Indirect size

Terry Larsell

60 gallon tub, 2 people, no other out of the ordinary loads. The cost of indirects is going up almost as fast as the price of gas. I can get a 45 or 50 for a lot less than a 60. Do I need 60? I hate to heat more water than I have to.

Terry

hot_rod

the answer is not that ease

but comes down to what the customer wants and expects. Can you get a handle on how much HW they currently get by with?

Do they have large dump loads at times of the day, a large tub for example? For that type of load either enough storage capacity, or an instantanous sized properly to generate it on a continous basis.

Once you get an idea of the amount of DHW they require the math is simple.

DHW= 8.33 (G) (Th-Tc)

DHW= 8.33 (gallons required) X (Th required temperature- Tc cold incoming temperature) this formula tells you how many BTUs are required to raise the temperature for a given quantity with a know incoming and supply temperature.

hr

Uni R_2

Capacity of boiler loop?

If you have a high mass boiler you'd probably have better savings with a larger indirect because in the summer you'd be stranding X gallons of XXX° water after each DHW cycle. With fewer cycles you would strand less heat.

Dave Yates (GrandPAH)_2

depends

on the storage temp. You can make the 50 work, but not at 120F because in the dead of winter, when they're most likely to use that tub, you'll need 49.5-gallons of 120F water from the water heater itself and won't get more than 38 to 42 at 120F due to mixing in the 40F during the draw.

Bump up the storage temp and add an ASSE certified 1016/1017 thermostatic scald-guard mixing valve on the indirect's outlet. The increased storage temp will use less than $10.00 in added fuel cost for stand-by heat losses. Overall, that offers a much safer hot water system - with or without the worries about filling a tub.

Measure the outlet flow in GPM from the tub (or any other fixture) once the temp has been adjusted by the homeowners. Let's say they have it flowing at 106F. To determine the percentage of hot water required, I default to our known low of 40F (we actually go lower than that, but not every year). 40F seems to be the nation-wide average from research I've done because lines are buried in depth according to local weather conditions. From Florida to Alaska, the recorded cold water inlet temps (municipal authorities) will be at 40F. Design for design conditions.

% = (106 - 40) / (120-storage - 40) = .825 or 82.5% of the hot water comes from the tank. 60 x .825 = 49.5 if, and only if, the hot water remains at 120F.

Try that at 140F & % = 66% 60 x .66 = 39.6 gallons required. The ASSE mixing valve mixes down the 39.6 gallons to make the required 49.5 gallons at 120F.

The beauty of this system is that even if you miss the mark by a bit, you can bump up the storage temp to increase capacity without compromizing their safety.

Chris S

indirect size

Boiler mass plays into this as well. In the summer, a cast iron boiler from a cold start may take longer to achieve a usable temperature than it will take to deplete the usable hat water in an indirect. ( i had this complaint on 2 installs). We timed a burnham V84 at close to 7 minutes from a clod start with the indirect calling before it was able to start putting heat back in. In the winter it was not a problem, because the boiler was most always warm. My current low mass boiler comes up to temp in less than 2 minutes, so with my 40 gallon indirect & 1" piping to & through the 1" coil, we never run out of water.
3 -4 young adults at home plus laundry etc. We never have to wait to either run the dw, or laundry- there is always enough water. And we do have and use a whirlpool- never a problem. Still, I offer customers a choice, and some opt for larger units. One insisted on an 80 gallon running off of a mod con (great recovery) for 2 people with no special draws in the home. No body sprays etc.

This is not my pitch for low mass, just what I happen to have right now. My old house had a V73, and an indirect, when we wanted to use the tub in the summer, I would tell my wife/ daughters to push a thermostat up until they heard the baseboards ticking, then shut the heat back off & fill the tub. Primitive, but it worked.

Chris

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Terry Larsell

Thanks GrandPAH that's what I was looking for. My dad was a math teacher but I find I really have to slow my mind down to get a handle on these formulas. It's clear as mud now.

Terry

Jim Franklin

Is there circuit logic to prevent heat going from the indirect into the boiler in those first few minutes when the boiler is cold?

jim