Are the tanks too big?

SpeyFitter

So I was looking at some specs of Bradford White Indirect Hot Water Tanks.

For their RTV series, a 75 Gallon RTV single coil can put out 225 Gallons of hot water at 135 degrees as a 1st hour rating. Thats with 113,000 BTUH input, with 8 GPM @ 180 degrees of boiler supply water, with a continuous rating at those BTUH's of 165 Gallons/hour @ 135 degrees.

And then it got me thinking about some houses I've been involved in. One house we did, about 20,000 square feet, we installed 4 of these tanks in series, each with their own mod-con IBC 15-150 (15,000 to 150,000 BTUH) boiler that has "opt out" mode to supply these hot water tanks.  Even with the 14 bathrooms I know this place has, I somehow think that there is WAY too much hot water capacity here when you consider the output of these tanks. If you're taking a shower, you're only going to need a faction of that 135 degrees of water usage to make your water comfortable. Even one of the ones with the fancy body sprays, etc. How many people are taking showers at the same time? Laundry, dishwashers, again? 

I'm curious if any of you guys have been surprised by how much you can downsize some of these indirects and still get great water output. Thoughts?

CMadatMe

Probably

I know what you mean. I can give you another example...Recently I was called where the customer had a 65 Gallon, 370,000 BTU nat gas water heater that failed in a school. The heater supplied the cafeteria area and janitorial area. After getting a price on the heater I scratched my head because of how much this htr cost.

After looking at the specs on the original htr and figuring out the demand. We were able to use a Vitodens 200 with an output of 212,000 and a 120 Gal Indirect. We tried to run the indirect out of hot water by opening every fixture and ended up flooding the floor drains. We couldn't get it to run out. While this installation did cost a few dollars more the customer did get Viessmanns Stainless Indirect and a highend boiler. The fuel savings will pay for the difference in less than 5 yrs easy. Belive it or not the cost difference isn't as much as you would expect. Hard to give you an idea as no pricing is discussed here.

Mark Eatherton

YOU have to ask the questions....

If this question is left up to the mechanical engineer to answer, then you can guarantee he will do it to the hilt.



If you sit down with the consumer, and ask THEM what THEY think the maximum potential DHW load is going to be, then and only then will you have a decent idea bout the potential of compounded loading.



Ask questions like, :How many people might want to shower at the same time, and how long will those showers last? What are the chances of someone wanting t fill a large soaking tub, and THEN have people jumping into the shower? What are the chances of a machine (dish, clothes, etc) calling for hot water during ANY of there other loads calling?"



Then, based on that information, design a system with the capacity to handle the loads.



I am curious as to why you piped your storage tanks in series instead of parallel? Seems to me the first boiler to see the loads is going to have a much short life than the last heater... If they are piped in parallel, the load is equally spread among the heaters, and they all contribute to the load. I am open to new ideas, and if you have a reason why you do it that way, I'm all ears.



ME

SpeyFitter

Not my call!

Hey Mark,

When we did this house, I was just an apprentice and did what I was told.There was 6 IBC 15-150 Boilers, 4 of which are dedicated to their own hot water tank (Bradford White RTV 75 single coil) on a priority basis if they called. The entire house was heated by radiant, a pool pack set up for an indoor pool, and snow melt. I guess the one benefit of piping it in series is you're less likely to lose all 4 boilers at the same time for the main load during extreme weather. These boilers have a built in cascade controller to rotate them with as I mentioned DHW priority opt out. Yes the one boiler will have to work harder while heating the most upstream tank but it's not like the other boilers won't be working as well. But looking at this objectively, factor that the dual coil rtv 75 has a heat exchanger that is almost perfectly matched to the IBC 15-150's capacity, and I'd imagine one of these tanks would easily be able to provide hot water for the entire house looking back on it objectively anyways.

I don't do the design for the company I work for, even though I'm a certified radiant panel designer, but I do question and scrutinize everything we install and I can see a lot of things I'd do differently.