaller units, then in the cooler months just running one.

 

13x5 = 65m² so Thai guesstimate would be 39,000-45,000 BTU - That's one big cooler, definitely time to look at several smaller units.

Yes, a bit of an upside down question indeed. The reason I ask though is I have the opportunity to purchase a Mitsubishi ceiling installed inverter unit (48,000BTU) for half sales price (friend's unit that is less than a week old). Saving 40k sounds great but I'm wondering if the unit would be too big for the space and I end up with what you identify as a potential problem. Outside unit is PUY-SP48YKA and inside unit PLY-SP48BA. Not sure what specs to look at to see if I'll have a problem or not.

[Crossy]

10 hours ago, Silencer said:

Yes, a bit of an upside down question indeed. The reason I ask though is I have the opportunity to purchase a Mitsubishi ceiling installed inverter unit (48,000BTU) for half sales price (friend's unit that is less than a week old). Saving 40k sounds great but I'm wondering if the unit would be too big for the space and I end up with what you identify as a potential problem. Outside unit is PUY-SP48YKA and inside unit PLY-SP48BA. Not sure what specs to look at to see if I'll have a problem or not.

 

I would think that unit would be a good fit, it's only really slightly over size. I can't find the spec. immediately but I suspect the minimum cooling would be about 8,000-10,000 BTU.

 

Any particular reason your friend is unhappy with it?

[Excel]

13 hours ago, Crossy said:

Interesting question, not been asked that before, most people want the smallest (cheapest) unit that will do the job.

 

If you have an inverter type unit there's no real maximum as the unit will throttle to the required cooling level, that said many inverters do have a minimum cooling level below which they shut off (check the spec.).

 

If non-inverter type then i wouldn't go more than 50% over the standard Thai guesstimate of 600-700 BTU/m² or you will run into issues with the compressor not running enough to dehumidify the space leading to a nasty "clammy" feeling.

 

It might be worth splitting the required cooling between two smaller units, then in the cooler months just running one.

 

13x5 = 65m² so Thai guesstimate would be 39,000-45,000 BTU - That's one big cooler, definitely time to look at several smaller units.

In my previous house I had an open plan living/dining/kitchen area 11 mtr x 9 mtrs with a high single pitched roof. All walls were shaded from the sun, two of which were glass walls. At the time the only way to have an effective a/c installation was by fitting 2 cassette types, the largest available at that time of around 54k BTU each. Now it was not the running cost ( which was high) the issue rather the inconvenience of the repairs which seemed to become too regular for my liking.  So after maybe 4 years out they came and replaced with 4 simple wall mounted a/c units. Easier to service and in my experience more reliable. The biggest plus being, and something that endorses Crossy's point of having smaller units, is that when one was down and needed servicing it still left 3 operational. In your case I would oversize the two to allow for intermittent redundancy during maintainence/breakdown periods, albeit seldom hopefully. Just my view.

[Yellowtail]

48,000 is not that much in that space. 

 

I love the cassette units, and would jump on it were it me. 

 

A lot of benefits with the ceiling units over the wall, and the only negative is the price. 

[Silencer]

Thanks all. Reason for availability is the office opened, unit installed, then office closed, all within a week. Selling all the stuff (I already bought brand new 3 bathroom sinks, 3 one-piece toilets, associated plumbing, 4 interior doors, and a large decorative urn for silly price). I'm building a house soon and debating the AC option. My plans call for small wall units in several bedrooms and this space but I might delay install of all those to save initial building costs.  I rarely use AC now where I rent and always have a sea breeze but maybe not in new location which is more inland. Fan used for sleeping has been fine even on the hottest nights. I've recently read that inverters are "always on" and best for rooms you want to keep cool often. Since I don't need that I think I'll pass and if/when I need AC use a less efficient wall unit for a few hours. It would have been a non-starter without your comments, so thanks for your input. 

[sometimewoodworker]

4 hours ago, Silencer said:

I've recently read that inverters are "always on" and best for rooms you want to keep cool often.

That is completely incorrect.
 

Our inverters get switched on when needed. However when on they cycle down when the temperature is at its set point. 

[Silencer]

2 hours ago, sometimewoodworker said:

That is completely incorrect.
 

Our inverters get switched on when needed. However when on they cycle down when the temperature is at its set point. 

I guess I should have clarified that the article stated that if you use air-conditioning all day, like at an office, a factory, store, or home ....an inverter is more cost efficient as it does not cycle on/off such as a regular AC unit. But as I do not have a requirement to keep a room cool for a long period of time, it would be a waste of money for me (even at half price) for a large inverter that works best when kept on to keep a steady temperature. I have not used AC at home for almost 2 years.

[sometimewoodworker]

6 hours ago, Silencer said:

I guess I should have clarified that the article stated that if you use air-conditioning all day, like at an office, a factory, store, or home ....an inverter is more cost efficient as it does not cycle on/off such as a regular AC unit. But as I do not have a requirement to keep a room cool for a long period of time, it would be a waste of money for me (even at half price) for a large inverter that works best when kept on to keep a steady temperature. I have not used AC at home for almost 2 years.

Again that isn’t telling the complete story. An inverter unless sized too small will almost always be cheaper to run than a non inverter, since an AC is usually used for more than an hour. 
 

of course having no AC is often cheaper than having AC but you need to be careful to check the power usage of the fans you use instead. 

[digbeth]

I've had calculated a btu invertor sized too large for a room and the a/c tech refuse to install it, saying when run it will always throttle, leading to not feeling as cold as non invertor unit, they had to take it back, and I went to the shop, got a refund for the difference and got a correctly sized unit for less.

 

They said that the 'not feeling cold' after changing to an invertor unit from many customers had lead to this policy, this is a big mitsubishi dealer in Bangkok 

[OneMoreFarang]

5 hours ago, digbeth said:

I've had calculated a btu invertor sized too large for a room and the a/c tech refuse to install it, saying when run it will always throttle, leading to not feeling as cold as non invertor unit, they had to take it back, and I went to the shop, got a refund for the difference and got a correctly sized unit for less.

 

They said that the 'not feeling cold' after changing to an invertor unit from many customers had lead to this policy, this is a big mitsubishi dealer in Bangkok 

Thanks, that is interesting.

 

I wonder about the correct size. What is the correct size or a proper calculation? I looked at several online BTU calculators which produce very different results with the same input. And most of those online calculators don't even ask about the climate.

I would think to calculate the required size it would be necessary to input the maximum outside temperature and the desired room temperature. But it seem no online calculator shows these details.

 

On 7/3/2021 at 4:45 PM, Crossy said:

the standard Thai guesstimate of 600-700 BTU/m²

Is that a good estimate?

 

I don't want to ask and trust the sales guys from the shops who get commissions..

 

This is a general comment/question. I don't need info for one specific room.

 

[Crossy]

11 minutes ago, OneMoreFarang said:

Is that a good estimate?

I don't want to ask and trust the sales guys from the shops who get commissions..

This is a general comment/question. I don't need info for one specific room.

 

It seems to work pretty well for a "typical" Thai house in a "typical" location, you could go smaller for a bedroom used only at night. It will certainly get you into the usable range of A/C sizes. We've  never had "cannot" from the installers when using those numbers.

 

There are so many variables that it's very difficult to actually "calculate" the required size and even when you decide that you need 10,345.8 BTU lo and behold you have a distinct set of sizes to choose from and no exact match.

[OneMoreFarang]

1 hour ago, Crossy said:

 

It seems to work pretty well for a "typical" Thai house in a "typical" location, you could go smaller for a bedroom used only at night. It will certainly get you into the usable range of A/C sizes. We've  never had "cannot" from the installers when using those numbers.

 

There are so many variables that it's very difficult to actually "calculate" the required size and even when you decide that you need 10,345.8 BTU lo and behold you have a distinct set of sizes to choose from and no exact match.

Thanks

 

I am aware that it does not make sense to try to get an exact number. But what I found on the online calculators were numbers which varied by more than 100% for the same input. I am sure it makes a difference if someone wants to cool down a room to 24 degree or maybe 20 degrees when it's outside 40 degrees.

[Yellowtail]

The number of variables for calculating heat-load is considerable, so any online calculator is going to be ballpark. When you consider that people are going to be unhappy with an undersized unit, and (generally) happy with an oversized unit, from a manufacturers standpoint, it is always better to err on the side of excess capacity. Particularly when you consider it typically does not cost significantly more to run an oversized unit than it does a right-sized unit.