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Thread: Understanding my tankless with recirc system

  1. #1
    DIY Junior Member AaronHartwell's Avatar
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    Default Understanding my tankless with recirc system

    Hey everyone,

    I'm new to the forum. This is my first post.

    I have a relatively large house (5000 square feet) with a natural gas powered tankless hot water heater paired with a recirc pump. We've been in the house for three years and have recently started to have some trouble with our hot water. This has led me to want to better understand the system as a first step towards diagnosing the problem we're having.

    The system:
    Takagi TK3 flash hot water heater
    Large'ish copper/brass recirc pump plumbed in to what appears to be a hot water loop
    Invensys SR301 3 zone relay which appears to be controlling the pump

    The problem:
    On occasions, when you turn on the hot water you get hot water for a while, then it goes cold for a while, then hot again and it will stay hot. This is particularly problematic in the shower scenario where you turn on the water, notice that it's hot, get half way through your shower, then get ice cold water for 3 minutes before the hot water returns.

    My Questions:
    1. What role does the 3 zone relay play when I don't have multiple zones? I assume the hot water loop is just a single zone and that there'd be a simple timer on the recirc pump rather than this big complex looking unit.

    2. I can imagine there being two different failures that might cause the behavior we're seeing. A) The recirc pump not running when it should such that there's a bunch of cold water in the lines. If that were the case though I would think we'd get cold water for a while before getting hot but once we got hot water it would stay hot. B) The hot water heater isn't kicking on quickly anymore for some reason such that we get the initial blast of hot water that was stored in the lines, then cold water as the heater is trying to cycle, and then finally hot water again once it successfully starts.

    I'm guessing it's B?

    3. If it is A, I assume I need to tweak the programming of the relay? I think it's programmable. If it's B, I assume I need to figure out why the hot water heater isn't starting quickly?

    I've read that recirc pumps and tankless hot water heaters often don't play well together and wouldn't be surprised if the builder's subcontractor did something dumb with my set-up. There have been a bunch of other things that we've discovered that are suboptimal and stemmed from going with the cheapest sub. Reading the TK3 manual seems to indicate that this particular unit will work with recirc pumps if set-up correctly.

    Sorry for the length of my post. I wanted to make sure I gave you guys all the information that might help.

    Aaron


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    Last edited by AaronHartwell; 03-22-2012 at 10:51 AM.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    I don't know the specifics of that tankless, but some of them just don't like recirculation. In fact, on some of them, it voids the warranty. If they do allow recirculation, it takes some fairly specific plumbing and maybe a buffer tank.

    Some tankless (and boiler) systems have a safety that will shut them down for a moment if they've been running for some period of time. This would be described in the user's manual. Also, over time, the tankless will become 'limed up' with mineral deposits, and is no longer as efficient on transferring the heat to the water through that caked on buildup. This could affect the sensors as well. They need periodic acid flushes to remove that buildup and maintain proper and efficient operation.

    If that dip in temp isn't caused by a safety cutout, then it may be one of two other things: you're running a cooler shower that isn't tripping the flow sensor because you're not using all that much hot water, or the flow sensor is sticky and not tripping properly.

    They may have used a 3-zone controller because it was easier to find. The price difference isn't much, and wiring it up is essentially the same, too. And, it gives you a couple of spare relays, should the one you're using crap out.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    DIY Junior Member AaronHartwell's Avatar
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    Thanks for the reply Jim.

    Yeah, I saw lots of information about tankless hot water heaters not being supported with recirc pumps for various reasons. The manual on this TK3 said it has a built in controller for recirc pumps and supports them. Of course that doesn't mean mine is set-up correctly for one.

    I've been wondering if there's some sort of maintenance I should do. It sounds like I should look into this mineral cleaning process. I lived in West Texas for a while and that water was AWEFUL for leaving deposits. I live in Washington state now and the water seems to be way lower in mineral content but I'll look into the cleaning thing. Thanks for that suggestion.

    I run a pretty hot shower and we used to not have the problem so I don't think cool shower is it.

    Good point on the 3-zone controller being easier to get as a possible solution for why it's there vs. a simple timer.

    Aaron

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    The controller is a pretty simple device, if it's what I think it is...it lets a 24vac thermostat/aquastat turn the 120vac power on/off to the pump...it could have a timer wired in, but in itself, all it is is a voltage changer/switch. It is normally used in a heating system. On the 3-zones, when any one zone calls for heat (or in your case recirculation), it also would turn on the boiler (and maybe your tankless system uses that input as well).
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    DIY Junior Member AaronHartwell's Avatar
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    Interesting. So it's really just a switch. That would explain why the manual doesn't mention anything about setting a temperature or time. I don't *think* the hot water heater has a timer built in but it does have a temperature setting I think. I'll re-read the manual.

    I wonder if I wouldn't be better off getting rid of the controller and just using a simple timer to control whether the pump is on or not. Basically I'd have it on for a few hours in the morning when we're likely to shower and in the evening for the same purpose.

  6. #6
    Moderator & Master Plumber hj's Avatar
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    There are two possibilities;
    1. The return line does not have a check valve, which could cause your symptoms, and
    2. You have a tempering valve so it #1 is not the problem then the valve has to be since it mixes hot and cold water together to get the proper temperature, (assuming the heater is actually running when you have the problem).
    Licensed residential and commercial plumber

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    I would start out by removing the controller and letting the tankless operate the pump and see if the problem goes away. Why do you have a controller operating the pump?

  8. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    There will be a pair of wires connected to the thermostat contacts in the controller...see where they go. That is what turns the pump on and off - it likely goes to an aquastat. Depending on how the tankless is setup would determine what you might do. The tankless' input could be a trigger telling it the pump is on, or it could be an output to turn the pump on (i.e., it could be using the internal aquastat as the control input to the pump's controller. Often, control voltages are 24vac, but the pump needs 120vac to run, so you can't just hook it up directly. You'd have to read and understand the manual and diagrams to sort this out.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  9. #9
    DIY Junior Member AaronHartwell's Avatar
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    HJ - I don't see anything that obviously looks like a check valve or tempering valve at the heater but it's possible that there's one there and I'm just missing it? Here's a better picture... What do you think?

    https://skydrive.live.com/redir.aspx...57C7FC0206!998

    Hackney - Good question on the purpose of the relay. I'm not sure honestly. This is the way the builder installed the system. I'm sort of wondering whether the relay box is needed.

    Jim - There's a skinny wire going from the heater to the relay. You can see it in the picture above. Then a set of larger diameter wires going from the relay to the pump. The wires going to the pump are probably power wires. The skinny wire going from the heater to the relay must be telling the relay when to send power to the pump. To your earlier point, it looks like the relay is really just a big switch that gives the pump power. It appears that the heater is actually the thing deciding when to turn the recirc pump on or off.

  10. #10
    Plumber Winslow's Avatar
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    I know that when piping a recirculating line on a flash heater nowadays the heater requires a storage tank (with elements), roughly equal to the size of the recirculating line to keep the recirculating loop hot. It doesn't cycle through the flash heater but circulates through the system. Rinnai has a couple of piping diagrams of how to do it.

  11. #11
    Moderator & Master Plumber hj's Avatar
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    That could be a check valve hiding behind the wiring, if so it could still be worn out. Hold on to the return pipe while someone is using the shower to see if it stays hot or gets cold.
    Licensed residential and commercial plumber

  12. #12
    In the trades Dana's Avatar
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    Quote Originally Posted by AaronHartwell View Post
    Thanks for the reply Jim.

    Yeah, I saw lots of information about tankless hot water heaters not being supported with recirc pumps for various reasons. The manual on this TK3 said it has a built in controller for recirc pumps and supports them. Of course that doesn't mean mine is set-up correctly for one.

    I've been wondering if there's some sort of maintenance I should do. It sounds like I should look into this mineral cleaning process. I lived in West Texas for a while and that water was AWEFUL for leaving deposits. I live in Washington state now and the water seems to be way lower in mineral content but I'll look into the cleaning thing. Thanks for that suggestion.

    I run a pretty hot shower and we used to not have the problem so I don't think cool shower is it.

    Good point on the 3-zone controller being easier to get as a possible solution for why it's there vs. a simple timer.

    Aaron
    You're right, unless you're on a particularly bad well, the water west of the Cascades has MUCH less mineral content than typical W-TX. It's likely this unit could run for decades without liming up.

    It's possible, even likely, that the hit of cold water you're experiencing is the standard "cold water sandwich" aspect of tankless HW heaters, which is not cured by recirculation systems. With every draw there is a necessary ignition delay that lets cold water pass through the heat exchanger with out heating it.

    With a recirculation system the hot water arrives at the tap and turns off the pump, so the water is always nice and hot when water is first drawn. But when the draw begin there are a few seconds of flow through the tankless prior to establishing the flame that will be unheated, but then followed by more hot water. To reduce or eliminate that feature a small amount of local hot water storage plumbed anywhere between the tankless and the tap works- even a 2-3 gallon point-of-use electric hot water heater (not wired up) provides sufficient dilution for that quart of cold or tepid water to bring it close to the average temp, smoothing the dip in temp experienced at the tap to near-undetectable levels.

  13. #13
    General Engineering Contractor ballvalve's Avatar
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    I have bottled water quality water. Do you have a hot water-electric type teapot? Mine has the element under a stainless steel bottom plate, and after about 2 months, the bottom is fully covered with mineral deposits. Imagine what goes on inside a pipe for 3 years with a gas flame on it.

    Pass thru water heaters are good for a little apartment or cabin with minimal use - unless you want to add into your life the pleasure of acid washing out a water heater every year or more. America had it made with tank water heaters that used to last 20+ years without much care. Autos have been gadgetized into repair oblivion, and now water heaters are being foisted upon us with hundreds of parts to go berzerk. I have a friend that took a sledge hammer to his flo thru after spending many thousands, and installed a 300$ electric job.

    Any good life cost anaylsis shows flo thrus in big houses to be a huge joke on the consumer.

    And the flo thru jokes requiring a tank water heater to make recirc work? That shows you how super salesmen screw you for their profit line. Why sell a tank rig with a 50$ profit, when the supplier can make 600$ on a POS flow-thru-plug-up-hot-cold-hot-cold mechanical nightmare?

    " Hey honey - instead of dinner and movie this Saturday, lets acid wash our water heater! then we can check all our tire pressures, the well expansion tank pressure, and triple floss our teeth" .... "Good idea love, we can also trim the cat and dogs toenails"
    Last edited by ballvalve; 03-23-2012 at 10:51 AM.

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    Quote Originally Posted by ballvalve View Post
    I have bottled water quality water. Do you have a hot water-electric type teapot? Mine has the element under a stainless steel bottom plate, and after about 2 months, the bottom is fully covered with mineral deposits. Imagine what goes on inside a pipe for 3 years with a gas flame on it.

    Pass thru water heaters are good for a little apartment or cabin with minimal use - unless you want to add into your life the pleasure of acid washing out a water heater every year or more. America had it made with tank water heaters that used to last 20+ years without much care. Autos have been gadgetized into repair oblivion, and now water heaters are being foisted upon us with hundreds of parts to go berzerk. I have a friend that took a sledge hammer to his flo thru after spending many thousands, and installed a 300$ electric job.

    Any good life cost anaylsis shows flo thrus in big houses to be a huge joke on the consumer.

    And the flo thru jokes requiring a tank water heater to make recirc work? That shows you how super salesmen screw you for their profit line. Why sell a tank rig with a 50$ profit, when the supplier can make 600$ on a POS flow-thru-plug-up-hot-cold-hot-cold mechanical nightmare?

    " Hey honey - instead of dinner and movie this Saturday, lets acid wash our water heater! then we can check all our tire pressures, the well expansion tank pressure, and triple floss our teeth" .... "Good idea love, we can also trim the cat and dogs toenails"
    The tea kettle typically gets way hotter than a water heater. The hotter you heat water the more minerals precipitate out of it.......if it contains minerals.

  15. #15
    Retired Defense Industry Engineer jadnashua's Avatar
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    Quote Originally Posted by Hackney plumbing View Post
    The tea kettle typically gets way hotter than a water heater. The hotter you heat water the more minerals precipitate out of it.......if it contains minerals.
    Yes, but do you make many 10's of gallons of tea every day?
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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