We bought the house (built in '61) a few months ago, and I notice that whenever we use the clothes washer, all pressure in the house drops to a third or less of normal flow. It doesn't do this with anything else (flushing, using dishwasher, etc. And opening a tap, etc.).

I did some investigating, and noticed that the water supply for the washroom (it was added on a few years ago) are tapped into the water supply ahead of anything else in the house, except the hot water heater.

Would this be the culprit, or should it really not matter? What do you recommend? Thanks!

First, it only appears your pressure drops, its your volume at issue. Is your incoming main supply a 3/4" or 1" line, or smaller? Tap for bathroom not the culprit unless the washer is in there too.

Bob's bang on. You have a volume problem. Most houses built in that time had galvanized piping. Over timer Iron Oxide,(rust) can build up in a 3/4" pipe and cut the opening down to the size of a pencil/ I had the same problem as you in my cold water. I ended up repiping the cold water in PVC. Problem solved. Good luck, Tom

Thanks guys. The water supply lines inside the house are all new... in the process of making some upgrades and remodeling, we ended up replacing every bit except for the washroom (laundry room) and the master bath, which was already redone with new lines.

It sounds like the problem may be the supply to the house... I think it's 1/2 or 3/4, but I'm not sure. It couldn't be a simple problem that doesn't require trenching my yard, could it?

If the service line from the meter is old galvanized then you have found your problem.

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It couldn't be a simple problem that doesn't require trenching my yard, could it?

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Sorry but NO! The solution would be to replace the old service line with 3/4" PVC. Good luck, Tom

At the risk of sounding ignorant, how do I check to see whether it is galvanized or not? I have a feeling that it is, but I want to make sure.

Ok. Find your main incoming supply pipe and scrape it with a knife. Shiny like a new dime then its galvanized. Shiny like a penny its copper