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Q.My remodeling customers are frequently skeptical when I replace their old 5-gallon toilets with 1.6-gallon toilets. They have heard of problems like "skid marks," multiple flushes, and so forth. Also, the word I have heard from the field is that pressure-assisted toilets create more problems than they are worth. Have the new low-flow designs solved these problems?

A.Rex Cauldwell responds: When 1.6-gallon toilets first came out, they did not work well because they were not designed for low flush: They were essentially 3.5-gallon toilets with a 1.6-gallon tank capacity. Everyone had trouble with them (you had to flush twice), so many consumers resorted to pressure-assisted units. Pressure-assisted units typically work well in the beginning (although many are noisy), but may eventually need repairs. In some cases parts may be hard to find.

I prefer to keep it simple and use a gravity toilet. Most of the new 1.6-gallon designs work fine. However, when replacing a 3.5-gallon unit with a low-flow, I try to keep the customer’s expectations in line. Skid marks are a problem with all the units and are just something they’ll have to live with. Some manufacturers give a written one-flush warranty and advertise quiet operation, but I make no personal guarantees.

To make the low-flows work better, many manufacturers reduced the toilet’s passageway diameter to around 1.5 inches. This gives the water the velocity it needs to evacuate the bowl, but the narrower outlets tend to clog more often. One manufacturer, Toto, advertises a much larger passageway — 2 1/8 inches — and other makers are following suit.

A larger passageway — or a higher price tag, for that matter — doesn’t necessarily mean the toilet will flush better. Some manufacturers have designs that are just downright stupid. One well-known unit has to have the handle in the down position for the entire length of the flush. You can’t just push the handle down, let go, and assume it will flush. If I’m asked to install this unit, I’ll make minor modifications to get it to work better.

In general, low-flow toilets are more sensitive to minor problems than 3.5-gallon units. For example, putting blue tablets in the tank causes a sticky coating to form everywhere. Older toilets can tolerate this, but with low-flows, it can slow down the water, causing "lazy flush syndrome." And even a minor misadjustment of the water level refill system in a low-flow unit may only allow 1 gallon to enter the tank — not enough for a successful flush. By contrast, if a 3.5-gallon toilet is misadjusted so that the tank only has 2.5 gallons, the toilet will probably still flush fine.

Ask your plumber for advice when you’re selecting a low-flow toilet. He or she should know which models give the best service.

Rex Cauldwell is a master plumber and electrician in Copper Hill, Va.