Payback on indirect DHW vs. std water heater

J.C.A._3

Of COURSE THEY___ recommend buying a bigger unit!

You think these things are getting the same 8% margin they were when they first came out? (cutting close to the profit margin leads to some real interesting theories). If the shoe fits.... Chris

Dr. Z

Payback of indirect DHW vs. standard (tank) water heater

I’m having a Weil-MClain 105 K BTU/hr mod con installed in a 4-unit apartment building in Buffalo. My question is in regard to the payback of having indirect DHW installed as well.

Space heating design basis, based on clocking the current 2x oversized boiler, is about 88 KBTU/hr at 0 F outdoor temperature. I think installing an 85K BTU unit is cutting too close, so I’m going with the next higher size at 105K. This gives me some additional capacity to use for DHW without having to upgrade to the next higher size boiler.

My current DHW consumption is about 100K BTU/day (6 residents total), using an off-the-shelf HW tank. Based on your experience (and I can see from this forum that there is a lot of experience out there), what kind of savings in usage am I likely to see with indirect? Is it enough to offset the capital cost of installation?

Any thoughts are appreciated.

ALH_4

DHW

What size/model/fuel is your current water heater? What is the flow rate of the existing shower heads? How many are there?

One nice thing about the indirect is you have a single, sealed-combustion, high-efficiency appliance doing everything. Plus, the heat output of Ultra is more than 3x the output of a standard 40-gallon water heater. Is this a hot water baseboard system? Unfortunately, I am not familiar with the logic of the Ultra's DHW priority function, so I cannot say exactly what happens if there is an extended DHW load when space heating is also needed. Generally there is a time limit on DHW priority.

Some of the stainless steel indirect water heaters, such as the Crown Megastor, have lifetime warranties. Replace a standard water heater, and the indirect would have paid for itself.

J.C.A._3

I'm with Andrew.

Buy it ONCE, and figure the cost of replacement of a thermos bottle with a fire under it,(at LEAST 3 times in 20 years) VS. lifetime warranty. Throw in the single appliance to be serviced/repaired...and deduct that over the lifetime of replacements and attached burner parts.

Oh yeah...the recovery rates are quite good compared to separate fired units also, and again...that warranty.

The cost of fuel isn't going anywhere but UP.....So this sounds like a "No Brainer" to me. JMHO. Chris

Brad White

Andrew has it right

The relative efficiency differences are dependent and depending on DHW demand and other variables, may be marginal or may be measurable. I would expect your garden-variety DHW heater to have an annual efficiency of anywhere between 0.40 and 0.65, maybe a shade more. Too many variables and this is no way to rate the equipment anyway. Combustion efficiency is likely 0.70 to 0.75 at the very best if no vent damper nor any controls but the aquastat. It also supposes a submerged combustion chamber, water all around it.

With an indirect, your combustion source determines your higher efficiency, say 0.90 or so to start. This will diminish as the tank comes up to temperature (boiler water and domestic water temperatures begin to converge) and condensing stops. You will still be in the 80's efficiency probably. With the indirect you can also give it a "time-out", off hours setback, if your tenant schedules can allow. Maybe not. Impractical with a conventional DHW heater.

The biggest point Andrew made is the best argument: Longevity. A good SS heater should outlast a conventional one by a factor of five or more. You still have to watch the warranty, replace anode rods and understand your local water quality, but all things being equal, at least five times if not ten times the life. Throw in replacement cost and labor and forget energy efficiency savings. They be gone.

Nick_30

Dr. Z

I may be incorrect, but if memory serves me I believe Weil Mclain reccommends using an Ultra155 as the smallest boiler to feed an indirect. However with that being said, your not going to lose money like the old oil units when they came on and fired full on. The 155 will still modulate down to 31k btu. You may want to double check me on this, and if I am wrong please let me know.

Dave Yates (GrandPAH)_1

Dr. Z

According to the life cycle cost analysis I've done, the indirect coupled to a hi-eff modcon can only be beat by solar for long-term cost savings. In my analysis, I used a 50-gallon per day draw of 120F water, a delta-T of 80F to heat the water and current installation & fuel costs for my area. I utilized a 5% increase in all costs per year. For standard water heaters, I added replacement costs every 13 years and for the SS indirect, i added replacement costs at the 20th year. Total time was a 34 year period.

The fuel was natural gas with a cost of $.01335 per cubic foot.

For a standard water heater with a .67 EF rating, the total cost came to $32,096.62 after 34 years.

For the indirect (not including the boiler) with a .87 EF, the total costs came to $26,330.30 after 34 years and that included the costs to operate the circulator.

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J.C.A._3

GrandPah,

What about "equipment depreciation" and amortized cost over the lifetime of the appliance... as opposed to the rise in cost of said equipment, over the same period?(Did you EVEN consider THAT?)READ: MANY factors that could be considered, in the lifetime of said equipment.

Well...wasn't that a mouthful? I'm still betting the savings will be MORE than your estimates.(Economics ain't my strong point...but I think the buyer wins in any case! over HIS lifetime anyway) Chris

Ross_7

priorty

Utilizing the priorty feature,which is built in to the boiler (MCBA)controller, you should not have to oversize the boiler. That 105 should keep up just fine.

Dave Yates (GrandPAH)_2

My Dad

Was a Certified Public Accountant all of his long career (retired at 82, but continues to work at 89!). When I was a young lad, I worked with him during summer time off from school. I clearly remember doing double declining balance sheets for days on end. Not my cup of tea!

No, I did not take the tax considerations of depreciation into account. That would really muddy the waters! I tried to keep the playing field as level as possible in my analysis.

J.C.A._3

Don't worry GranPah,

I was just riding the economic side of the waterslide! I like it on the slide side...Chris