Why does my apartment's water pressure drop so dramatically?

Question

Apologies in advance if this isn't the right place to ask this question (or if I haven't provided enough info; my plumbing knowledge is definitely lacking).

I recently moved into a new apartment (in Chicago). Sometimes our water pressure suddenly dramatically decreases in the entire apartment (all faucets/showers become mere trickles, or put out no water at all) for periods of usually 5 minutes or less, but sometimes longer.

The apartment is owned by a relatively sizeable management company. When we reached out to our property manager, they sent a company to pump the pipes and clear out sediment, and when that didn't really help, they said "The building is old, and has old pipes; you just gotta deal with it, there's nothing more we can do." When pressed about what "old pipes" means, and why that causes this issue, they had no explanation.

We're guessing that this happens when someone else uses water in the apartment building (6 identical units, 3 floors). For instance, there's a noticeable effect on water pressure when we turn on a faucet and a shower at the same time.

I'd understand a normal drop in water pressure at random times in the day, but to not have any running water at all seems unreasonable to me. It happens quite frequently; basically we don't have reliable water between 7am-2pm and 7pm-11pm.

Here's some more information about the issue, in case it helps: we're on the third floor. 2 of the units in the building are AirBnBs and often aren't occupied. Apparently every fixture in our apartment is connected directly down into the main line. Management has sent out a company to pump the pipes for sediment 6 or so times in the last half year, meaning our neighbors probably have similar complaints. And again, all fixtures in the apartment are affected by the water pressure drops when they happen. Both hot and cold water is affected. Also, I believe the building is about 90-100 years old, but I'm not sure.

Our questions are as follows:

1. What exactly might cause something like this? We want to know so that we have something to say to management when we press further. Like what is the mechanism of this sudden decrease in water pressure? Does "pipes being old" actually make sense as an excuse, and if yes, why?
2. If you have a guess about the cause of the problem, how costly/difficult might it be to fix?
3. This is unreasonable, right? To not have running water randomly throughout in the day? Or are we actually just being whiny and this is a common issue? (I need a sanity check)

Any advice would be really appreciated. Thanks very much for your time!

Answer 1

My guess is you are on the third floor. This problem is fairly typical of many apartment buildings especially in upstairs units. However, the low pressure you are describing is excessive and not normal. You might ask the plumbers to measure the water pressure as it enters the building (curbside pressure) and then after the pressure reduction valve (PRV), if there is one. Usually, curb pressure is about 125 PSI plus or minus 20.
If it's much lower it could be a sign of corroded or leaking supply lines that could be the responsibility of the city/town or the apartment building owner, depending on the location. If the curbside pressure is in a normal range and the apartment has a PRV it may be set too low to provide adequate water pressure, especially to upper floors during high usage periods. In a typical home the pressure after the PRV is in the 50 to 75 PSI range. If it's less than that, there are things that the property owner can do to boost pressure to upper floors, including adjusting the PRV, booster pumps, etc. Whether they would is an open question.
In an older building you could have corrosion that has built up in the pipes over the years that could be part of the problem.

Edit: If you have a sillcock somewhere in your unit (clothes washer, deck) you can purchase a screw on water pressure gauge very inexpensively to measure your pressure and record the readings along with time of day to show the owner.