Washing machine overflow clog could be due to a blockage, poorly vented line, or a small drain line. Try the following. Don’t connect the discharge directly into the drain without an air gap.
A customer said the standpipe that the washing machine discharge hose goes into overflows with water and onto the floor. So he attached a rubber hose connector that fits onto the mouth of the drain and seals the discharge into the standpipe.
I told him that direct-connecting a discharge line to a drain without an air gap is probably against code. Sealing the discharge line to a sewer pipe will allow a sewage backup to run into your washing machine. Something that is very possible given your clogged drain situation.
Three potential conditions causing your drain to overflow are:
Partially clogged drain line (my first guess).
Undersized trap or drain line.
A poorly vented line.
The problem could be a combination of these conditions:
After years of use, the drain line may be 80% clogged.
The new machine may be pushing more water than the old one.
The direct connection may force water through it but there may be consequences.
The pump on the washing machine is not designed to force water against a clogged line. It may wear out prematurely.
If the drain clog is in the main sewer, water may be forced out of another area, like a sink, a toilet, or a floor drain. Water may be forced up the vent pipe from the washing machine. When the water is turned off, it may drain back into the washing machine. And it will not be clean water. Or the drain may clog and water from the above floor could fill the washing machine. And finally, a siphon could be started and water can be ‘sucked’ out of the machine as fast as it fills.
I told him he should correct the problem as soon as possible. Snake the standpipe and drain line and make sure it drains properly. Hook a hose to the laundry tub and flush the standpipe. For maintenance, use an enzyme drain cleaner every night or so for a week. Skip a day here and there but do it for at least seven treatments.
If you cannot find the time to do it right, when will you find the time to do it over?