indirect water heater

Paul S_3

How do i size an indirect water heater.? ....also how do i verify how many btus a indirect heater needs?....customer says building ran out of hot water all the time....the job has 6 full bathrooms. ...they have an weil mclain 80 in there already....supply and return piping to indirect is 3/4"...way to small....not wired for priority ..boiler is atmospheric cast iron....the heating is all in floor radiant....5 zones....staple up...no boiler protection....boiler is a 165 k.....my heat loss is around 125k...i will be changing boiler and indirect because boiler failed from low return water.....boiler leaking.... flue pipe deteriorated...will be using tekmar 360 with 4way valve and actuating motor for full reset with boiler protection ....i do not want to size boiler to heatload and not be able to adequately heat DHW.....i was looking at a weil mclain 110 plus

Paul S_3

you mean boiler piping 1"? the 110 has 1 1/2" connections and requires 25 gpm...wouldnt that be 1 1/2"....do i use 20 delta T for pipe sizing to indirect?...no hot tub....basic shower heads i think 2.5 gpm.... at least 2 showers going on at same time...average shower 15 minutes

Ironman

You need to determine what the peak demand for hot water will be in gpm. It's usually between 6:00am - 8:00am and about the same time in the evening.

A modern shower head with a flow restrictor passes 2.5 gpm; about 2 gpm is hot water. So, if three concurrent showers are running, that's 6 gpm x 20 minutes duration = 120 gals. during peak. Next, find an indirect that has a first hour rating that's higher than that (I'd go at least twice that). Now see how many btus that indirect requires to achieve that rating. If your boiler can't deliver that, then you'll need to factor in the difference or go with a larger indirect that will provide more storage.

Remember a btu is a btu no matter what is used to produce it. One btu is what it takes to raise the temp of one pound of water one degree (usually expressed in btus per hour). A gal. of water weighs 8.33 lbs.

In my example above you would have 1000 lbs of water to heat (120 gal x 8.33). Typically we figure a 77* temp rise (50* to 127*) to heat water. So, 1000 x 77 = 77,000 Btus are necessary to reheat the 120 gals. that was used, but remember, the boiler would produce that in an hour, not in 20 minutes (if that was the boiler's output). That's where the additional storage is needed. A basic rule of thumb: it takes about 40,000btus to continuously heat 1 gpm. So, a 100,000 btu boiler (output) will continuously heat 2.5 gpm, if the indirect is capable of that.

The 80 gal. indirect MAY be able to do the job if it had the proper piping, pump and boiler output, and if it was given priority. I'd consider a 119 gal. indirect with the boiler you're proposing.

Another caveat to consider is that you're required to have a tempering valve that meets ASSE 1017 on that indirect. With that, you could turn the TANK temp up to 150* and set the valve to 125* to prevent scalding. That would kill legionella in the tank and also give you about 12% more stored hot water.

This has been a somewhat rambling reply from a tired mind, but you can also contact any boiler/indirect rep or manufacturer and they will size it for you.

Paul S_3

thank you guys

Aaron_in_Maine

I would save the Indirect run it to 150 install a mixing valve and put it on priority.

Aaron_in_Maine

Hatterasguy said:

Aaron_in_Maine said:

I would save the Indirect run it to 150 install a mixing valve and put it on priority.

...........and change the piping to 1".

Yes at least 1". I run whatever the pipe tapping is they make them that size for a reason.

Paul S_3

Pipe tapping 1 1/14 ....i will try to save indirect. ...but customer may want to go all new

Eric_32

Does anyone know if that Weil Mclain 80 has a dip tube in it? I wonder if it's broken and dumping cold water near the top the of the heater?

Ironman

Isn't the WM just a rebadged TT Smart tank?

kcopp

The old one was but not the newer series. Its not the tank in tank type. The new W-Mc are a stainless tank w/ a stainless coil.

bob eck

If the W/M is a older Smart TT IDWH I believe the 80 gallon model only has 60 gallon of potable water in storage. Take shower heads down to 1.5 GPM run 1" or 1 1/4" Supply and return insulate the piping. Make sure the pump is sized right. Give the water heater priority. Store water temp at 160 and mix to 125 or so.