Hot water priority in apartment building
March 15, 2021 5:01 PM   Subscribe

I live in a 3-unit apartment building. All the apartment units receive hot water from the shared boiler (I'm pretty sure it's a boiler, which I believe is in the basement). Which unit gets priority in terms of hot water (that is, the first to receive hot water) if hot water is low and beginning to run cold? I believe it would either be the top or bottom unit.

For instance, sometimes there is a drastic decrease in water temperature - I'm not sure if this is from a washing machine cycle or something else. Then it returns back to normal within several seconds.
posted by thesockpuppet to Home & Garden (8 answers total) 2 users marked this as a favorite
 
If it's in the basement, I would think the closest unit to the basement. However, all units could experience a drop in pressure and/or a drop in heat fairly simultaneously if another unit turns on their hot water.
posted by amanda at 5:17 PM on March 15, 2021 [1 favorite]


I think it depends on how the piping runs. I'd guess there's one hot pipe from the boiler and it goes to apartments 1, 2 and 3 via branches. So there wouldn't be a priority given to any single floor - when the hot water is there it goes equally to all and when it's gone then the unheated water would go equally to the three apartments. Of course, if say apt 1 was nearer to the boiler than apt 3 then 1 would get hot water sooner after turning on the faucet than 3 would, but they'd both run out when the boiler runs cool.

When there's a sudden drastic temperature drop, that's more likely due to a pressure drop at the boiler, thus the water at the shower would be cooler because less hot water is being mixed in.
posted by anadem at 5:52 PM on March 15, 2021 [1 favorite]


The other 8 ball in this is "is there a circulation pump?"
I know on modern systems where fixtures might be distant from the water heater, we put in circ pumps that keep hot water flowing in the pipes - a loop - so you don't have to run cold water for a long time before you get the hot.
In that case, no one has priority.
posted by rudd135 at 6:03 PM on March 15, 2021 [2 favorites]


I see two questions - first, who gets hot water is very much depends on how the system is configured - a "Trunk and Branch" system is what anadem is describing, and is the traditional configuration. Zoned systems update that to include active flow control, but both of these lend themselves over time to uneven distribution of hot water. Repairs, corrosion and build up alter the flow, even over short runs. A manifold system is more modern and should include devices that limit water hammer and pressure differentials. And recirculating, as rudd135 notes, loops the hot water through the system back to the heating element.

As for the second issue, a rapidly closing valve, like those on a dishwasher, or washer, can cause water hammer and it can also draw cold water out of the standing water in the hot water pipes. That should be a brief 'shot' of cold water. A small arrestor can be had for 15$, and easily installed inline, and full sized can be had for 45$.
posted by zenon at 6:58 PM on March 15, 2021 [2 favorites]


Honestly, it depends on how it's piped. It also depends whether there's a recirc system for the hot water.

Either way, it sounds like the system is undersized and/or has water hammer issues.

Source: I am a plumber
posted by cnidaria at 1:47 AM on March 16, 2021 [1 favorite]


Also, fun and possibly related fact: ideally fixtures like showers *should* have a special valve designed to prevent scalding when water pressure changes.
- Pressure-balancing valves rebalance the pressure so you don't get scalded if, for example, someone flushes the toilet and the cold pressure drops.
- Thermostatic shower valves are a bit fancier and maintain both temperature and pressure.
posted by cnidaria at 1:52 AM on March 16, 2021 [1 favorite]


If hot water is running out, ask the landlord to increase the temp. If the water is hotter, each apt. uses less hot, more cold, and the hot lasts longer.
posted by theora55 at 6:45 AM on March 16, 2021


If the water is above 120 F, scalding is a concern. In the US, it's generally prohibited to have the water over 120 F. Some more sophisticated systems have it at 140 F for Legionnaires prevention, but they have a thermostatic mixing valve setting the hot water temperature back down to 120 F before point of use.

This may sound silly, but people have been injured and died from scalding, especially babies and elderly folks. Setting the water heater above 120 F in a multi unit situation is not a safe plan (or likely legal, depending on your location).
posted by cnidaria at 7:18 AM on March 16, 2021 [2 favorites]


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