DHW recirculation design

[roundrightfarm]

Greetings,
I'm designing a DHW recirculation system for a building that includes three separate apartments. The hot water pipe will split into three separate branches right near the DHW tank. The first branch will serve one kitchen which is very close and will not be recirculated.

The other two branches (about 50' each) will be recirculated. My main question is whether is should combine these two branches into one long (about 100') recirculation loop or if I should have them meet, share a common return pipe to the DHW tank and install a balancing valve to control flow rate through both branches. The building's layout makes it convenient to do either.

The first branch will serve two full bathrooms, two kitchens, one half bath, and a washing machine. The second would serve one half bath and one full bath.

I have no experience with recirculation systems and I'm not sure if I am just imaging that the common return setup would even be a consideration, so let me know if I'm overthinking this. The only real advantage I can imagine to it is that the hottest water from the DHW tank will get to all the fixtures sooner and perhaps allow a recirc pump schedule that turns on less often. Otherwise, it is more costly and complex to install, but I'm mainly thinking about the lavatories at the end of the long loop that may never see water direct from the DHW tank. What if they are used right before the pump kicks on. Will they often be slightly cooler that the other fixtures? Any thoughts or advice from someone with experience would be appreciated.

 

[Jadnashua]

Some recirculation systems utilize a temperature controlled valve at the point(s) of use. Often, the circulation pump can remain on, and the valves will modulate so that each branch gets what it needs to get hot. Those valves tend to shut when the water temp gets to about 105-degrees, and some are adjustable. Those probably do not need balancing valves, as the flow would be self-directed as things warm up at individual outlets.

If you design things properly with adequate slope, you might be able to achieve what you want without a pump, and only rely on convection loop(s).

Good system design says on hot water, the flow velocity should not exceed 5fps, and consider that many valves will not achieve the maximum capacity, so things would be slower. Then, without recirculation, it would take even longer to get hot where you want it, plus, purging out all of that room temp water is a waste of resources when you want hot, not counting if you're then paying to run it down the sewer. Given all of that, on a 50' run, even if you had maximum flow rate, that' at least 10-seconds to flush the line, and in reality, it could be 5-8x longer since something like say a bathroom faucet has no where near the maximum volume to achieve that 5fps maximum design rate (on a 1/2" copper line, that equates to about 4gpm, and on a 3/4" line, about 8gpm) - a vanity faucet is flow restricted to about 2gpm, and often, people would not turn it to all hot.

You'll need at least one check valve in the system to prevent drawing hot water from both the supply and recirculation line - how much of that mixing you'd get would be determined by the length and ultimate restriction differences between the two lines. While initially, the bottom the WH might be the same temp as the top (outlet), that quickly changes during use as cold gets added to the bottom of the heater (where the recirculation water goes, or might be pulled from without a check valve).

 

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[rjbphd]

Proper installed re circ line do not needs a pump and check valve.

 

[Jadnashua]

Without a check valve, when you're drawing hot water, it can flow through two different paths...how much flow you get with each line depends on numerous factors. But, consider that the 'return' line normally goes into the bottom (cold) side of the tank. Any flow from that port cools off the water at the point of use. It might not be much, but it entirely depends on the resistance of each path. Too much resistance, and a gravity fed path may not flow, and your recirculation fails to make things warm. A 50' path will need some minimum flow rate to overcome heat lost out of the pipe along the way, so insulation is critical, but insulation only slows the movement of the heat, not stop it. Depends on how and where the pipes are run. Someone living in say Florida verses Maine will have different design challenges.