QI have a 7-week-old son, and my air conditioning hasn't been working for four weeks. I told the maintenance office about this problem three weeks ago and they said they would have it fixed. Nothing happened, so I wrote the maintenance supervisor a letter explaining my situation. Someone attempted to fix it the next day, but they couldn't because of a temporary technical problem. They said they would return but never came back. What should I do?

A Writing a letter and creating a paper trail is the right idea, for this problem and many others. Keep reminding the maintenance office that the problem has not been fixed. Also send a letter documenting what has happened in the past few weeks to the overseeing property management company.

In buildings that have central air conditioning, landlords must follow basic minimum standards for air conditioning set by local building codes for habitability. Where residents have window units, the responsibility of repair might fall to them, depending on what their lease stipulates.

Generally, however, apartment operators have a duty to provide adequate living conditions, such as conditioned air, a roof and a safe living situation. So, if the rental property has an air-conditioning system, then your landlord is required to fix it within a reasonable period of time.

In the summer in the Washington area, presumably a reasonable period of time to fix air conditioning would be "as soon as possible."

"If you go for a night without air conditioning, there's going to be a reasonable time built in for an owner to respond, but certainly after two, three, four, five days, the owner's going to be in some hot water there and the resident can seek to take some action," said Michael Semko, legal counsel for the Alexandria-based National Apartment Association.

Semko said apartment owners have to follow a legal doctrine known as "implied warranty of habitability," which essentially means that even if a lease doesn't specifically address it, an apartment operator must ensure that a rental property is livable. Either by judicial decision or statute, almost every state has adopted the implied warranty of habitability.

"It's a warranty from owner to resident that the property is fit to live in and will remain so throughout duration of lease," Semko said. "For example, take a high-crime area, or really any area of Northern Virginia, Maryland or D.C. You're going to need to have a lock on a building in a high-crime area. It arguably makes a property non-livable if a resident has no way to keep out the people they want to keep out," he said. "In a place where it's 85 to 95 degrees every day for three months, air conditioning is certainly something the owner must provide. Otherwise, it's unbearable and you can't live there."

Although there are plenty of people who live without air conditioning, in buildings where it is supposed to exist, landlords would be violating their duties if they did not provide it.

So, if a landlord fails to fix a broken air-conditioning system, after you inform him about it, you could file a complaint against your apartment operator with your local housing office, which would probably send someone to investigate whether your landlord is in violation. That process is likely to spur a landlord to action. Also, by talking to your local housing authority, you can also find out whether your specific scenario allows you to withhold rent until the problem is fixed.

Are there any hard and fast rules on what modifications to an apartment may be done without obtaining the landlord's permission first? For example, the thermostat in my place is easily 30 years old, and while it "works," it is far from energy-efficient. Because my energy bills are quite high over the summer, I would like to install a more modern, energy-efficient thermostat with the ability to program usage patterns and avoid keeping the place cooled while I am away.

The management of this complex doesn't know from "energy efficient' and tends to replace broken appliances and fixtures with ones that are equally old, so I don't believe asking them to replace the thermostat is a productive choice.

Am I likely to have problems if I hire an electrician to come in and do the installation? Or would I be better tossing it to the management and seeing if they will permit it?

The usual rule is that tenants are expected to pay for repairs they cause and landlords pay for everything else. But just in case you and your landlord had another agreement when you moved in, first look at your lease to find whether it contains any information about making unnecessary repairs or changes. Many leases prohibit you from making any alterations without the landlord's permission, and changing the thermostat would certainly count as an alteration.

Remember that because your cooling system is not broken, your landlord is satisfying the requirement to keep your place livable. That means that you have the option of talking to your landlord about doing (and paying for) the upgrade yourself or about possibly shifting the responsibility of paying for utilities to the landlord.

Instead of presuming that your landlord will not understand energy efficiency, you can help him understand your predicament -- big air-conditioning bills -- quite easily.

Depending on the kind of place where you live, your landlord may be receptive to your making such an upgrade. Many property owners would be amenable to a tenant organizing and paying for an expensive, time-consuming change. If you present the idea in the right way and keep the path of communication open, you and your landlord may work out something that benefits everyone.

We have asked our landlord to replace our heating/cooling unit as it is very old and has been leaking freon for more than two years. She has yet to do so. In addition, we have a small yard and some of the trees are growing out of control and are potentially harmful. We've asked our landlord to have someone come and trim, but she says it's our financial responsibility, even though we rent the house, and do not own it. Is this fair?

Again, where this is an issue of habitability, your landlord is responsible. If your lease does not specify that tenants do the yard work, then, no, it's not really fair for you to have to trim the trees and pay for it. But you have to read the lease to find out if you agreed to that arrangement.

Also, exposure to freon could endanger tenants, and that possibility alone violates the essence of landlord-dom.

An old heating/cooling unit that works may not warrant replacement, but one that leaks freon does justify a technician.

Next time, do not wait two years to take action on a problem that irks you unless you want your landlord to think she can get away with negligence.

Do you have questions, comments or ideas about apartment life? Contact Sara Gebhardt via e-mail at gebhardts@washpost.com or by mail, c/o Real Estate Editor. The Washington Post, 1150 15th St. NW, Washington, DC 20071.