Indirect replacement for direct fired

BigEHead

I'm looking to replace my direct fire hot water heaters with an indirect because they are getting to the end of their lives. I currently have 2-75 gallon 75k BTU heaters and was wondering what your opinion is on sizing. The larger indirects are 119G but not sure if that's overkill. I'm not sure how to compare to the above. I have a 165k heating capacity boiler currently with 5 zones. I will eventually replace the boiler but right now I will most likely add a new zone for the indirect before replacing the boiler. Thanks for your insight!

mattmia2

No one ever complained about too much hot water. The only risk is that it costs more to install and has slightly more standby loss. It also means in the summer that maybe the boiler runs less often so you have less losses from the boiler itself cooling after every cycle.

BigEHead

thanks matt. oh i don't mind going too big, but i wasn't sure if 119G is "too" big compared to what i have now. the lochinvar squire 119g has a 431g 1st hour rating. i don't know how to compare that to the 2x75s i have. i believe it's .8 x capacity so .8 x 150 = 120g? not sure if that means i could get away with going with even a 30g indirect which says it has a 184g 1st hour rating.

mattmia2

The first hour ratings in the manual will have some specifications about what boiler they are connected to. You need it to be connected to a boiler with that output, supply water temp, and flow to get that rating.

hot_rod

Any way to get a handle on how much DHW you actually need, or use. You can use past history or a load calculation. Or out a meter on the system for a month to measure actual consumption.

Larry Weingarten

Hi, I agree with understanding what your hot water demand is first. What’s the biggest “dump load”, like tub filling and anything else that’s happening at the same time? How many gallons?

Yours, Larry

BigEHead

Good question, it's a relatively large house with 5 showers, a jetted tub, 15 sinks, 2 dishwashers. I'd like to size for "peak" usage for what is in the house, not for what we use only because based on our usage, it'd be pretty low. Is there a way to compare the capabilities now with the direct fired vs indirect needed?

EBEBRATT-Ed

I looked up a 75 gallon direct fired. first hour is 125 gallons after that it is 80 gallons. You can work those # against the indirect.

Of course, knowing your actual consumption would be better. Putting a meter on as suggested would be best

BigEHead

Thank Ed. So since I have 2 piped together, I'd get 250 gallons first hour and then 160 gallons after that. A Lochinvar Squire 52 Gallon indirect is rated at 255 gallons first hour, so I could just get a single one to replace the 2-75s that i have now. Am i thinking that correctly? So a 119G would be way overkill.

Hot_water_fan

@BigEHead the trick is that indirect ratings depend on the boiler attached - a 165kbtu boiler can get you close to 255-52 = 203 gallons for the first hour. The rough calculation is 8.34 x gallons/hour x (outgoing - incoming water temp). So 8.34 x 203 x (122-32) = 152kbtu, which your boiler may or may not be able to provide based on its efficiency. Besides cost, not much downside to going bigger. 

BigEHead

Thank you for your insight!

EdTheHeaterMan

To calculate the first hour rating of a tank type water heater, you take the recovery rate of the water heater and add that to 80% of the tank capacity. As an example I use a 40 gallon electric water heater that I personally tested after installing it for a customer. The tank took about 4 hours from the time I filled it with cold water and turned on the electricity until the bottom thermostat was satisfied. With this information (4 hours to make 40 gallons go from about 60°F to 120°F) I came up with a recovery rate of about 10 gallons per hour. On a 40 gallon Gas water heater, i did the same test. It took 30 about minutes to satisfy the thermostat at 120°.

So a 40 Gal electric has a 42 gallon first hour rating. 80% of the 40 gallons is about 32 Gallons + the 10 Gal recovery rate = 42 Gal first hour rating

A 40 gallon Gas has a first hour rating of 112 gal. Recovery of 40 gallons in 30 minutes = 80 Gallons in one hour + 32 gal in the tank = 112 gal.

Your 75 Gallon tanks with a 75K BTUh burner might have a first hour rating of about 135 Gallons. Two of them would be about 270 first hour rating

An 119 gal indirect connected to a boiler with 165K BTUh input might have a first hour rating of 200 gal if connected with 1" pipe to the boiler. If you connect with 1-1/4" pipe to the boiler you can get about 300 First Hour Rating. The reason the pipe size is important is the amount of heat a 1" pipe can move from the boiler to the indirect heat exchanger is much less than 1-1/4" pipe can transfer.

Your tank has a specification sheet that will be able to determine the amount of hit water can be generated based on the amount of heat the boiler can provide to the tank. a 5million BTU Boiler can only move the amount of heat the connecting pipe can handle. A 1" pipe can move 8 to 10 GPM. A 1-1/4 Pipe can move about twice as much water to the tank HX.

If you look at these charts from Weil McLain you can see that different size boilers connected to different size tanks may have the same capacity based on BTU input of the boiler, Pipe Size, and tank size. In your case a smaller tank (less expensive) may provide what you need.

I hope this helps with your decision.

Larry Weingarten

Hi, To everything above I’ll add that it would be nice if manufacturers talked about what your heaters could deliver in fifteen minutes, because you probably don’t want to wait an hour for the tub to fill. Adding recovery rate to 75 or 80% of storage volume will give you that “dump load”. I’d look at realistic worst case for all usage and get some number of gallons that will get used in fifteen minutes. Base your system on that. Sometimes just putting in lower flow shower heads has a dramatic effect on what equipment is needed.
Yours, Larry

EdTheHeaterMan

Based on a boiler that is over 90% efficient and has an output of about 145K to 150K BTHh the indirect that is 80 gallon all the way up to 119 gallon will provide a first hour rating over 300 Gal. This is more than the 270 gal @BigEHead already has.

BigEHead

Tremendously helpful, thank you so much!