"Air conditioner" is often shorted to "air con" or "AC". It is a machine that keeps the air at a comfortable temperature.
When the weather is very hot, some clever shopkeepers will lure people into their store with a "Cool inside" sign. In Winter, they may use a "warm inside" sign.
When a machine is turned on and is functioning, we say it is "running". A well-known joke that plays on this is "Is your fridge running? Because you better go and catch it!".
"Air conditioner" とは、しばしば "air con" や "AC"に省略されます。空調を快適な温度にする機械です。
天候がとても暑いと、賢い店主は、お店に "Cool inside"という看板をつけ、冬には "warm inside" にし、客を引き付けます。
機械が付いていること、動いていることを "running"と言います。
有名なジョークは、
"Is your fridge running? Because you better go and catch it!"
冷蔵庫が走って（動いて）いるの？捕まえに行ったほうがいいよ！
です。
Well, you could just place a sign up every time the AC is on - as in the first example sentence. No explanation is necessary but people understand they must close the door. The second example explains that it is to the benefit of all inside the building that the door remains closed.
Air-conditioned room. Please keep this door closed.
Please close the door.
Keep this door closed at all times.
Air-conditioned room, please close the door after you. - This is commonly seen on shop/restaurant doors. - It implies that the air-conditioning will not work to the full potential if the door is left open. Also it is polite and to the point. It is asking people to close the door when they enter and also when they leave. Make sure to have it on both sides! :)
Air-conditioned room, please close the door after you.
（冷房中、ドアは閉めてください）
-これはお店やレストランなどのドアでよく目にします。ドアを開けておくとエアコンがあまりよく効かない、ということですね。また、丁寧で的を得ています。出入りする時にはドアを閉めてください、とお願いしています。ドアの両側に貼ってくださいね！ :)
These sentences ask kindly that the door be closed at all times.
Remember to use the word, please.
Keeping the doors and windows shut when the air conditioner is on helps to keep all the cool air in the room.
If we leave the doors or windows open, we are letting the cool air created by the AC to escape and waste energy.
"Shut the door, Save energy!"
Shut: closed
これらの文は、親切に常時ドアを閉めることを伝えるものです。pleaseを使うのを忘れずに。
エアコンがついているときに、ドアや窓を閉めておくことは、部屋の冷気を保ちます。ドアや窓を開けっぱなしにすると、エアコンの冷気が逃げ、エネルギーの無駄になります。
"Shut the door, Save energy!"
（ドアを閉めて、エネルギーを節約しよう！）
Shut:閉める
Please keep this door closed as we have the air conditioning on.
We have the A.C on, please make sure this door says closed.
All of these sentences above will let someone know that you want them to keep the door closed because your "A.C" (Air Conditioning unit) is on. Also you could add "Thank you for your cooperation" to the end of each of these sentences.
これら全ての例が、誰かにエアコンがついているのでドアを閉めたままにしてほしいことを伝えることができます。
"Thank you for your cooperation"（ご協力ありがとうございます） を文の最後につけることもできます。
Come in we are open, the door is shut because the AC is on
Save the environment, shut the door behind you
I would put a sign up to invite people in but let them know the reason the door is shut is only to save energy. Please shut the door behind you because the AC is on is acceptable and I've seen the below examples.
Close the door to save the environment.
Close the door after you to prevent energy waste.
私だったら、人を呼び込み、またドアが閉まっている理由が節電のためだと分かる看板を立てると思います。「Please shut the door behind you because the AC is on（エアコンが入っているのでドアは閉めてください）」は使えますし、以下の例も見たことがあります。
Close the door to save the environment.（環境保護のためドアは閉めてください）
Close the door after you to prevent energy waste.（節電のためドアは閉めてください）
If there is AC on in the workplace or at home, it is important to make sure that it is not wasted by leaving a door or window open.
If we want to inform others about being careful with this, we could use some expressions such as:
1) Keep Door Closed-AC On
2) Door should be closed at all times.
3) Please close the door. AC is on.
職場でも家でもエアコンがついていたら、ドアや窓を開けっ放しにしてはいけませんね。無駄になってしまいます。
これについて人に伝えたいなら、例えば：
1) Keep Door Closed-AC On（ドアは閉めておくこと。エアコン入っています）
2) Door should be closed at all times.（ドアは常に閉める）
3) Please close the door. AC is on.（ドアを閉めてください。エアコン入っています）
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