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Help me size my hot water recirc pump

david_cary
9 years ago

I have a 5 yo house with a 3/4 pex hot water loop for instant hot water. It currently has a small pump that takes about 10 minutes to reach the final faucet. My best measurements put the loop at at 200 feet.

I want to switch to a more on-demand situation with a faster pump to reduce the heat loss.

It takes 1 minute 45 secs to reach the final tub fixture if I just turn it on. Presumably this is a 2.5 gpm fixture head. That seems a little off since I think the volume in the loop should be 2.3 gallons and this fixture is really at about 170 feet from the hot water heater. I'm guess the fixture doesn't really pull 2.5 gpm.

The current pump is Grundfos 1/33 hp model I believe. My plumber suggested that I couldn't speed things up with another pump and that the pipes were limiting. Maybe for all practical purposes he is right and that is my question.

I'd love the fastest pump I could get that could turn on when people return home or enter a bathroom/kitchen.

Comments (11)

  • SaltiDawg
    9 years ago

    See below....

    This post was edited by saltidawg on Sun, Aug 17, 14 at 9:13

  • jakethewonderdog
    9 years ago

    Wow! Your tub is 170-200 ft from your water heater! That's really bad design.

    Ok, so the issue isn't the size of the pump. Your plumber is correct.

    Do you have a return line or does it go back through the cold water line?

    How is the pump activated now? (timer, thermostat on pipe, button)

    Here are a couple of ideas:

    1. You want to insulate the entire hot water line and return line no matter what else you do.

    2. Consider a small, point of use electric water heater in series with your main heater. This could fit under the bathroom sink or in a closet near the tub/sink. This would give you ~ 3- 5 gal of hot water immediately until the hot water gets to the fixture.

    3. If POU heater isn't doable, look at a circ pump that uses a thermostat mounted on the pipe. This will keep the water hot in the pipe at all times.

    4. If you want to limit heat loss through the loop, besides insulating the line(s), consider using a motion detector to trigger the pump. The motion detector would activate when you walk into the bathroom to anticipate your water usage but would otherwise leave the pump off. See link for an example.

    Here is a link that might be useful: Motion detection switch

  • jackfre
    9 years ago

    Metlund Design is the very best recirc system on the market. You will want the motion sensors. Www.gothotwater.com I would suggest that you put another water heater at the far end of the house.

  • david_cary
    Original Author
    9 years ago

    Right - It is a loop through the entire house. No fixture is 200 ft from the hot water heater but the last fixture is close.

    Hard to do motion sensors or demand switches when it takes 10 minutes to heat up.

    Hence the original question - what size pump to get?

    The pipes are insulated and the source is solar making a POU and 2nd tank less of an issue. But minimizing pump time on would still be helpful. I already have a timer and thermostat.

  • jakethewonderdog
    9 years ago

    Sorry, I'm more clear on the issue now. I missed that it takes 10 minutes for the pump vs a little under 2 if you just let it run.

    So Taco has a 1/8 hp unit that will give you just under 6 gpm at 10' of head. It was built for "on demand" use and has remote & motion controlled activation options.

    If you can determine the amount of head on your system, you can use the flow/head chart on any pump to see what you are in for. At 6 gpm, you should be down to something less than half a minute.

    Here is a link that might be useful: Taco On Commmand

  • david_cary
    Original Author
    9 years ago

    Great link. Thanks. How does one determine the head?

    Would really be helpful to know how fast I get before plunking down $300 on a new pump.

    30 seconds would be fantastic! Would make motion detectors etc completely doable. Especially since guest room is last and they can wait

  • jakethewonderdog
    9 years ago

    Use the link below to estimate your head loss on your run.
    The Friction Loss chart is on page 80 and the Fitting Equivalent Length of Tubing is on page 79.

    Since this is a closed loop, you don't have to consider the actual vertical rise. Also, only look at the fittings and such in the loop, not those that branch off the loop that the circ pump uses.

    So, for example, let's start at an initial guess of 5 gpm. A 100' of 3/4 PEX pipe shows a head loss of 5.9' in the Friction Loss chart on page 80. If you have 200' you have a loss of 11.8' just in the straight pipe.

    Then count the number of fittings in the chart on page 79. The number assigned to the fitting is the equivalent number of feet in straight pipe. So, if you had 4-90 degree elbows you would have the equivalent of another 37.6' of straight pipe equivalent that you plug into the Friction Loss chart - giving you another 1.48 ft of head.

    Add the straight pipe and fittings together and you would have 13.28 ft of head.

    At that head, the largest pump will move about 5 gpm (so our initial estimate on GPM for the pressure loss table is good).

    If you come up with a number of GPM from the pump chart that's significantly different than your GPM that you use for the initial estimate on the friction loss chart, you have to re-calculate the loss based on the new GPM number. It's possible you would have to do a few iterations before you get a good estimate.

    My guess, without knowing the fitting situation and based on 200' of 3/4" pipe is that you are probably talking about 5 gpm - so something about 30 seconds.

    Here is a link that might be useful: Feet of head pressure loss chart

    This post was edited by jakethewonderdog on Thu, Aug 21, 14 at 9:35

  • david_cary
    Original Author
    9 years ago

    The loop clearly wasn't built for speed. It actually goes upstairs and back down twice. I have to go back and look at photos but my first mental run though is 14 90 degree fittings.

    So looks like 3 gpm is closer.

    I have an option to bypass the guest room - either have them wait a little for water or flip a valve when we have guests. That removes 40 feet of pipe and at least 4 90 degree fittings. Would probably get me back to 4 gpm.

    Anyway - thanks for the help. At that speed, it is definitely worth the money. Even just to have the pump off when no one is home. But I think I can make motion detectors work also. Big project ahead. All to save $50 a year - completely irrational....(solar hot water and PV and 5 cents a kwh off peak rates)

  • Happy Hill Farm
    9 years ago

    My grundofos had temp setting on it. It runs until return temp reaches what you set it at. I put a timer on it to turn off at night but otherwise it runs whenever needed. Maybe that type of setup would work for you.

  • jakethewonderdog
    9 years ago

    maplegarden,

    That was what he was trying to get away from. He wanted to move to an "on demand" system which would require a more powerful pump.

    There is an amount of information out there that shows recirculation systems are very wasteful - far more so than the amount of water they save.

    An on-demand circulation system can provide the convenience, save the water and not waste nearly as much energy.

    Of course insulating the pipes is essential and will reduce energy loss as well.

    Here is a link that might be useful: Recirculation system cost in propane.

  • pima74
    9 years ago

    We use X-10 controls to turn our circ pump on and off as needed. It had a timer built in but we don't have ridged schedule that makes the timer useful. I timed how long it takes to push hot water to the master bath, kitchen, etc. and run the pump for that long when hot water is needed at that location. The exceptions are when the clothes or dish washer need hot water and we we leave it on for the appropriate cycle.. It's a small pump also so the down side is it can take up to 8 minutes to push hot water to the back bath room. The up side is that that bath is used rarely for guests. Since we have a septic system our water saving is related to not "over working" it and not using any more treated water than necessary (a social good since water rates are relatively low.)