Recirculating pump return on cold?

[Qwertyjjj]

With a recirculating pump and the return on the cold line, how does the pump pump against the pressure of the normal cold inlet into the water heater? Is there some sort of valve switch to prevent the main cold from interfering with the pump return line?

 

[Breplum]

a special crappy bypass valve is installed on a far sink in the cabinet below.
It allows water to push into the cold, making the cold water at that location quite warm.
A very poor but viable option to those who like to brush their teeth with hot water.

 

[Sponsor]

 

[Phog]

When the pump turns on, it draws hot water out of the heater and pushes it into the hot pipes. Since there is hot water leaving the water heater, some new water must flow in (to replace what's being lost). That in-flowing water comes from the cold water pipes. When recirculation is happening, flow is going backwards in the cold water pipe at the same time it's flowing forward in the hot water pipe. So you can hopefully see that the water just goes around in a loop. It's not pushing against the city water pressure, it's just going in a circle.

 

[Qwertyjjj]

I have access to the crawl space so could potentially put in a dedicated hot water return instead.
The only place it's really a concern is the kitchen. The rest of the house I could leave the water running a bit before using showers etc.
Any other options? I'd have to check the plumbing runs but maybe a more direct route from the heater to branch off. Heater has 3/4, kitchen has 1/2 reducing to 3/8. Seems like the bottleneck is probably the faucet size

 

[Qwertyjjj]

When the pump turns on, it draws hot water out of the heater and pushes it into the hot pipes. Since there is hot water leaving the water heater, some new water must flow in (to replace what's being lost). That in-flowing water comes from the cold water pipes. When recirculation is happening, flow is going backwards in the cold water pipe at the same time it's flowing forward in the hot water pipe. So you can hopefully see that the water just goes around in a loop. It's not pushing against the city water pressure, it's just going in a circle.

Sort of understand. Just confused by this image showing city incoming on left and return incoming from the right.
http://www.diyplumbingadvice.com/images/ACTExistingHomeDiagram.jpg

 

[Bannerman]

Just confused by this image showing city incoming on left and return incoming from the right.

With the pump shut off and no water use, the city pressure will be virtually equal in both the hot and cold lines. The pump will then only cause a pressure differential across the pump to cause flow from the hot to cold lines.

Heater has 3/4, kitchen has 1/2 reducing to 3/8. Seems like the bottleneck is probably the faucet size

The recirculating water will not be flowing through the actual faucet, but the hot line will typically connect to the return line directly prior to the 3/8" faucet connections so the recirculation will flow through the 1/2" lines.

 

[Qwertyjjj]

Ignoring the pump, if I increased the pipe size at the sink to 3/4 would it help with flow and therefore deliver hot water faster or am I always limited by the 3/8 pipe to the faucet?

 

[Reach4]

Ignoring the pump, if I increased the pipe size at the sink to 3/4 would it help with flow and therefore deliver hot water faster or am I always limited by the 3/8 pipe to the faucet?

If you supply the sink with 3/4 inch pipe instead of 3/8, the hot water will arrive slower. For the same flow at the faucet, water travels about 1/4 as fast in actual 3/4 than with actual 3/8.

 

[Qwertyjjj]

Then I have no option but to have a pump or make the 1/2 PEX shorter?
Doubt there is a 3/8 line I can use.
Routing goes
3/4 supply into heater
3/4 hot out
Converts to 1/2 run to sink
Then 3/8 faucet line

 

[Reach4]

Then I have no option but to have a pump or make the 1/2 PEX shorter?
Doubt there is a 3/8 line I can use.

3/8 pex exists, but usually costs more, and is harder to find, than 1/2 inch. Not a lot more nor a lot harder. But might need a tool you don't have also.

 

[Bannerman]

Then I have no option but to have a pump or make the 1/2 PEX shorter?

Unless you relocated your faucet closer to the WH or installed a WH nearby to the faucet, physically shortening the Hot supply line distance is probably not realistic.

By using a recirculation pump and return loop, the Hot or Warm water arriving at the faucet will only need to travel from where the Hot line crosses over to the return line. If the cross over location is directly before the faucet 3/8" connectors, then only perhaps 2' of cold water within the 3/8" faucet line will need to be cleared out before the faucet flow becomes warm/hot.

A dedicated return line is preferable. In using the cold water line as the return, a thermostatic valve or switch is typically installed where the Hot and Cold lines converge so as to stop recirculation flow once the water temperature reaches 95-100F. This will then prevent the cold water lines from becoming too warm and leading to the situation breplum mentioned in post #2. When using a dedicated return line, the return temperature will not be a concern so recirculation flow can continue non-stop if desired, and the water within the entire recirculation loop can be hotter.

 

[Jeff H Young]

I did install the system Breplum was against. at my Dads house which was brand new in 2001 , he was very happy with it. in master bath at his lav. some use a motion sensor but he opted for a simple decora light switch , guess he would get cold water first hit the switch and quite quickly have semi warm and then hot.
Yes not a very good system . never had a problem though in 5 years or so. we could have ripped the house to pieces and made a real good system. or done what 99 percent of cheap skates do and that is nothing at all. But to those that would like to make things a little better there are options. One problem with many houses plumbed in PEX have is running a circ line
I thought about installing one in my house but I'm too cheap myself. it takes minutes for hot at my kitchen coming from manifold in attic back down to first floor kitchen. I got plenty other projects than to bust open walls here

 

[Jadnashua]

Using a recirc system generally requires a few parts:
- a pump. Note, it must be either bronze or SS body, not a cast iron one used for hydronic heating systems.
- a check valve to prevent water from flowing from both the return line and the hot line at the same time to mix and become only warm when someone tries to use hot.
- a return path. This could be a cross-over valve if using the cold line, or a dedicated line.
- often, a control for either the flow or the pump.

Depending on how your home is plumbed, you may only need one return at the furthest point from the WH. But, if you have branches going multiple directions, you may need more than one, and to get things to work, you might need a set of balancing valves. Water will take the path of least resistance, and the likelihood of each branch being equal is slim.

It's inefficient to leave the recirculation running constantly. To stop it, you can either do it manually with a switch, or use a temperature controlled valve or switch. The system I have, shuts the pump off when it senses the water is about 95-degrees. Hot enough so that full hot isn't far away, but (using the cold line), you're not pushing much warm water into the cold line. Some of the cross-over valves shut off at about 105-degrees, again, for the same purpose.

You don't normally want to use a big pump, especially if you plan to leave it running constantly. The copper institute calls for a maximum velocity in copper lines for hot water of 5fps - on a 1/2" line, that's only 4gpm, and doubles when you go to a 3/4" line. Pex allows it to flow a little faster, but not all that much. With a smaller pump, it tends to work best if you have an automatic system that might run on a timer so that it can shut off when not needed, or the temperature has been achieved. On mine, the pump only draws 9W, but it can take nearly a minute the first time it runs in the morning (it's on a timer) then it shuts off. It only runs maybe 3-4 times an hour to keep that water warm. If you don't insulate your lines, that could be more often, and you'll be spending lots more for energy both for heating the water and moving it around.

Say you do not have a recirculation system and it takes a minute to get hot water in the morning. A shower head is going to flow a bit over 2 gpm, so you'll be wasting about 2 gallons of your available hot water supply. If you have a recirculation system running, the water in the tank AND the water in the pipes will be hot, and there will be little lost hot water when you want some, so you'll actually have more hot water to use. So that time you throw out two gallons of water you heated but can't use, especially if you're paying for your water and sewer, if you set it up to only run during normal use times, depending on your water and sewer charge rates, it may be cheaper to just run the recirculation system than not...and, it's certainly a lot more convenient to have hot when you want rather than having to wait.