Let's get hitched: boiler to indirect water heater union

Wayne and Jean are like most of our retired customers: They are concerned about rising fuel costs, are on a fixed income and they want some ideas about how to cut costs. They've already buttoned up their house by installing insulation and now they're giving serious consideration to replacing the boiler. The couple lives in a rural area and burn oil. Their existing boiler is approximately 20 years

Wayne and Jean are like most of our retired customers: They are concerned about rising fuel costs, are on a fixed income and they want some ideas about how to cut costs. They've already buttoned up their house by installing insulation and now they're giving serious consideration to replacing the boiler.

The couple lives in a rural area and burn oil. Their existing boiler is approximately 20 years old and it incorporates a domestic coil. The flue piping travels horizontally through an unheated crawlspace to a masonry chimney and, therefore, includes a draft fan to paddlewheel the exhaust gases.

I pointed out that if we did any work on the boiler, or installed a new boiler with a domestic coil, we would require an ASSE listed 1016/1017 scald-guard mixing valve to regulate the potable water delivery temperature. They noted the water is scalding hot. Noise has been an irritating issue and that, too, can be virtually eliminated with newer model oil-fired boilers.

It turned out they already were familiar with indirect water heaters, but were not quite clear on how they worked. After explaining the benefits, which included lower fuel usage, they wanted pricing options with options.

Alternatives

Here's the deal: They'll save fuel by lowering the boiler's firing rate by .30 GPH. They'll also save fuel because the new boiler's efficiency rating is 10% better, and if they go for the indirect instead of a coil, they'll save even more fuel via lower standby heat losses.

They'll also be eliminating the additional overdraft created by the in-line paddlewheel fan (it pegs the barometric draft damper wide open), which will save Btus from being prematurely sucked away. And last but not least, by bringing in combustion air from outside the building envelope and eliminating the combustion draft fan, they'll save fuel by lowering the home's infiltration rate.

If they choose the pricing option that includes an indirect water heater, we can eliminate the chimney and flue through the crawlspace, too, with an upgrade option for a direct-vent boiler. Upfront costs will be higher, but with projected fuel savings, this installation has the potential to pay for itself in less than 10 years.

Balancing act

There's just one hitch: With the home's added insulation, the new boiler's Btu output will be substantially below the minimum input required to meet the indirect water heater's recovery rating. Our salesman was quick to point out that the two don't meet — on paper — for determining domestic hot water recovery rates. But, if you read the fine print for the indirect water heater's recovery ratings, they never really meet — in the real world — right from the beginning!

The assumed working conditions are based on three things: 58°F incoming cold water; 135°F domestic storage; and 180°F from the boiler. I can only “assume” the last two are accurate because we all see 40°F municipally delivered water (55°F for well water) during the coldest weeks of the year. That's the domestic water design condition needed for accurately projecting hourly ratings. The manufacturer lists a required minimum net boiler output of 99,000 Btus in its calculations, and I'll have just 79,000 Btus to play with in this application.

This has become an all-too-frequent challenge as the required Btus for domestic hot water often loom much larger than do the Btus required to meet design heating conditions. Should you size for the domestic or for the heating load? Should you add Btus to the heating load for domestic hot water production and upsize the boiler?

If you size to meet the domestic load, then the heating efficiency suffers, and if you follow the listed indirect ratings without doing a little bit of homework, you're going to catch a liability bullet and end up giving away work to satisfy an angry customer. You're the one who stepped up to stand squarely in front of the bull's-eye. Everyone else has literature to present that clearly shows it's your fault — you didn't apply your conditions to properly size the water heater!

Flow rate is key

And, if those issues don't give you second thoughts, there's one more condition you need to meet that's typically ignored — choosing the right circulator to meet the flow rates required to transfer energy from boiler-to-indirect so that the recovery rates don't suffer. Any given indirect water heater has a required GPM flow rate and pressure drop (resistance to flow) to overcome.

Combined, the two will dictate where a circulator's performance will fall — if you utilize the manufacturer's performance chart. Here again, ignorance is not bliss — it's another bullet loaded in a hair-triggered liability gun. Domestic hot water production represents 15% to 30% of a home's overall energy consumption, so choosing the lowest wattage circulator that meets flow rate and pressure drop ratings can have a significant impact on yearly savings.

Sounds complicated, doesn't it? Tune in next month and I'll share my methods for making this an easy and fun task that will help you increase sales while building trust with your new or existing customers.

Dave Yates owns F.W. Behler, a contracting company in York, Pa. He can be reached by phone at 717/843-4920 or by e-mail at [email protected].

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