Need Help Choosing an Indirect

Mike T., Swampeast MO

Boiler: Vitodens 6-24 (fine to suggest the appropriate Viessmann but would like alternates--read somewhat less expensive)

Worst case reserve capacity for indirect: 3.8 gpm flowing through space heating system @ 1½' of head loss. 20 mbh remaining burner capacity.

Reasonable reserve capacity for indirect: 2.0 gpm flowing through space heating system @ 1½' of head loss. 50 mbh remaining burner capacity.

Physical requirements of indirect: 1" connections for domestic. Must be suitable for use in a gravity recirculation system. No real restriction on size/shape of tank.

Special consideration: Will use my current stand-alone water heater as a "tempering tank". You can assume incoming water to the indirect at about 60° minimum.

Utter worst-case DHW load: 12 gpm @ 130° for 5 minutes, or 60 gallons @ 130° in 5 minutes. This for a "car wash" shower. Have measured one household member's shower temp at 130°! Not important to be able to have this much water this hot and this fast in extremely cold weather. Some cooling towards the end of the five minutes is acceptable.

More reasonable DHW load: 6 gpm @ 130° for 12 minutes or 72 gallons of @ 130° in 12 minutes. This for filling a large tub with ¾" Roman filler (90 gallons) at approximately 110° bathing temp. Includes 12 gallons from cold side over period @ 55°.

hr

Sounds like you

are talking about more than one tank to cover the least to worse case! Maybe an instantanous just for the BIG (carwash load) case?

Not many of the small indirects have 1" DHW connections. But a 3/4 X 1" reducer (or increaser) is a simple way around this. Very little pressure drop, or restriction, in that short section of fitting. I doubt it would bother your gravity recirc system.

hot rod

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jeff_51

use instantanius as a booster

will only come on as temp from indirect falls

larry

Mike,

What is your water quality? Do you have a lot of hardness minerals? Is ease of cleanout a concern?

Shawn_9

indirect

TURBOMAX has by far been the best indirect I've installed so far.My company installs the Buderus S120 32 gal. tank in a typical install.Higher needs and demand get the TURBOMAX 23,which is a 26 gal. tank.Aquastat set at 160 degrees,and a tempering valve at 120.
TUROMAX is the opposite of standard indirects being the domestic runs through the coil exchanger.
High output demands,I would recommend a TURBOMAX45 ,48 gal.,32.7 sq.ft. of exchange surface.1 1/2 domestic and boiler tappings,and your boiler better be able to provide the BTU's for an excellent recovery rate.

Mike T., Swampeast MO

Thanks All

HR: Am coming up with absurdely sized tanks--think I need to lower the expectation!

Larry: Very alkaline and quite hard. I have a love-hate relationship with water softeners. Not sure if one will be used. None at present.

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Am I approaching this correctly:

Assuming that whopper shower load, am consuming 500# of water with a temp rise of 70° or 35,000 BTUs in five minutes, or a rather incredible 420,000 btu/hr.

Quite cold (for the area) weather gives me 40,000 btu/hr available from the boiler or 667 btu/min. If I assume that the boiler will run for 4 of those 5 minutes, the boiler can only contribute 2,667 btus.

35,000 - 2,667 = 32,333 BTUs that must be available from storage.

Say tank starts at 140°. Allow temp to drop to 110° after 5 minutes. Doesn't that give me 30 btus of storage per pound of water? 32,333 BTUs required/30 btu available per pound = 1,078 pounds or 129 gallons.

YIKES!!!!! Am I doing this correctly? Does stratification in the tank help that much?

Mike T., Swampeast MO

GOOF UP ABOVE

Forgot that Vitodens has a built-in diverter so all output can go to DHW.

So, I get about 80,000 btu/hr or 5,333 BTUs in 4 minutes.

29,667 req. from storage.

With same allowable drop tank drops to "only" 119 gallons. UGGH!

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Looks like I need to size for the tub and say, "If you run out of hot water in the shower, turn off some sprays or turn down the temp!"

Mike T., Swampeast MO

Using same method for tub but allowing 30° temp drop in the tank (from 140° to 110°) tank works out to 91 gallons.

Recommendations for a 90 gallon indirect that can transfer 80mbh (1,333 BTUs per minute)? With 40° temp drop across indirect, need 4 gpm @ 8½' (3.6 psi) max head loss.

jerry scharf_3

I'm being dense

Mike,

I read what you were saying, and I don't get the approach. Maybe we justthink about sizing DHW demand differently.

First, I think the shower at 130 is just unsafe and should not be planned for. I didn't think you could get shower fixtures that go above 120F any more. Also for the car wash, maybe you could find a way to plumb in a GFX waste heat recovery system. It reduces the demand and saves money down the road. Just hook it up to the shower and feed the cold water for the shower back through it. Won't be optimum, but it makes the setup as easy as possible.

Second, Im not sure I get the tempering tank idea as described. Why would you run a lower efficiency device all the time? Seems like it should only operate at peak demand times.

If you look at the 72 gallon demand for the tub, the question that follows is "how soon do you need to be able to have how much more water?" So you get full heat for at least the first half of the tank, then water temp starts to drop as there is inevitable mixing. You probably get better than 3/4 of the tank worth of hot water. So before any heating, my rule of thumb says you need about 95 gallons of tank to fil the tub.

On the heating side, you've got 80 MBH of heat to deliver with around 80 F gain needed (1000 BTU per hour per degree.) with an ideal HX, the first hour recovery is 100+ gallons, but I would derate that by the tank transfer efficiency and the time you need to take away to heat the house.

Now if you raise the water temp to 145 (makes Dave Yates happy too) you shrink the tank size a bit at the cost of slighty lower efficiency.

I have a T50, which puts out 80 MBH in indirect mode on a 105G dual coil tank. I also have a 90 or so gallon tank. With 60F incoming water and 105 F bathing water, I was shocked how quickly the tank recovered, and the boiler uses the smaller of the two coils.

My Stiebel Eltron tank has all 1" connections, but I agree with Hot Rod about the reducer not being much of a problem. It also has 3" of foam, so loss aside from the pipes is not bad at all. It's not stainless, but I think it is going to last. It has a large cleanout port as well.

jerry

Mike T., Swampeast MO

Thanks Jerry. I'm most likely the dense one here. First time I've sized an indirect--they're almost unheard of around here. Probably wouldn't bother myself 'cept it's the first step in my solar integration system.

Always heard that indirects gave very fast recovery (and usually less capacity than a stand-alone), but the new Vitodens has less input that my current stand-alone (110 gal) and WAY less than the old boiler.

Have Grohe thermostatic mixers that are about 10 years old. They actually go up to 135°, but you do have to press a button to get temps above 100°. I can't handle that high temp, but some actually seem to like it even if they do look like a lobster coming out...

Tempering tank (old heater) just hooks in series to the indirect supply. It won't be operating, but will give a little "free" heat as the incoming is warmed to basement ambient. Have done that in student rental houses and it seems to work nicely.

Hadn't thought of using a GFX dedicated to the big shower by feeding back to its cold supply. That will actually work with the piping.

If you look at the 72 gallon demand for the tub, the question that follows is "how soon do you need to be able to have how much more water?" Not much and not too soon. Piping can handle the flow, but I'm already being enough of an energy hog as it is. Tub has an in-line heater, so at least you don't have to add more hot water for a long soak.

Guy_5

why?

I'm not trying to be critical here, but how come my toilet is limited to 1.6 gallons per flush, yet I can drown myself in the shower? Wasn't at least a part of the logic to conserve fresh water?

Unknown

Good point

Where's the conservation when you have to flush twice to get rid of the deposit ? Well , I do have a cheapo HD model for now .

Mike T., Swampeast MO

Well understood Guy. Truly highest possible flow and not frequently used that way. Usually just one 1.5 gpm shower head used, quick scrub with the back brush, then a few bracing (doesn't even have time to get hot) seconds from all the body sprays for a quick, complete rinse.

Constantin

May I suggest....

....looking into the us of a NTI Trinity 400.

It has 16-1 modulation and a firing range of 25-400kBTU/hr. You can heat a lot of water at the upper end of the firing range while modulating down low for the heat load. That would reduce the size of your indirect needs somewhat and speed recovery.

Mike T., Swampeast MO

Need to use the Vitodens as DHW is an integral part of my solar scheme to greatly reduce cycling when the domestic heat load is tiny. At least it looks like I'm getting a good-sized "dump" for any excess solar heat...