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For contractors, a domestic hot water conundrum

Aug. 7, 2014
Options are in order from least to most expensive: Reactivate the boiler and have its DHW coil feed into the electric water heater. Add a second electric water heater; install a gas-fired water heater to work with the electric tank model. Install a tankless gas-fired water heater to handle the whole DHW-load.   

Time travel for a throw-back way-back to the days of yore when fuel, and labor, were cheap. Oil was often a fuel-of-choice because natural gas was not available and oil was far more economical than today because the annual average increases have caused a wide gap to develop-over-time for the net cost-per-Btu.

Thirty years ago, oil boilers typically incorporated a DHW coil and were set up to run 24/7/365. Poorly insulated, the mechanical room often doubled as a warm place to proof dough for bread baking.

I can close my eyes and smell that wonderful odor from Mom’s large bowl of rising dough. Once natural gas was available, my parents had me install a gas conversion burner, gas water heater, and change the boiler to heat-on-demand-only. Good-by mechanical-proofing-room for raising dough: hello ECV (energy conservation value). 

Today, customers clearly see dollar signs for the stand-by heat losses and the higher costs associated with fuel-oil. Conversion burners are once again a hot-ticket item for folks who don’t have the budget to change out their boiler for a modulating condensing model. The opportunity to reduce their operating costs by 30% or more quickly offsets the cost to convert from oil to natural gas.

Installing a simple relay that cuts off power to the boiler unless there is a call for heating from the thermostat enables customers to turn their middle-aged boiler into a heat-on-demand-only appliance, which greatly enhances the ECV of your work.

Five years ago, we installed a gas power-burner on an older oil-fired steam boiler. Break-even point was projected at three-years, so they’re ahead of the game. An electric water heater was already in place with valving set up to either have cold water pass through the boiler’s DHW coil, or go straight to the water heater. We added a relay for heat-on-demand-only.

As it turns out, our clients moved to another state, but decided to retain ownership of their home due to the lousy real estate market. Absentee landlords, they rented the home to a large family of seven with number eight on the way!

The tenant family takes their showers in the morning and, as you correctly surmised by now, that 50-gal. electric water heater is only able to handle three back-to-back showers. The tenants complained, the landlord called and asked for a solution.

Is the water heater working properly? Current conditions: 65°F incoming cold water; 120°F storage temperature in the water heater; 105°F adjusted bathing temperature; 5.5-GPM in the master bath shower-head for eight minutes; 5.2-GPM in the children’s bath shower-head for eight minutes; 110°F 1-GPM at both lavatory faucets for four minutes total; and 110°F 2-GPM at the kitchen sink for two minutes. Too many complaints over too many years led to the development of the spread sheet I use to illustrate the water heater is working properly (see chart). Chart is in PDF format.

The landlord wants to resolve this issue and satisfy the DHW needs for the tenant-family. “Should we install a second water heater, or consider a tankless model? We need an economical solution because the tenants are only going to be there for the next 11-months.” 

Options in order from least to most expensive: Reactivate the boiler and have its DHW coil feed into the electric water heater; add a second electric water heater; install a gas-fired water heater to work with the electric tank model; install a tankless gas-fired water heater to handle the whole DHW-load. Lots of other combinations but ruled out due to costs.

A gas-fired water heater will require a new breech into the chimney or enlarging the existing one for a larger flue pipe to carry the combined exhaust and, given the fact that the tenant family has a daughter soon to be a teenager that may not resolve the issue. Teenagers disappear into the shower at 13 and don’t reappear until they turn 20!

“We really don’t want to spend more than a few hundred dollars to resolve this issue.”

That ruled out the tankless model, but there is a way to turn their DHW production into a powerhouse and do so economically while giving them options!

“How much more will it cost me to heat DHW via the boiler than a second water heater for the year?”

The tank-style water heater has a .63-EF rating, but the boiler has no EF-rating and, as you know well, boilers are not well insulated. For operating cost comparison purposes, if we grant the boiler a .50-EF (50% efficiency), then the costs for 68.8-GPD of 120F hot-water is: $297 for the twinned electric/gas tank-style water heaters; or approximately $374 for the DHW coil and electric tank water heater combination; and if we include all three (coil/gas/electric) into the mix the cost has the potential to be less than $374 if we place the electric tank last in the lineup (see drawing), but the final result is entirely dependent on how much DHW the tenants use. 

A less efficient, but less expensive up-front-cost solution, is to reactivate the boiler’s 24/7/365 operation and add the DHW coil to the mix. The boiler’s DHW production will feed into the cold water inlet of the tank-style water heater and an ASSE-listed thermostatic scald-guard mixing valve added to the hot water outlet to ensure enhanced safety because the boiler never had a mixing valve and the potential is there for delivery of hot water well above 120°F from boiler-to-tank.

When the tenants vacate next year, the landlord can revert back to heat-on-demand-only for the boiler with the ability to either allow cold water to be tempered as it passes through the coil or by-pass the coil entirely.

All Dave Yates material in print and on Contractor's Website is protected by Copyright 2014. Any reuse of this material (print or electronic) must first have the expressed written permission of Dave Yates and Contractor magazine. Please contact via email at: [email protected].          

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