Q - I have made my home more airtight to save energy, but I worry about carbon monoxide poisoning from my water heater and furnace. What causes carbon monoxide to form and how can I check for it?

A - High levels of carbon monoxide (CO) are caused by incomplete combustion of the fuels that fire your furnace and water heater. An inadequate supply of combustion air or a clogged flue are common causes. Have your appliances and flue inspected regularly by a qualified technician.Making your house more airtight may increase the chances of higher CO concentrations. Most furnaces and water heaters draw combustion air from inside the house. Therefore, in a very airtight house, there may be an inadequate supply of air for complete combustion at times.

One quick check for proper combustion is to look into the opening by the burners and observe the flames. Properly burning gas flames should be blue and have uniform shape. Flames with yellow tips, poorly defined edges and broken shapes indicate incomplete combustion.

Black soot around the burner opening, excessive window condensation and weak draw into the flue draft hood are bad signs. These symptoms are often present when flue gases with CO backdraft down the flue into your house.

Supplying outdoor combustion air to your furnace and water heater reduces the chances of high levels of CO and saves energy. Already heated indoor air isn't drawn up the flue and chilly drafts in your house are reduced. You can often set your thermostat lower and still be comfortable.

A rule of thumb is that you need about 10 times as much combustion air as the amount of gas you burn. If you are venting combustion air into your utility room through a horizontal duct, you should have one square inch of vent area for each 2,000 Btu/hr. total input rating for the appliances.

CO is ordorless and colorless, so it is impossible to detect with your senses. Symptoms of CO poisoning range from mild headaches, blindness or death at high concentrations. If you suspect high CO levels, leave your house and have it tested by your utility company or health department.

You can mount inexpensive CO-sensitive testing tablets in your home. Their color changes to warn you before CO concentrations reach a hazardous level. You should place them near your furnace and water heater and in a bedroom where the symptoms of CO poisoning can creep up on you unnoticed.

You can write to me for UTILITY BILLS UPDATE No. 367 showing an inspection checklist for proper combustion in your furnace and water heater, and information about and a list of manufacturers of inexpensive CO-testing tablets. Write to James Dulley, The Deseret News, 6906 Royal Green Dr., Cincinnati, OH 45244. Please include $1.00 and a self-addressed envelope.

Q - I am getting replacement windows and I now have marble sills. The window contractor said that it is more energy efficient to replace them with wood. Is he right or does he just want my marble sills? N.R.

A - He is correct in saying that wood sills are more energy efficient. With the position of a window sill, there is no insulation between it and the cold outdoors. Wood is a better insulator than marble.

In cold climates, sweating on marble sills can be annoying. If you plan to sell your home some time soon, marble sills are usually more attractive to prospective buyers. The increased value and interest in your house may more than offset the cost of the lost energy through the sills.