\"Reverse Indirect\"/ Combined buffer-indirect tank

Keith_21

So what's the final verdict on the superior product? ErgoMax or Turbomax? Turbomax has better marketing, but you really can't base a design decision on looks alone.

Thanks a lot
Keith

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Keith_19

Mass buffer. .

I know I hate answering questions from people if they haven't bothered doing research of their own at first, so I took the time out to read up on this particular topic before putting together this email. I was contacted by a working class yet mechanical and educated customer in East Meadow who has put it to me to give him an estimate on the most efficient system that he could possibly have. A simple description of the situation -
- The home is approx. 40 year old two story dormer, extremely (extremely) well insulated, with a design heat load of only 65 MBH.
- The occupant lives alone during the week while his wife stays in only for weekends.
- There is an original monoflo tee system emitting heat worth 27 mbh.
- A basement baseboard loop of 22 mbh
- A remodeled bathroom/kitchen with under tile radiant tube of 10.7 mbh.
- The remainder of the design load is in a currently unheated garage, which mayl be heated in the future.

After pouring over the textbooks, I found a reference in Siegenthaler's "Modern hydronics. . ." on page 526 of what he calls a "combined buffer/DHW tank" system. Other people call it a "reverse indirect", where domestic is piped through the coil of the indirect, while the submerge water fullfills the role of a buffer tank. The customer is keen on this idea, seeing that his hot water requirements are quite low, and it seems as if very small equipment can be installed, namely a Burnham MPO84 or Buderus G115/21, both with DOE ratings of 74 MBH - well above the actual space heating load of 60,000 btu/h. It also makes for less worryisome operation of the low temp circuits during spring and fall when they may be on alone.

The serious question I have about designing the system is about guaranteeing that hot water will be available to the occupants. Even on design days, an excess of 14 mbh can theoretically heat up to 22 gallons per hour to 120 degrees F, but I'm not sure how to determine if that will actually happen from the literature that is available from the manufacturers. Burnham provides "coil heat transfer area" in square feet, but that's about all. According to Siegenthaler, I shoud be sizing the actual buffer to be about 25 gallons, perfect for the Burnham Alliance 26A(which has 6.5 sq. feet of transfer area), but again, DHW is concerning me. Will just winging it with a 40A(with 8.6 sq. feet tranfer area) be enough?

I realize that there are times when a person can paralyze themselves with accuracy, worrying about every last little number, and I normally would play it safe by oversizing just a tad, but in this case I really have no idea what a reasonable number would be.

Oh yeah, will outdoor reset even work in this system, given that the buffer tank needs 180 or better at all times to generate hot water for the occupants?

Anyway, I appreciate you taking out the time to help me along with this. The customer seems like he really cares about efficiency and long term operating costs, but I'm a little sheepish that I can't snap to attention and tell him exactly what he expects. The truth is you don't run into that many people like him. . . In terms of operating efficiency, if this isn't the best system for his application, maybe you can steer me in the right direction.
See you soon and thanks again,

Keith


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KAG

Alternatives

Have you thought about a tankless on-demand unit, like a Rinnia. They only make hot water when you use it. If they want the highest effiecent system this would be a good option for their needs.

hr

as for the reverse indirect

our host here at this site keep four girls in hot water with an Ergomax in his own home

I've installed a few in carwash applications. Their output info is real, if you have the BTU power to drive them as the tables show.

I would also prioritize the DHW load so when you have a DHW call all the heat zones shut down for a short period. This assures you get the full boiler output to cover the DHW load.

Plenty of threads in the archives about that style tank and it's use.

This link has a favorite of mine. http://www.pmmag.com/CDA/Archives/3a1d8c312dfc7010VgnVCM100000f932a8c0____

Maybe contact Mike and get his opinion after 4 years of running on those systems.

My biggest gripe regarding that tank is the need for additional piping connection ports.

Also the kleenex thickness insulation they ship them with. Way to thin for a tank that sits at 180 or more all day!

Fairly easy to remove the jacket and take a few laps around the tank with bubble foil, however.

hot rod

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Guy_6

hmmm

22 GPH only comes out to .36 GPM- hardly enough for MY shower.
I would think that a good full mod. boiler with outdoor reset would eliminate the need for the buffer tank.

Joe Brix

SInce we're talking oil

Mod/con is out. But to get decent recovery for 2 people taking back to back showers, I'd want 60K+ BTU to feed the indirect. Maybe the TR36 indirect from Phase 3 would be a better option. A regular indirect with larger heat transfer surface area and it holds 12gal of boiler water. Now you're able to use simpler piping and outdoor reset with priority.

Maine Doug_37

Ergomax

I have two 80K Biazi's feeding an E44 Ergomax. Usually only one boiler runs the building and DWH but the huge clawfoot tub requires both if the heat is on. The Ergo provides boiler water mass, min temp is 140*.
This indirect supplies DHW water for my studio and home. It supplies much hot water.
The Ergo tank insulation is minimal, I mean really thin. The thermo sensor well was not properly installed at the factory and the tank has serious rusting as a result. So if you get one, take the jacket off, tighten all factory fittings and add insulation.

But it does make a lot of hot water.

Fred Schaefer

If mod/con is out, a 2 stage would do the job...

Jed_2

That's why

I specify "Turbomax"(Thermo 2000) rather than "Ergomax". Turbomax rates standby loss at < 1/2°F/hr., which is the same as the styro-insulated indirects on the market. It has 4 amply sized tappings and commercial grade gauge/PRV.
Lifetime warranty residential, and 10 YRS. COMMERCIAL. Ever hear of a 10 year warranty on a commercial water heater? Outputs are amazing, and relative to the HGS. Up to 9 parallel monster coils( from 19.6 ft² - 58.9 ft² heat transfer area). I have a few out there "doin the job".

Jed

Garry_3

Have anyone replaced the aquastat on the Ergomax with one that has a wider differential? In a buffer usage situation, I would like to use an aquastat with an adjustable 30F degrees differential instead of the 7F shipped with the unit. Can anyone recommend a replacement?

hr

Any basic setpoint control will work

I have used the Ranco brand as well as HeatTimer, tekmar Honeywell, and Johnson Controls. Most have a 30 or 40 degree adjustability on the differential.

Johnstone Supply has an excellent selection. Find one with the same diameter thermistor or you may have to change out the well in the tank.

I still have a few prototype RTI's with 100 degree adjustability. I think HeatTimer build a handful for them once upon a time.

hot rod

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Ron Schroeder

I use a Tekmar 150 on my indirect with the differential set to 40 degrees.