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Community and Q&A

Hot water recirculation tempering questions

user-314050 | Posted in GBA Pro Help on

I am currently living in my house’s finished basement while I am renovating the upper floors. I just passed rough-in inspection for the upstairs baths and kitchen, and I mentioned to the inspector that we were sometimes running out of hot water for our shower when the basement slab hydronic heating had been running for any extended time. I said we would be installing a thermal mixing valve so that we could turn up the temperature of the water heater and that we would also be installing a hot water fixture recirculation pump for the basement and a separate pump for the upper floors (both systems have each been plumbed in series with a return line at the farthest fixture).

The inspector told me he had been a plumber for 20 years before he became a City inspector and very generously gave some advice. He said that we should consider upgrading our water heater element to a 180 degree type. He also said we needed to install the recirculation line with a bypass line back to our cold water line to help guarantee the system would not allow scalding water to enter the pipes due to cold water stagnation or something to that effect. He attempted to briefly describe where all this should occur, but I did not comprehend what he was saying. He ended by saying that code does not require a tempering valve but we should certainly install one if we intend to raise the hot water heater temperature.

I did a Google search and only found 180 degree water heater thermostats and no reference to higher temperature elements, and those thermostats clearly state that they are for commercial applications only. I checked my water heater’s top thermostat, and it is set to about 125 degrees currently and can go as high as 150 degrees. Both settings are too high for scalding protection, so I’ll wait until I install the mixing valve before I adjust the temperature settings.

As a side note, when we complete our upper floor renovations, which will include in-floor hydronic heating, we’ll be installing a ground source water-to-water heat pump – the ground loop is already installed – so for now, the electric water heater is a stop gap heat source.

After an internet search on mixing valve performance problems, all I could find was this video by Caleffi on YouTube called “Installing Thermostatic Mixing Valves & Recirculating Pumps in Plumbing Systems” .

My questions are these:
1.I’ve attached a diagram showing my best interpretation of the Caleffi engineer’s recommendations, and I’m hoping someone can comment on whether I’ve designed it correctly? One complication I see is that I have two separate recirculation systems, and I’m not sure whether they will adversely affect each other’s performance.
2.Is there a better (and possibly cheaper) motion activated recirculating pump than a Taco 006-CT pump paired with Taco’s 554-4 motion sensor? I liked the fact that the pump includes a temperature sensor that would turn itself off when the return water reaches the set temperature.
3. The current heating need is for the basement in-slab a three zone pex hydronic system. Is there a reason the loops water temperature should not be higher? For the basement concrete slab, this may not be an issue, but the upper floor are wood with suspended Pex, and response times and overall performance, I assume, will be very different. In order to prevent the air temperature swings that result from the basement slab heat transfer lags, I installed a dual thermostat that shuts off the hot water when the slab at the thermostat reaches temperature, – it allows for more comfort too, because the slab remains at a more constant temperature with more pump cycles, though those cycles are still several hours long each. I understand that we could achieve similar results by pumping lower temperature water for longer periods of time, but we get into the same complication as with the fixture water tempering. So, with higher temperature heated water, is there a better way to modulate temperature for the hydronic system than the Caleffi engineer’s recommendation?

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  1. GBA Editor
    Martin Holladay | | #1

    Briefly, you are in over your head.

    I have some advice to offer -- not enough to answer all of your questions, because some of your questions would require me to do some research. But whatever advice you receive from me in the future, or other advice from kindhearted GBA readers who are smarter than me, will be no more or less than free Internet advice.

    If you don't have a thorough understanding of hydronic systems, you should hire a mechanical engineer and an experienced plumber rather than relying on the advice you will receive here.

  2. user-2890856 | | #2

    Brint , If I may ask a couple questions ?

    Why 2 circs for recirc as opposed to 1 and a zone control ?

    Why motion activated ? Every time someone enters the room these will activate and move water throughout the circuit . Taco smart plus is far superior or just a simple switch that activates the single circ .

    Why the 006 model ? Is this a Huge home ? The 003 is probably more than sufficient for all but the largest of homes . Research the Smart Plus and get rid of the second circ , you don't need it . Also look at the simple switch concept , much better all around , turn it on return colder water to source and replace with hot , turn off . No need having auto operation for hours at a time while you are at work or sleeping . This should only take about 30 seconds to refresh the hot piping completely and only when you need it .

    As far as the inspectors recommendations , They are not the best advice . 180* in a residential type water heater will void every manufacturers warranty . you should store at 150 - 160* highest , that is enough to eliminate any harmful bacteria such as Legionella and the like . Most quality mixing devices require a Delta of around 27* between the source water and the mixed temp also . Make sure that you purchase and install one that is pressure balanced also a sthis will lessen the chance of an accidental scald injury . Look at Caleffi 5231 series or Taco 5000 series mixers , they are safe and they are the best .

    To tell you whether the water in the slab can or should be hotter we need to know what temp it is now . Did you have someone design this hydronic system or did you do it ?

    Suspended tube usually requires higher water temps than a heat pump is capable of since it must heat the airspace between the insulation and the subfloor before any heta can go to the rooms it was meant to heat . Long story short , bad idea , not efficient and WILL present problems that you will hate thus giving Martin's argument against radiant more undeserving validation .

    Was a proper heat loss calculation performed for the radiant heating on a room by room basis . This is not rocket science , it is more actually much more difficult . Throwing loads of devices at a system to make it do things that can be easily achieved by letting nature do what she does is how these things fail . If these things were not done or done poorly your system will not be what you bargained for , i hope you did not bargain . There is usually only one chance to do this right and at first glance it appears you may be in trouble .

  3. user-2890856 | | #3

    You should heed Martin's advice Brint . It is well stated and well intended . Your drawing shows many hydronic circuit mistakes and possible disasters . I will comment on that no further .

    Martin , good observation and well said .

  4. charlie_sullivan | | #4

    It sounds like you have a short term problem that you are sometimes running out of hot water, and a long term problem of wanting to design a good system. The short term problem could be fixed with a tempering valve and a setting the tank to 150. Nothing complicated there.

    But the full design including the heat pump is quite complex, and getting high enough performance to justify the investment in the loops will require getting a lot of details right. So I have to agree with Martin and Rich that you should find a good geothermal/hydronic designer to work with.

  5. Expert Member
    Dana Dorsett | | #5

    If there is vertical drain pipe down stream of the shower accessible in the basement, installing the biggest gravity-film type drainwater heat recovery heat exchanger that fits would likely cover any showering performance shortfalls while decreasing the net energy used, since it's getting half or more of the showering heat out of the drain outflow. That's WITHOUT bumping up the storage temp (which would be worse for efficiency.)

    EFI ( )used to wholesale one vendor's lineup in the US (Renewability PowerPipe) but there are others. The big orange box store carries them too, but at a dramatic upcharge from EFI's wholesale price. I'm not sure if they're still carrying them, but it's pretty easy to open an account and get wholesale pricing from EFI.

  6. user-2890856 | | #6

    Dana ,

    Storage temps are a health issue . You may want to check out some ASSE standards . I don't often vehemently disagree with you but it is a proven fact that increased storage temp kills Legionellosis bacteria , increase the storage capacity of a smaller tank while giving the same or in most cases better outputs than the usually spec'd larger tank , the differential will also take longer to reach thus holding out the burner longer .

    Why should folks noit enjoy the same protection in their home as out in public ? There is a reason commercial equipmernt operates at higher temps , this is no accident .

  7. Expert Member
    Dana Dorsett | | #7

    Bumping it up to 180F instead of 140F to get the capacity is not a health issue.

  8. charlie_sullivan | | #8

    Dana has a good suggestion--not much more expensive than a tempering valve.

    The big orange price is not too bad...better than what my plumber tried to charge me...

  9. user-314050 | | #9

    Thanks for the advice Richard, I never considered a zone valve – I’ll look into it. My existing hydronic system valves open very slowly, so I’ll have to find a quicker acting type, if the system is to have an on-demand response.

    After reading about scald temperature times, I’ll shoot for keeping the water heater we have and see what 140 to 150 does for us (after we install the tempering valve!).

    The tempering valve I specified in the diagram was an ASSE 1070, which the engineer in the Caleffi video recommended as having better response and a narrower temperature range over the 1017 models you’ve listed. Though, all of the other system components are Taco, so I’ll look into their models per your recommendation.

    The system hydronic heating was designed by an engineer. He also sized and located the horizontal ground loops and there are only two things I would not have done per his designs: First, he had us putting all six pipes at 1’ o.c. in five foot wide trenches on our relatively small city lot as he designed it – there was not enough space to place the excavated soil, so, after the first 75’ feet or so, we dug two 2’ wide trenches (first digging one and backfilling, then digging the next one(s) parallel to it) and putting three pipes in each trench; Second, I would have put a heating loop below the center of both basement walk-in closets – they are located in the building corners and cool off over time, and it’s one of the most likely places I’m going to be barefoot, so it feels really cold compared to the bedroom floors.

    I had assumed we would need the sheet metal conduction plates for our upstairs pex loops, but the engineer said we would not require them with joists at 16” o.c and a loop in each bay, and, more importantly, because we are super insulating the house (double 2x4 depth walls with blown in cellulose, replaced the basement foundation walls with Quad-Lock ICF with 2-1/4” outer EPS and 4-1/4” inner, etc.), heating the joist cavity will more than suffice. I hope he’s right, but I also heed the warnings of easily overheating a well-insulated house. And, yes, I did all the work myself (and, boy, stainless, copper, and brass are expensive stuff, even ordered on-line). I will not be installing the heat pump myself – we delayed that expense because all we’d be heating is the basement until the upstairs is renovated. So far, the system has worked wonderfully.

    Dana, we are going to put in a heat recovery drain as soon as the upstairs is finished, thanks. This had the inspector scratching his head, wondering why all drains except the toilets went to the utility room. The three upstairs baths are not centrally located near each other, so I spent a lot of time drilling holes through 2” old growth joists over my head – gave me a new respect for the work plumbers do – it was exhausting. Unfortunately, there was no way that I knew of to recover the heat from the basement below slab drains.

    So, back to my primary question about the tempering valve. Is the engineer from Caleffi wrong? He seems so convinced of the potential problems. I’m sure to you professional engineers, plumbers, and HVAC guys, the solution he points out is a no brainer, but this is the first hot water recirculation system I’ve attempted to install, so I had to watch the 3rd quarter of the video four times before I got what the engineer was saying – without back pressure from the cold water side, just a return inlet to the heater will not allow the introduction of tempering water.

  10. user-2890856 | | #10

    Unfortunately your engineer should have paid better attention in some classes or at least researched some things as opposed to believing he knew it all . I believe you probably built this way to conserve energy and your hard earned dollars , am I correct ? You know that air space your engineer said is not a big deal ? For every 800 sq feet it totals the equivalent of a 16.6 sf room you are heating but cannot live in . Your engineer should write a letter to the school of higher education he attended and request a refund , as should many . Probably an M.E , right ?

    You should also amend your drawing to put the recirc water back into the device as opposed to the mixer . The mixer should receive fresh water from the main and the recirced water should be rehaeted to disinfect through the heater . Forget the zone valve , that was off the cuff sarcasm . You need no zone valves , you only need the pump sized to bring the water through the highest pressure return line and when the pump comes on when either room's control is activated the whole hot system should refresh . Throwing mechanical solutions at problems that do not exist is futile and why so many things quickly get out of hand and are frowned upon .

    Dana , Maybe you could tell me why higher temps are required in places of public accommodation and we must mix down at hand sinks and the like if it has not to do with health reasons . It could not possibly be that higher temps are required to sanitize could it ? What reason do we have to sanitize if not a health concern ? I'll have to inform ASSE , IAPMO , ANSI and the others that we can change our mission . In the meantime maybe you could work on the public water systems of which >45% contain the Legionella bacteria .

    The Plumber protects the health of the Nation ! Has been that way for quite some time and will continue to be . We are not all just ass cracks and thieves .

  11. charlie_sullivan | | #11

    Brint, I agree with Rich that the suspended tube is not a good idea. Yes, it can work with a good envelope and a moderately high water temperature. But the heat pump would be much moreefficient producing a lower water temperature.

    On the recirc, all the fancy stuff is to prevent a constantly running recirc from making the temperature creep up beyond the mixing valve setpoint. If you only run it on demand, e.g., with a pushbutton, the issue goes away.

  12. fitchplate | | #12

    Frankly who ever designed your system was not experienced in integrated DHW/hydronic heating systems. This is a basic community college hydronic heating system design class 101 issue. Boilers, thermostats and flow controllers should be piped and wired to prioritize DHW and its target temperatures and volumes, period.

    I would be going after the designer/installer. These not incompatible systems (DHW and hydronic in-floor radiant), they just need to be done right. One is an open system, one is closed and an HX connects them. Your designer is responsible for this screw up and its a common screw up in this line of business.

  13. user-314050 | | #13

    Richard, thanks again for your candid advice. We have not installed the hydronic piping upstairs yet. I’m not looking forward to it either – endless hours of hole drilling, pipe pulling, and good strategic thinking. The engineering was done seven years ago. We did not begin the project for several years and just occupied the basement last winter. Before I contacted the engineer, I read a lot on line, got Siegenthaler’s books and ultimately decided to hire a mechanical engineer. I have not done much reading on the subject of heat transfer since then, but after reading your response, I did an internet search on suspended tube vs. heat transfer plates, and it looks like the jury is out, both from predictive calculations and real-world failures that people are reporting. Even if my engineer did do his calculations correctly, I’m not sure it’s worth the gamble to not use transfer plates.

  14. charlie_sullivan | | #14

    Glad to hear you haven't installed the tubing yet. I can see both sides of the argument on suspended tube vs. plate ... but only if you are using a boiler. A boiler can supply high temperature water efficiently. A heat pump can't. What temperature water was it designed for?

  15. user-2890856 | | #15

    Charlie , that is not entirely true . A cast iron boiler can do it but only at design temps , at any other time it will short cycle to death and , it's a cast iron boiler . A mod con MUST have low return water temps to achieve anywhere near it's rated AFUE . Seeing as radiant floor heat requires a Delta T of at least 20* in any floor application and the fact that I have never witnessed a suspended required water temp below 140* even in a tight house it would require 150* out and a return of 130* . Barely condensing and under 90% efficient . This poor man has been screwed and did not get any of the fun .

    Besides that , there is not a heat pump I know of that will make that temp and have anything like what any of us would consider an acceptable COP . He would be much better off using that GCHX for cooling only if it even is designed well enough for that , I have my doubts by the use of a couple key words . The heating if the required suspended tube temps remain below 150* should probably be handled by the water heater , of course just not any water heater buy a quality piece of equipment
    specified by someone other than the Jamoke responsible for this soon to be nightmare .

    in all fairness , maybe I misunderstood the terminology used for the GCHX . Would the Op like to tell us exactly how that was designed and installed ? Ithink it was a horizontal loop config that was not spaced properly to dissipate or collect heat well .

  16. user-314050 | | #16

    The engineer did not provide any detailed calculations on how he derived the heating loads. It looks like he used software provided WaterFurnance (he was recommending WaterFurnance) to size the heat pump and plugged in the heat loss design load btu/hr and DHW design load down to 7 degrees OAT (Portland, Oregon). It does not list expected heating supply temperatures. Somehow I doubt if he did a Manual J or equivalent calculation. Although I understand the trust but verify mantra, I can't imagine that a guy who supposedly had designed many ground source systems would design an inadequate system. It looks like now there are many online heat loss calculators - eight years ago, before I contacted the engineer, I used a subscription software for a month. Anyone recommend a particular calculator? My original calculations were for me to decide how much insulation made sense for my climate, in terms of dollars spent and payback times, but I never really pursued it to any conclusions. But now, there has been lot more thinking on this subject, especially with the passive house movement, so I'll revisit it.

  17. user-314050 | | #17

    Update on the engineer's design: Tucked into the pocket of the binder the engineer gave me was a sheet with a building load summary - not room by room, but square footages per envelope type, and it has windows, light loads, people, etc. included.

  18. user-2890856 | | #18

    All those things are nice . Unfortunately with radiant 20,000 BTUh at 160* cannot be achieved with a machine that is only capable of producing heating fluid at 130-140* but has a size that would suggest that it can produce those BTUh .

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