I don't know for sure about the washer, I expect it might be a problem. I would urge you to reconsider the whole tankless heater idea. These are expensive to install, expensive to maintain, and often can not supply sufficient hot water in the winter if you live in a cold climate. If this is a gas unit, you may find your gas supply is not large enough and so you will have to have a new line run in and new meter. If it is electric, it may require an new, larger electric service. It has been reported many times that over a long period of time, the conventional water heaters cost less. I know the hype the tankless heater sales folks give you make it sound like this is the only way to go. but this is a vocal minority.