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Community and Q&A

Tepid Water Tank Storage?

lance_p | Posted in Mechanicals on

CZ6A, Ottawa ON

So after reading through an excellent thread full of good info on water heaters and Legionella here:

https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/104870/sanden-water-heater-split-co2-heat-pump

I have a question regarding tepid cold water storage. My idea was to put all water entering the house through the Drain Water Heat Exchanger (DWHE) and then have the cold water line go into a cheap electric tank type water heater to be used solely as storage for room temperature/tepid water. The idea would be to eliminate that super cold ground water from making it to the faucets during hand washing etc. which would keep cold water off hands and reduce the number of uses of the hot water tap. Ground water here gets pretty cold.

Does anyone see an issue with a cold water storage tank sitting at 65-70F? We will be on a well, and drinking water will bypass this storage tank, having its own filtration and supplying a drinking water faucet as well as the water input on the refrigerator.

Thoughts? Are well water systems at risk for bacteria like Legionaire’s etc., and would a storage tank at room temperature foster growth?

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Replies

  1. Expert Member
    Michael Maines | | #1

    Lance, my guess is that water sitting at room temperature is not a good thing; I believe E. Coli and other nasties present in small quantities in well water would be able to multiply, but hopefully someone else has some solid info on why or why not.

  2. this_page_left_blank | | #2

    You should test your well water. If you don't have coliforms, you should be fine. If you do have coliforms, you have a problem in any case. When you get your water tested, there is no acceptable limit for those bacteria. I don't know a lot about Legionella, but Wikipedia says the growth range is 25-42C, and says its dormant below 20C. I'd say your risk is pretty minimal, although somewhat elevated compared to just having the water in your pressure tank, which in most cases will not reach room temperature. I'm not sure I really understand the advantage of bringing all the incoming water through the DHRU, but if you decide to do that I'd suggest you try to keep it on the down low from the building inspector. Since the DHRU is a code requirement, they might also insist that it be installed in accordance with the manufacturer's instructions.

  3. Expert Member
    MALCOLM TAYLOR | | #3

    Trevor,

    I think you mean E-coli. Coliform are just an indicator and there are acceptable limits for them in all drinking water sources. There are no limits for E-coli, mainly because it indicates sewage contamination.

  4. calum_wilde | | #4

    Lance,

    I keep tepid water in my house, for hand washing. I've been doing it for 14 years so far, since we moved in. But I've never installed a tank, I just use the water that in the pipe... I know that likely sounds sarcastic, but I'm serious. The water in the pipes comes up to room temperature pretty quickly, it's vary rare that we've run the water so much that we get ground water temp water to the bathroom faucets.

    Sorry to derail your post, but have you thought about water conversation instead of a tepid water tank to help reduce hot water usage? I've installed Niagara Earth 0.5gpm faucet aerators and 1.25gpm shower heads, and the dishwasher is run on it's energy efficient cycle. We don't use hot water for laundry but we've recently upgraded to an energy star rated HE washer that uses very little water, the toilets are set to their lowest fill setting, and we don't use hot water for hand washing. All told it's a very rare day that we have ground water temperature water at the taps while washing our hands.

    Then again, if your water distribution piping is made for such low flow devices than it seems more likely that it would be an issue. Mine is all 3/4" ...

  5. lance_p | | #5

    Testing the water makes sense, and then treating accordingly. I guess periodic water testing would point out an issue if one existed, and further investigation could be done if necessary.

  6. lance_p | | #6

    Trevor, the idea would be all fixtures minus the kitchen sink, refrigerator and drinking water spout would be plumbed with tepid water. Those drinking water sources would have a feed taken before the tepid water tank.

    DWHE manufacturers point out that exchange efficiency goes up with flow rate, and that the cold side of the shower valve should pull water from the DWHE as well as the hot water tank inlet. This way 2.5 GPM of flow through the shower head (and down the drain) is matched by 2.5 GPM of cold water flowing up through the DWHE. Plumbing only the water heater inlet to the DWHE results in something less than 2.5 GPM flowing up through the DWHE and efficiency dropping slightly, despite higher water exit temperatures.

  7. lance_p | | #7

    Calum, all valid points. I guess it depends on your plumbing layout, and even your plumbing as you point out with 3/4" lines.

    Our current home is plumbed with 1/2" lines for everything but the main trunk. If you flush the toilet in the master bedroom, the farthest point in the house from the water meter, the effect is that ground water has pretty much flushed through the whole system and is never more than a few feet away from any faucet. 20 minutes later the water will have mostly acclimatized, true, but I don't usually wait 20 minutes after flushing to wash my hands and brush my teeth. I have recently developed a tooth that's sensitive to cold, and rinsing after brushing can be quite uncomfortable sometimes.

    I have a low flow aerator on the powder room faucet. Unfortunately, that faucet is the closest one to the water meter and the farthest from the water heater. With the low flow aerator it takes a long time to get hot water to that faucet, long enough that you feel you're wasting your time waiting for it. I'm not sure what the flow rate of that aerator is, but my wife noticed it immediately so it's likely half or less of normal. It's pulling water through roughly 30 feet of 1/2" PEX before getting a signal from the water heater, and even when it does arrive it's fairly slow to warm up as it loses heat to all that pipe along the way. With a better plumbing layout this issue could be mitigated, no doubt, but it doesn't favor the eco aerator.

  8. calum_wilde | | #8

    Lance,

    You wrote "DWHE manufacturers point out that exchange efficiency goes up with flow rate". That's incorrect. From a post by Dana Dorsett "Efficiency is higher at lower flow rates, lower at higher flow. Efficiency is also lower at higher incoming water temps, higher at lower temps."

    *I thought about looking up a more credible source but I'm pretty sure we can all agree that Dana knows what he's talking about and is more than sufficiently credible for this discussion.

    You might be familiar with the thread I copied that from. :) https://www.greenbuildingadvisor.com/community/forum/mechanicals/104728/dwhr-and-hpwh-vs-electric-total-efficiency

    In response to your reply to my comment... Yes, I totally agree that waiting for hot water with the low flow devices is a pain. I looked at some information about on demand hot water circ systems, but I don't think the cost would be worth the convenience to us. I hope my previous post didn't come off the wrong way, I just wanted to make sure you considered the other options.

    Oh, and you might want to see a dentist. I put off a sensitive tooth too long once and ended up with a root canal. I would not recommend it.

  9. this_page_left_blank | | #9

    Malcolm,
    I thought that was what my wife (PhD in microbiology) told me, but I may have misunderstood or didn't remember correctly. But the takeaway is the same, if you have a contamination problem, it needs to be addressed irrespective of whether you have tepid water storage.

    I just did a search on the guidelines for drinking water in Canada and found this blurb:
    "The presence of any total coliform bacteria in water leaving a treatment plant indicates a serious breach in treatment and is therefore unacceptable." There are several other references indicating any level of total coliforms at all is a problem, for example:
    "If total coliforms are detected after implementing these corrective actions, a boil water advisory should be issued" (this is in reference specifically to well water)

  10. dickrussell | | #10

    "You wrote "DWHE manufacturers point out that exchange efficiency goes up with flow rate". That's incorrect. From a post by Dana Dorsett "Efficiency is higher at lower flow rates, lower at higher flow. Efficiency is also lower at higher incoming water temps, higher at lower temps."

    Let's not confuse efficiency (however you want to define that) and heat transfer rate. In a heat transfer device with fluid flow on both sides, film coefficients increase with velocity and thus turbulence at the boundary layer, so that overall heat transfer rate increases. But spending less time in the device due to higher velocity decreases the amount of heat absorbed or given up. The two effects work against each other. Assuming efficiency is the fraction of recoverable energy in the outgoing warm water that gets transferred to the incoming cold water, then slower flows would indeed give greater efficiency. If the flows are somewhat fixed, higher efficiency is obtained by a combination of greater heat transfer area and configuration that increases velocities and/or residence times. Greater velocities come from more but narrower passages, but that incurs greater pressure drop, which reduces pressure at the exit on either side. The warm side outgoing flow, presumably a drain, is limited by static head of the shower drain above the heat exchanger, while the cold side pressure is a matter of well pump or city main pressure.

  11. Jon_R | | #11

    Other things being equal, higher flow on the cold water side of a DWHE makes it more efficient. Don't agree - think about how few BTU will be extracted as cold water flow approaches zero.

    But the common case is that DWHE cold and drain water flows are matched - then "efficiency is higher at lower flow rates".

  12. Expert Member
    MALCOLM TAYLOR | | #12

    Trevor,

    I think the important distinction is whether the coliform is in the source, or in the water being used in the building. I'm a small water systems operator and until recently managed three community systems, two of which had surface water sources. It is pretty common to have coliform in the source, especially in the fall when accumulated debris is washed into the aquifer. Coliform is measured not because it is a problem, but because its presence can indicate the presence of more problematic things like e-coli.

    What's important isn't whether the coliform is in the well or stream, but whether it is still measurable after treatment. So as the links you read say, you don't want it in the house water-lines.Treatment is typically a combination of 1 micron filters, UV, and drip chlorination which keeps the lines disinfected.

  13. charlie_sullivan | | #13

    Jon is right about the different flow rates and their effect on efficiency: For a given drainwater flow rate, you capture more heat with a bigger cold water flow rate. On the other hand, for matched incoming and outgoing flow rates, you capture more of the heat when it flows through slowly.

    As for the overall idea, I find that with our DWHR unit, the cold water in the coils of the unit itself, plus in the piping from there to the faucets, is enough to make the hand-washing water room temperature rather than our very-cold groundwater temperature. Two-person househould--if you have 6 kids lining up to wash hands before dinner the results might be different. Another caveat is that I haven't had my hands tested for bacteria after washing so it's possible that I'm not washing them thoroughly enough, and that if I did the water would be too cold.

  14. this_page_left_blank | | #14

    Malcolm,

    I think we agree, but have just been talking about different things. When someone on a well gets their water tested, the source is an indoor faucet. Most homes on well water, at least where I am from, have no filtration or disinfection system at all. So that being the case, in order to pass the test at the faucet, the well itself would have to be free from coliforms also.

  15. Expert Member
    MALCOLM TAYLOR | | #15

    Trevor,

    That's interesting. I didn't know that was still common. Where I am here in BC all new wells get tested and no occupancy permit is issued until the results are clean. A lot changed post-Walketon. There was a general tightening of requirements, but more recently a backing away by local government to avoid liability - which I find a bit worrying.

    Being married to a micro-biologist must be both fascinating and a bit scary at the same time.

  16. this_page_left_blank | | #16

    Malcolm,

    A clean result is required for occupancy here too, it's just that a clean result is usually obtained without treatment of the water. Periodic (quarterly, semi-annually?) re-testing is recommended, but I've never heard of anyone actually doing it.

    The only scary thing is wondering if she's going to give me salmonella again. She was immune to the effects of the under cooked chicken because she had a low level exposure to it for years in her lab, but I was not so lucky.

  17. Expert Member
    MALCOLM TAYLOR | | #17

    Trevor,

    LOL.

  18. lance_p | | #18

    Jon R (and a few after) got what I was saying. Looking back I could have stated it a bit more clearly.

    The two most common scenarios for installing a DWHE are:

    1. Plumbed to the water heater inlet only
    2. Plumbed to both the water heater and the shower cold side

    With a fixed flow rate and temperature draining down through the DWHE, scenario 1 has a lower cold water flow rate through the DWHE, therefore a lower average Delta-T through the exchanger and a lower overall heat recovery. I'm sure the difference is small, but if the option is there you may as well take advantage of it.

    In heat exchangers where both flow paths are variable, absolutely, the lower the flow rate the higher the efficiency. This is the case with HRV and ERV cores which are much more efficient at lower flow rates.

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