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Community and Q&A

Hot water re-circ line & vertical head loss

unknownco | Posted in GBA Pro Help on

Hi all,

I’m trying to size a pump for our re-circ line. We have a 1 story house, with a tankless water heater in our attic at one end of the house. Our 2 bathrooms are at the other end of the house.

The hot water runs from our 6 gallon reserve tank horizontally for a few feet and then down probably 10’ into the slab, and then across the house in the slab for about 55’ to our master bathroom shower. Then the return line begins around the shower and goes up about 10’ back into the attic and then runs back the 55’ through the attic, back into the tank. I have already done all the head loss calculations for the piping, with a target flow rate of about 1GPM and the head loss from friction is around 3.41 feet, and that’s probably high because I was running a worst case example.

My question is, how come all the head loss calculators I have found say nothing about the vertical head loss in systems? Is this something that I need to factor into my calculations for a pump? I found one article that said something about the vertical lift is not important because the return line is brought back with gravity. But that’s assuming that the water heater is in the garage or basement level pushing water up and then the return line is coming back down – which is not how our system is designed. Should I be factoring in the 10′ push from the master shower back into the attic – where the return line begins, if there is also a 10′ drop from the water heater down into the slab?

Any clarification regarding vertical head loss and residential re-circ pump sizing would be much appreciated.

Thanks

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Joe,
    When sizing a recirc pump, you have to calculate the head (or head loss). To do this, you have to know the total length of the pipe, its diameter, the number of fittings, and whether or not there are any check valves. The vertical distance from the top of the loop to the bottom of the loop is irrelevant.

    For more information, see:

    http://www.instructorworkshop.org/App_Content/instructorsworkshop/files/PRESENTATIONS/2010Presentations/09_GRUNDFOSHOTWATERRECIRCULATIONPHCC.pdf

    http://www.clevelandaspe.org/Orlando%20Symposium%202013/High-rise%20Design%20-%20Hot%20water%20Recirculations%20and%20General%20Pumping%20Practices.pdf

    http://shop.solardirect.com/pdf/water-heaters/solar-hot-water/active-solar/pumps/grundfos-guide.pdf

    http://www.pexuniverse.com/content/how-size-circulator-pump

    http://www.taco-hvac.com/uploads/FileLibrary/100-41.pdf

  2. charlie_sullivan | | #2

    I agree with Martin.

  3. unknownco | | #3

    Martin,

    Thanks. This is exactly what I did - I actually used the taco-hvac calculator.

    It sounds like I need a new plumber. He sized the pump way too high because he said the water has to push against 10' of head pressure to make it up the shower wall into the attic where the return line starts.

    Purely academic - can you explain 'why' the vertical distance is irrelevant? Something to do with gravity or the system being pressurized? I found some info in the links you posted - the Grundfos presentation, that I actually found yesterday - but it's still not clear to me because our re-circ loop is not the same as most examples I find of the water heater in the basement etc.

    Always appreciated...

    Joe

  4. GBA Editor
    Martin Holladay | | #4

    Joe,
    If you're pumping water from a deep well, and the top of the water column is exposed to the atmosphere, you have to lift the water up. Water is dense, so that takes a lot of energy. You're bringing water from down there to up here. If you had to pull it up with a bucket at the end of a rope, your arms would be tired.

    The opposite situation is a reservoir of water open to the atmosphere at the top of a hill. If you stick a pipe in the reservoir and run the pipe downhill, and put a hydroelectric generator at the bottom of the hill, you can generate electricity -- because the flowing water has energy which can be extracted. Instead of applying energy (as you do when you need to pump water uphill), you can get some "free" energy.

    When water circulates in a closed loop, the water flowing downhill balances the water being pushed uphill. You don't have to add energy to the system to pull any water to the top of the hill, because it doesn't stay there. You just need a little bit of energy -- the energy required to overcome the friction of the pipe and fittings.

  5. charlie_sullivan | | #5

    Another way to describe this: because it's a completely closed system, the water flowing down creates suction at the top that helps lift the water flowing up.

    If you were pumping water up for a decorative fountain, letting it flow down in the open over some rocks, there would be no opportunity to create suction at the top, so then the pump would need to do the full work of pumping it up to that height.

  6. Expert Member
    Dana Dorsett | | #6

    And the recirculator pump sizing issue all beside the potential efficiency problems/losses of having a tankless water heater in the attic instead of inside conditioned space, and located at the opposite end of the house from the bathrooms...

    Maybe there's a valid reason for putting it there, but it's hardly optimal.

  7. unknownco | | #7

    Thank you all for the explanations... Most appreciated.

    Dana,

    We just moved in to this home a year ago. The hot water heater hook up was always at this end of the house - where the kitchen and washer are. Even if we moved it to the other end of the house - at a large expense, we'd still want a recirc line / pump to bring hot water to the sink in the middle of the house and the kitchen. We installed the small 6 gal reserve tank to run the recirc line.

    Also, the attic will most likely be converted in the near future and thus eventually be conditioned.

  8. Expert Member
    Dana Dorsett | | #8

    The money is definitely going to be better spent on conditioning the attic than moving the hot water heater!

    In an ideal world the HW heater would be located something like halfway between the kitchen & bathrooms, but there's zero ROI on moving it if it all ends up in conditioned space.

  9. unknownco | | #9

    Dana,

    Yes - I had debated putting the tankless in the middle of the attic at least, but the furnace is in the area we'd put it, and the other free space is prime attic conversion space.

    In a perfect world, I would have like to gut the entire house and redo everything exactly how I wanted. And the funny thing is - we love the house and the layout, and for the most part things were done right. But I'm noticing all the time that I like things done right and the contractors I've been dealing with just like things done.

    Thankfully for everything else there's Green Building Advisor ;)

    If only you had a network of contractors who are as informed as you all are, and could send them our way.

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