Washing Machine Relocation Issues

I am in the process of moving my laundry room from the main floor to the second floor off the master bedroom. I have already relocated the washer and plumbed in the new supply and drain. Unfortunately I could not access the stack from this location and only have access to the 1.5" drain line. It wasn't much of a surprise to me that upon running the first full load of laundry that water backed-up in the kitchen sink downstains while the washer drained. Due to the design of the house it is not feasible to access the stack.

The drain lines are fully clear. When the water backs up in the kitchen sink there is only 3 gallons at max. My question is would a drain check valve under the kitchen sink safely prevent this back-up (and if so where can you get a check valve for 1.5" abs). I am assuming at this point that the amount of water backing-up in the kitchen sink can be contained in the piping up to the second floor washer. I realize that it may indeed causing an overflow at the washer.

I have already considered the option of installing a laundry sink but I am very limited on space and even a space saver 18" wide sink is a bit much.

Any advise is appreciated greatly.

Earl

I don't know that the check valve will help or not, but it sounds plausiable. These are not made from ABS or PVC, just brass. You will need 2 male adapters to make the connection. Be sure the valve is level and don't do anything that can't be undone if it doesn't work. At least it won't cost much to try it.

Improper washer drain

You may not have vented the washer upstairs.

This automatically overflows the downstairs fixtures.

You have also undersized the line,
And you have used the vent from the kitchen, which may not even have wasted fittings to handle what the washer puts out.

Nothing is impossible,
Some things take more time and money, but that doesn't make it impossible.
If a plumbing contractor had done the job, the kitchen would have retained it's vent,
The washer would have had a 2" waste line tying in below the kitchen sink trap arm
And the vent for the washer p-trap and the kitchen vent would not have tied back together until they were six inches above the flood level of the washer drain.

What you have describe is typical "handyman plumbing", the kind that gives real plumbers a bad name and "drain cleaners" fits.
Having three gallons of used greasy water back up into the kitchen sink does not sound very sanitary.

When you sell the home, there should be some warning to the next people of what problems to expect.
Either that, or they may just wind up suing you.
Legally, you must disclose problems like that.

Actually the washer drain is vented - I realize that the 1.5" is really too small for the washer drain but it is the only drain available without major renovations.

I've decided to install a laundry sink (although not ideal given the limited space), and I think this should solve my problem. The space saver laundry sink I am going to install holds 18 gallons so this should provide plenty of safety margin.