Recirc or storage tank?

GroundUp · February 7

This pertains to my personal home, being built in the spring. All heat is to be hydronic and I'm planning to employ a reverse indirect for both buffer and domestic usage as there are a few micro zones as well as wood, gas, and electric boilers. This is a slab-on grade home and the mechanical room is 50 feet away from the master. The master shower (and occasional bath) will be the highest DHW load in the house while the master sink or adjacent guest bathroom seldom uses any hot water. Laundry is also on that end with a similar distance but water temp is of lesser importance for those first 30 seconds. I'm toying with the idea of a small electric point-of-use tank for both sinks (cold inlet fed from the hot line, if usage goes past the tank capacity), and putting a recirc pump on the 3/4" line feeding the showers/tubs with a push-button timed switch to bring it on only before bathing for 1 minute to avoid wasteful recirculation. Does it pay to go to a larger storage tank with a recirc on a temp sensor, or even a second indirect since I'm running large diameter hydronic piping over there anyway (4 zones on this end so one main w/ zone valves makes more sense to me than home runs back to ZV in the mechanical room for each one)? I don't generally play with domestic plumbing so I'm not sure what the proper channel might be for something like this. Our current setup uses the switch/recirc approach for the master shower and it works pretty well but it's only half as far and uses a 1/2" PEX home run system which I would like to avoid in the new house as I prefer a copper main with branches to each fixture. Insight would be appreciated, thank you!

Larry Weingarten · February 7

Hi, not sure I'm answering the question, but here goes 🤠 I'd use demand controlled recirc, but keep the recirc loop at 3/4". Use a pump big enough to prime it with hot water quickly. Run this line no more than ten feet from all points of use. Use small branches off the main, like 3/8" where you can… which should be most everything but any tubs. This arrangement gives quick hot water with minimal energy or water waste.

Yours, Larry

hot_rod · February 7

I did see the new Grundfos Alpha recirc pump at AHR, it has a lot of features built in to minimize run time, and customization

Temperature control, timer, and ability to call it on with the app

I have a crossover valve on mine, but running constantly warms my solar preheat tank, so I am trying one of the new recirc Alphas

GroundUp · February 9

Nobody? Surprising considering the wealth of knowledge here

Jon_blaney · February 9

I use a manual system. Turn the hot water on in the tub, count to 10, your good to go.

bjohnhy · February 10

You didn't mention what size primary tank you were considering. I cannot imagine you would ever run out of DHW with a reverse indirect with the home you are describing. What would the extra tank be for? or just upsizing the primary tank?

When you said 2nd indirect, Did you mean a 2nd indirect tank (smaller?) near the master shower? I think that scenario would result in standby losses with less efficiency than a Point of use electric tankless heater. Oh, you were not thinking of point of use tankless. You were thinking point of use small tank, 1-2 gallon? I think this small electric tank idea is really good in theory, but it is one more thing to corrode and leak, and I would not recommend it when you have a dedicated recirc line as an option.

Your return recirculation line would be 3/4" copper? That'll lose heat fairly quickly. It is great that you have the "on demand" tech knowledge. I would insulate those hot pipes anyway if it was my house. I would probably use 1/2" pex return recirculation line, and if i needed the hot water there faster just bump the circulator up to medium (or high) instead of low.

I am actually considering the Point of use tankless for my kitchen. I think overall that may be more efficient than recirc for my house. But not necessarily more efficient for you, especially in the wintertime.

GroundUp · February 10

https://forum.heatinghelp.com/discussion/comment/1887101#Comment_1887101

40 seconds is a bit obscene, otherwise that's exactly what I'd do.

GroundUp · February 10

https://forum.heatinghelp.com/discussion/comment/1887188#Comment_1887188

Running out is not of concern- waiting 40 seconds for it to get to the master bath is a big concern. The POU tank was my solution for the two sinks, since HW is almost never needed there but possibly fed with the hot line from the reverse indirect rather than the cold, in the event that we need a little hot water after a shower or whatever without using additional electricity to heat up the POU tank. When I said 2nd indirect, yes that was a potential solution to the wait time and standby loss of heating up the lines (which will all be insulated copper- no PEX). If it's possible to utilize a small storage tank in addition to the recirc, I'm open to that as well. Just looking for some real world input from an actual plumber with experience in this department.

bjohnhy · February 12

POU tank can be a very reasonable solution but can come with some headaches that are worth noting.

Most homeowners prefer the tank hidden inside the vanity but many of the bathroom cabinet openings are just not big enough to accommodate some of these small tanks (even the 1 gallon models), particularly the cabinets with a center stile. You have to ensure the tank dimensions match your clearances. Well, just install the tank as the vanity is being installed!… but…when it needs replacing…that is a real pain.
Inevitably, the tank will corrode and leak, sooner than you want it to. I find the average lifespan is shockingly lower than 40 plus gallons with an anode rod. These POU leaks are not like leaking in the mechanical room next to the floor drain. There can be a lot more potential for water damage in the master bath. Just make sure the appropriate drain pan is there …and leak detector.

IMHO, the on demand recirc pump alone would be enough to meet your needs for hot water at sinks and/or shower. And, if you wanted something more, go with a tankless fed by the hot, IMHO. (Stiebel Eltron direct coil, at least 30 amp model). New construction, easy to run that circuit.

Recirc or storage tank?

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