Benefits of Small vs. Large Indirects

larry

If you have a whirlpool tub you need enough hot water to fill it in one shot. Depending on the size of the tub that could amount to 40+ gallons of hot water used in a 3 to 5 minute period. Some people might also have showers with body sprayers or multiple teenagers who take very long showers.

Steve Rockwell_3

Indirect Sizing

When I look at the specs for WM, Viessmann and Buderus indirect DHW heaters, it appears that even the smaller sizes (<40 gallons) can produce tremendous amounts of hot water. I can't imagine a scenario where we (family of 3, hopefully soon-to-be 4)would use over 150 gallons of 140* water in one hour, even on busy mornings. All these units produce that and more.

Given these specs, why do people opt for 80-200 gallon units? It seems that they are just paying to keep gallons and gallons of water hot that they will never use. By the way, assume use of a 150,000 BTU input condensing boiler.

Am I missing something?

Thanks for continuing to educate this analytical homeowner!

Todd_9

large indirects

We sometimes "oversize" the indirect to give a large dump load to fill a whirlpool, or if the boiler is on the smaller size a larger tank gives some leeway on recovery. You're right though, we took out 2-80gallon electrics and replaced with a 40 gal Buderus tank and they have yet to run out.

Unknown

Like Larry said...

Picture a 60 gallon Whirlpool with a 12 GPM imported faucet.

It'll fill the tub in 5 minutes.

Interpolate that out to an hour, and you need 720 GPH recovery or better, because the water cools as you introduce new water into the tank.

Or you could store the 60 gallons and use the boiler you have.

Noel

Steve Rockwell_3

So, no huge whirlpool tub, no huge dhw tank?

Sounds like the whirlpool tub is the only reason to get anything more than a 30-40 gallon indirect for a single family home.

Any other opinions?

Ron Schroeder_2

The size of the tank effects standby losses too. And maybe not in the same way as you might think. A well insulated large indirect might have LESS standby losses than a small one in some cases. The standby losses is not just from the tank but from the boiler too. With a large mass boiler and a small tank, the percentage of residual thermal energy in the boiler could be higher than the difference in standby loss of a large tank vs. a small tank it's self.

If the time between fires of the boiler is long enough for the boiler to cool off compleatly, all of it's residual thermal energy is lost and a large tank might have less number of times between fires to lose the thermal energy from.

I hope this makes sense. Does any body else think this might be possable?

Tom Giedraitis

I agree. We have a munchkin 140 with a superstor 45, problem is the 85 gal capacity whirlpool my wife and kids love. We can't just turn on the faucet and let it fill at full blast. Can't even get enough water in to cover the jets without stopping to wait for the tank to recover. It's worst in the winter (when you would typically take a bath) because the incoming water temp is so low.

I've got to be careful with the shower and body jets too since i pulled out the flow restricters.

Other than these really high demand items, I would have been fine with the 45 my contractor talked me into instead of the 60 or 80 I felt I needed.

Ron Schroeder_2

Just out of curiosity, has anybody tried an Ergomax that is smaller than the hot tub under similar conditions?

Constantin

Ergomax, Thermomax...

...as best as I can tell, these are beautiful HX's masqerading as water tanks. You can put unbelievably small tanks into large buildings, as long as you have millions of BTU's on tap to make the Thermomax/Ergomax sing.

IMHO, if you intend on having a small boiler, size the IDWH according to the largest dump load you can think of and keep the boiler smaller (i.e. closer to the heatloss). The dump load is defined as any whirlpools, body showers, and other high-flow appliances in the house.

How many of those appliances may require water at the same time is left to your imagination. An interview with the HO may help, but nothing is every definitive... teenagers, for example, can take forever to get showered so your flow requirements may be a lot higher than you'd think initially.