Highrise - Hot water domestic issues

[Eng_Director]

Good morning everyone,

I'm in charge of a small 22 story building with an ongoing hot water issue that I can't seem to resolve. It's a hotel that has approximately 250 guestrooms, each with a shower/tub and sink. It's supplied from the bottom up from 2 hot water tanks (each averaging around 175F internal) and a pump to each of three zones. We are clocking in around 140F to 150F at each zone with a balancing valve for each stack of rooms. The issue I'm having is that we are constantly getting hot water complaints from a variety of rooms around the building. Everything on the BMS is showing good, but individual rooms in no apparent pattern are getting ice cold water when the shower is running hot. This can be as many as 5 to 10 rooms a day or none at all. It tends to happen more frequently in the morning when the demand is highest, but we're not getting any errors for heat or pressure on the risers at each zone. I'm at a loss for what could be the issue. Usually these problems will resolve themselves within the hour, but it's creating a lot of stress for our customers and our team. I've brought out some plumbers before to take a look at our system and they can't figure out what the issue is. So I'm hoping some peers might have some more insight on what I can look at.


Kind Regards,


Justin

 

[moon161]

Is a whole stack affected? I think you answer this by saying 'individual rooms/no apparent pattern'

Are there tempering valves at each outlet? Could they be failing to cold only for some reason?

 

[Eng_Director]

It does sometimes affect the entire zone's stack at a time, but it doesn't affect the entire zone. The building does have some scaling issues, but I don't think the building would be old enough to have significant internal clogging on the supply pipes. We don't use any tempering valves, just a balancing valve at the top of each stack in the zone and a PRV at the beginning of each zone branch from the main riser. Of course there are also random individual rooms across the building around the same time that doesn't seem to have any pattern.

 

[Compositepro]

There is probably a hot water loop used which maintains some flow in the hot water pipes at all times so that there is always hot water available quickly at every room. This requires return of the water to the bottom of the water heater tank. When water usage is high there will be a larger pressure drop in the supply pipe and water flow may reverse in the return line, causing cold water to flow from the bottom of the tank to rooms that are at the end of the hot water distribution loop. The recirculation can be done simply by natural convection, but in a commercial application I'm sure a pump will be used. These pumps are designed for low flow and low pressure differential.

Another factor that could be causing or contributing to your problem are air-pockets that accumulate at high points in your hot water line. If your piping goes up and down to get over structural elements, a series of small air-pockets can add quite a bit of pressure drop in the supply to a room at the end of the loop. This would cause highly variable problems with time due to gradual air accumulation that gets flushed out only at high flow rate.

 

[itsmoked]

Is the problem accompanied with a flow variation in the shower or is flow consistent and only the temperature is varying?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

Unless your water heaters are running out, there shouldn't be cold water, so there's some additional valving in the system. CP's idea about the recirculation loop makes lots of sense, since they're even used in single-family houses to minimize the lag. Without a recirculator, it can take more than 5 minutes to get hot water through about 60 ft of pipe.

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[LittleInch]

Any sort of diagram or schematic you can post?

Plus details of pressures and pumps?

We're a bit blind without some decent information

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[btrueblood]

"with a balancing valve for each stack of rooms."

Whose product? These things are notorious for clogging or jamming in a closed or near-closed position. Restriction due to a malfunctioning balancing valve in combo with the hot water loop piping that Cpro suggested would cause similar backflow issues at higher flows. It might be that the balancing valves are set to restrict flow even if they are working properly, i.e. they were set for an assumed maximum flow that is being regularly exceeded by people showering in the mornings. You could look at a pressure independent valve for each zone, with an actuator controlled by water temperature in the return loop.