Cost of indirect tank vs gas and electric water heater

[Nanker Phelge]

So I was all set to buy an indirect hot water tank to replace my electric that just went kaput, adding it into my primary loop as a priority third zone in my two-zone boiler set up that heats my home.

So any of the three indirects I have narrowed my search down to will cost ~$1000-$1200 (plus ~$100-$150 to add a third zone). But my wife, being the smart one, pointed out that a seemingly comparable natural gas fed water heater can be bought for ~$450-$650, and further that an electric powered replacement could be had for as little as $275-$375.

I was a bit surprised. First of all, why are indirects on average priced higher than natural gas fed water heaters that require a burner, igniter, gas hookup valve, etc.? Second of all, even if I pay twice as much for electricity as gas to keep the water in the hot water tank heated, how long would it take before we start to realize efficiency savings going with the indirect? 5, 7, 10 years? Then how much longer before the tank/heater needs to be replaced? But even if we end up buying a natural gas fed water heater, how much difference might there be in efficiency overall, and perhaps the initial cost of the indirect is never overcome over the lifetime of an indirect?

Appreciate any thoughts or opinions that are surely more insightful than my own on the subject.

 

[Jadnashua]

At least some indirects have a lifetime warranty and generally, have better insulation and because they can utilize the full power of the boiler feeding them, you can usually live with a smaller one and recover faster.

In most places (there are exceptions), NG is the least expensive way to heat water, so there's a run-time savings you must take into account as well.

 

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[Plumber101]

Efficiency

Storage tanks range from 3 – 5 star efficiency.

Continuous Flow water heaters range from 5 – 6.7 star efficiency.

Condensing Continuous Flow units range upwards of 6.5 star efficiency.

There are a number of factors which influence the selection process. For more information into the selection process and influencing factors there is a guide for selecting water heaters https://www.conradmartens.com.au/selecting-gas-hot-water-heater/

 

[Dana]

Most home boilers are oversized for their space heating loads (or even the zone radiation in the home), and the additional duty cycle of a hot water load improves the net efficiency of the system. (If you want to figure out just how oversized your boiler is for the space heating load, run a fuel-use based load calculation on wintertime- only gas bills.)

The standby losses of the boiler are happening whether it's serving up domestic hot water or not. Adding an additional standalone water heater adds to the total standby loss. The standby losses of an $450-$650 atmospheric drafted tank type water heater are quite high compare to those of an electric water heater or an indirect, due to convection through through the center flue heat exchanger 24/7.

The improvement in net space heating + water heating efficiency is high enough with an indirect that in many locations there are subsidies available through the utilities/governments for making that change.

Methinks the use of "primary loop" isn't being used here in exactly the same way hydronic heating designers use it. In hydronic heat systems the primary loop is the short loop nearest the boiler, a loop that intersects with the secondary loop to the radiation zones. The interecting common pipe is referred to as the "hydraulic separator" or "point of hydraulic separation". It's sometime a purpose made device or a short section of fatter pipe, but in small systems it's often just closely spaced tees. This topology allows the pumping rates through the boiler to be different from the pumping rate through the radiation, eg:

While it's often the case that an indirect will have it's own secondary loop, separate from the space heating zones, it's still a secondary loop. With cast iron boilers it's sometimes possible to run the indirect without hydraulic separation, making it essentially a parallel primary loop.