Tankless Recirculating Loop

Discussion in 'Tankless Water Heater Forum' started by ChuckS, Mar 17, 2009.

  1. ChuckS

    ChuckS New Member

    Messages:
    96
    Location:
    Aurora, CO
    I recently had this full house tankless water heater installed and it was taking just over a minute to get hot water out of the facets. I installed this recirculation system which helped a little but I think it can be improved.

    This design uses a flow switch in series with an aquastat to turn the pump on as soon as any hot facet is turned on and does not turn off until the water in the pipes get hot. The advantage of this recirc system is I can quickly turn on then off a facet when I go to use the restroom and the water is hot by the time I go to wash my hands. The bad part is it didn’t really reduce my initial time to get hot water as much as I hoped.

    Studying this as I have, I know the length of time it takes me to get hot water is a product of all the water sitting in the pipes plus about 6 seconds for the heater to begin kicking out hot water. Because I have mostly ¾ pipes, according to this calculator which is just about dead on it was initially taking just over a minute to get hot water.

    The recalculating system cut the time down to about 45 seconds but I think it can be improved. According to the calculator, 45 seconds over 35 feet means my flow is about 1.5 gpm. I measured the output from the facet by timing how long it takes to fill an empty 1 gallon jug and it comes to exactly 1 minute. I believe this means I gained exactly .5 gpm flow rate by adding the loop.

    Since my pump is rated up to 6 GPM, the loop is completely ¾ pipes and I have ½ pipes to the facets I know I should be able to get this loop to move more water. I believe the problem with my recirculation loop is that the water has no where to go. Yes it circles water back around and through the heater but it competes with the city water which by my guess means I am getting .5 gpm city water and .5 gpm from the loop which equals my 1 gpm out the facet.

    What I think will work is some kind of tank about 2.5 gallons in size which will quickly suck in the water from the pipes when I first turn on the water then slowly release it to the input side of the water heater as the water is being used. I was hoping the expansion tank would do this but either I don’t have the air pressure just right, it needs to be relocated or I am not getting an increase in pressure when the pump turns on.

    Can someone suggest some kind of tank, it can be mechanical, electric (30 second timer for example), float valve or anything that can be rigged to take in about 2 gallons of water from the loop then via a pump or whatever, release that water back into the system as we shower etc…

    I hope this makes sense but if not please ask questions because I desperately need your help…

    PS... The way I adjusted the expansion tank was I took it to grease monkey and had it filled to the max pressure of 80 psi. I installed it in the loop and make sure no wanter was going in the tank. I then slowly started releasing air until I could feel a little water in the tank. I figured I was right on the edge and that any increase should put more water in the tank. I am a DIY layman so don't laugh too much...
  2. SewerRatz

    SewerRatz Illinois Licensed Plumber

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    1,705
    Location:
    Chicago, IL
    Last edited by a moderator: Mar 4, 2014
  3. ChuckS

    ChuckS New Member

    Messages:
    96
    Location:
    Aurora, CO
    Yes, I saw that in one of the threads when I began snooping here. Unless I am reading it wrong, it runs on a timer. I wanted to stay away from a timer because I only want to make hot water when I need it. Otherwise, I don't see much savings over having a tank. You are really just using your recirculation loop as a low efficiency tank.

    I wished I could find more on how exactly it works so maybe I can model something after it and even order that tank like a replacement part.
  4. SewerRatz

    SewerRatz Illinois Licensed Plumber

    Messages:
    1,705
    Location:
    Chicago, IL
    The way the suggest you run it is with a timer that is built into their remote. You can still use a flow switch and an aquastat to turn the pump on and off.

    In your case what some people do is install a 6 to a 12 gallon electric water heater in the recirculation loop and use an aquastat to turn the pump on and off as needed.
  5. ChuckS

    ChuckS New Member

    Messages:
    96
    Location:
    Aurora, CO
    I see, too bad I didn't see that model before I had mine installed. unfortunately I don't have another $2600 for a new one.

    I thought about putting a small electric heater in the loop but I am hoping I can do this with nothing running until I turn on the facet. I'd be happy if I could get it down to say 20 or 30 seconds.
  6. SewerRatz

    SewerRatz Illinois Licensed Plumber

    Messages:
    1,705
    Location:
    Chicago, IL
    Was the recirculation loop there before the tankless heater? If not how long did you have to wait with a tank?
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,133
    Location:
    New England
    I think part of the problem is that there's a flow restrictor in the tankless. If you put an expansion tank in there and lowered the pressure it could be worse since it would store more water that can cool off. I agree, you don't want to have the pump run continuously or even on a timer. The best way is probably to install a small heated tank fairly close to the main point of use to provide hot water until the tankless can purge the lines.
  8. SewerRatz

    SewerRatz Illinois Licensed Plumber

    Messages:
    1,705
    Location:
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    The units I have put in do not have a flow restrictor, they naturally have a pressure drop due to the water having to pass through the heat exchanger. I know some of the lower end models do restrict the flow to ensure they can perform as advertised.
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,133
    Location:
    New England
    WHether the pressure drop is because of a dedicated flow restritor or the tortuous path through the heat exchange, the end result is the same...it will limit the flow.
  10. SewerRatz

    SewerRatz Illinois Licensed Plumber

    Messages:
    1,705
    Location:
    Chicago, IL
    Going through the path of the heat exchanger is not much different than going through a tank with its extra piping. I have tested the flowing pressure in my home as well as a few others, seems to be a 5 PSI differential between the cold and the hot. Also checked with a flow meter there was no noticeable change in the flow rate. That was with tanked heaters, with tankless like the Navien, and Nortz the pressure differential was around 5 to 9 psi. Again no noticeable change in the flow rate. Now with the Bosch units I came across there was a 15 to 20 PSI drop and the flow rate was cut nearly in half.
  11. ChuckS

    ChuckS New Member

    Messages:
    96
    Location:
    Aurora, CO
    No, the tank heater was installed much closer to the facets so there was no problem then. This tankless system was installed at the far end of the crawlspace which had to do with my natural gas service and the fact that the heater uses up to 199K BTU.

    I also linked to the wrong heater in my initial post, I actually have this one. It is a Paloma 7.4 Series which wasn't cheap if you ask me...

    http://www.tanklesswaterheaters.com/ph28rdv.html
  12. ChuckS

    ChuckS New Member

    Messages:
    96
    Location:
    Aurora, CO
    This is true, I first installed the expansion tank with the 40 psi it came with and found the tank stayed full of water. It was doing no good. That is why I had it run up to the max then reduced it until I got a little water in it. I may try putting the expansion tank on the output of the pump and using a check valve to separate the pressure from the city water.

    Thanks for confirming my thoughts, it doesn't seem like you're gaining any efficiencies with those methods.

    I may do this as a last resort but I would have to put one in two bathrooms, laundry room and the kitchen. I was hoping there was a better way.

    My main concern are the dishwasher and clothe washer. By the time the water gets hot, they stop taking water in.
  13. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,133
    Location:
    New England
    The reason you don't want the pump on continuously is that the flow through the tankless would cause it to essentially run all the time the pump was on. There are some configurations approved by (at least some) tankless manufacturers to add a recirculation system. What does your installation manual say about it? That would be the best way to do it. If not covered in the manual, call or e-mail the manufacturer. You may find that it voids your warranty.
  14. ChuckS

    ChuckS New Member

    Messages:
    96
    Location:
    Aurora, CO
    That is where I lucked out and another reason for a recirculating system like mine. I should have checked the warranty before installing it but looking now it appears I am still covered.

  15. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    Well, you did this backwards. One of the advantages of tankless is the ability to locate the units closer to the demand locations, so that the delay problem and the inherent energy and water waste is minimized. You should have run the gas line necessary to put the tankless in a more appropriate location.

    But that is water over the dam. Now you will spend condsiderable time and money trying to fix the problem. Without a PhD from MIT to calculate this head pressure/flow problem, the obvious answer is a 2 gallon electric up close to the demand location.
  16. ChuckS

    ChuckS New Member

    Messages:
    96
    Location:
    Aurora, CO
    My wife reminded me what he said, he said we would have to get the gas service upgraded by the utility company if we wanted to run a longer gas line. He also said there was no clear space to vent our the side of the house because of proximity to windows, doors etc... Talking to a guy today, he said the utility company would have upgraded the gas service for free... Is that true?
  17. SewerRatz

    SewerRatz Illinois Licensed Plumber

    Messages:
    1,705
    Location:
    Chicago, IL
    Only people that can answer that question is your gas company.

    I know here in my area, the gas company has a form for us to fill out with the current gas meter installed and current BTU demand, and the proposed demand with the install of any new appliance. They will tell us if it needs to be upgraded or not, and if it does need upgrading and it is due to the higher demand of a Tankless system, the will upgrade the gas meter at no charge.
  18. ChuckS

    ChuckS New Member

    Messages:
    96
    Location:
    Aurora, CO
    Please look down at tank 59401-001 - PRECHARGED WATER SYSTEM TANKS.

    How does this in line tank behave differently from my expansion tank?

    Something I see as a good thing, there is no chance of water going stale by remaining in the tank too long. My real question is does it expand with different characteristics than my standard tank?
  19. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,133
    Location:
    New England
    An expansion tank for a typical WH is probably only 2-gallons or so. It is designed to basically have very little to no water in it normally and the bladder only flexes a little bit to account for the water expanding. A storage tank is designed to actually store a fair amount of water (nowhere near the total volume...or you'd overstretch the bladder). Because the pressure is not cycling like it would with a well system, the storage tank would fill up and basically stay there if used on a municipal water system. It would maintain a supply if the main went off for some reason, though.
  20. clocker33

    clocker33 New Member

    Messages:
    2
    Re: Recirculating Loop

    I didn't read the whole thread here so this may be old news.

    Some people, with room temperature space, remove the insulation from their tank heater and use it inline as a preheater or replace it with storage tank.

    This is another idea that won't speed up hot water delivery but is a cheap way to increase efficiency: Gravity Film Exchange.

    It uses waste water to preheat the tankless. It needs access to 5 feet of vertical waste pipe to work but is said to rase incoming water 10-15 F degrees.
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