DEAR MR. MELTZER: My husband has been doing so well with his business these past few years that we've been planning a luxurious winter vacation. We expect to be away for three months.

Our question: Will it harm the house to shut down the heat while we're home? We don't want to return home to find high heating bills when we haven't even been in the house.

ANSWER: It is definitely not a good idea to turn off the heat in a house that is occupied all year. When a house remains unheated during cold weather, it often results in wallpaper loosening, swelling of floors, moisture seeping into woodwork and doors sticking.

Many other things can happen when heat is shut down. If you plan to be away for three months, lower your thermostat to 55 or 60 degrees. In this way you will protect your house, and yet your heating bills will not be so high.

DEAR MR. MELTZER: A local carpenter I hired installed a new front door and three interior doors. The work took about two days. After he finished, he presented me with a large bill and left.

When I inspected the work, I noticed he put three hinges on the front door, but only two on the new inside doors.

I told him I won't pay the bill unless he comes back and installs the extra hinge. Do you think I am doing the right thing?

ANSWER: No, you are not. An exterior door such as a front door weighs much more than an interior door. Most interior doors are hollow. The number of hinges required is governed by the weight of the door.

DEAR MR. MELTZER: I have lived in the same apartment for almost five years. For all of that time I paid my rent on the first of each month. Never once did I deviate.

However, last month I was 15 days late because I was in the hospital. I suppose I could have arranged to pay my bills from there, but I felt so awful that I just let everything slide.

Remember, this was the very first time I was late! In spite of that, when I paid my usual rent to the agent, he said I was required to pay a 5 per cent late charge. I told him that my lease does not include anything about a late charge. He replied that all tenants who pay their rent after the 10th of the month must pay a late charge.

Do you think it's fair, in view of my nearly perfect record?

ANSWER: If nothing is said about a late charge in your lease, you are not obliged to pay one. Often agents try to get away with something like this, but you are certainly not required to pay it.

DEAR MR. MELTZER: We found the house we wanted, signed an agreement of sale and attached a $5,000 deposit. For several weeks I have been asking the broker to send me my copy of the agreement. But in spite of my numerous requests, I still have not received it.

What about my deposit? Do you think I have anything to worry about?

ANSWER: No, I don't think you do have anything to worry about. What probably happened is that the broker has not yet been able to get the signature of the owner to the agreement of sale. What he is undoubtedly doing is trying desperately to keep the deal alive.

Give him a little more time.

DEAR MR. MELTZER: I'm having a house built, and you know what that costs today! Among the many things that are concerning me is the cost of fuel.

I want to be certain that this new house will be well insulated. Though I have questioned supposed experts, no one seems to agree on the type and thickness of the insulation to be applied.

Can you straighten me out on this? I want to be sure of doing the right thing.

ANSWER: Three-inch-thick insulation in the walls and six-inch-thick insulation in the ceilings is idea. The best types, in my opinion, are fiberglas or cellulose fiber. My personal preference is cellulose fiber. But you won't go wrong on either.

Bernard C. Meltzer, who does not answer letters personally, is a realtor, engineer and appraiser. His address is suite 615, 1420 Walnut St., Philadelphia, 19102.