Hybrid technology can work for home water heating

Joanne Schlafer

This article was written by Joanne Schlafer, a sustainable building consultant and project manager. To contact Joanne for help in making your next remodeling project green or for planning your new green home, email her at joannes@certifiedgreenconsulting.com. Is your furnace nearing the end of its useful life, or does your sixth sense tell you the hot water heater is going to die any day now – most likely in the middle of your morning shower? Perhaps it’s time to consider going hybrid. No, I’m not talking about your car. I’m talking about a very clever solution that converts your existing gas forced-air furnace into a very efficient, hot water heating system. 

Here’s how it works. 

Step 1) Replace the existing water heater with a tankless water heater (also referred to as ‘instantaneous’ or ‘on-demand’ heater). Note: It can be installed either on the exterior or interior of the house, depending upon space and venting requirements.  

 

Step 2) Take the ‘guts’ out of the old furnace, leaving the cabinet and fan.   

 

 

Step 4) Connect the tankless heater and the exchange coil via pump, relay module, and expansion tank.  Voila! You now have a very efficient system that heats your domestic hot water and produces heat for your home.  

 

 But does it really work?  

 

Thanks to Dave Emmitt of Direct Drive Service . I recently had the privilege of visiting an existing home installation and picking the brain of the homeowner. Here’s what I found out:

 

The homeowner’s furnace was an older furnace and the conventional-style water heater was aging. To replace both with like-in-kind efficiency (e.g. 80% efficient furnace and average tank-style heater) would have cost about $4,000. An upgrade to a 92% efficient modulating furnace would have cost approximately $4,500 and the addition of a higher-efficiency water heater would have added another $1,000-$1,200 to the price tag. The homeowner chose to do the furnace conversion, which cost $5,000 installed, and achieved an integrated high-efficiency system that was properly sized for his home.

 

Additional Benefits (Hint: It’s a greener solution)

The homeowner felt good about re-using the old furnace and not sending it to the landfill. The parts that are removed from the furnace are melted down and recycled.  In addition, the higher efficiency saves energy. After a year of living with the system, the homeowner was in the process of figuring out his savings. Industry data shows 22-38% in savings can be realized, depending upon usage and lifestyle habits. The system is also more economical in the long-run (more on that in a bit). Very little can go wrong with it, it requires minimal maintenance (draining/cleaning once a year), and it lasts for 20 years or more. Health-wise, the homeowner no longer has to worry about carbon-monoxide poisoning, as combustion appliances have been eliminated. The heated air is also not as dry as that produced by a conventional heat exchanger. And, looking to the future, if the homeowner decides to take advantage of solar tax incentives, this system (unlike conventional high efficiency systems) can easily accept solar-heated water.

 

Nothing is the proverbial silver bullet, and there are a few things to be aware of.   Firstly, with any tankless water heater, there can be a slightly longer wait time at the showerhead or faucet for the hot water. There can also be a bit more of the “cold-water sandwich” syndrome in between showers. This is because the water is heated at the source only at the time of demand. (Note: A reputable professional installer will ensure these issues are minimized. Do-It-Yourself installation is not recommended). Once the hot water reaches the fixture, there is an endless supply, as there is no tank that will become drained. For those who love their hot showers (writer included), this could promote longer showers and, therefore, greater water use. 

As I mentioned earlier, this system must be drained and cleaned periodically – at least annually. In areas where the water is hard, containing many minerals, more frequent cleaning may be required. The good news is that it is easy for the homeowner to learn how to do.

Lastly, the homeowner I spoke with was having an issue with the temperature of the hot water. In order for the hot water heat to operate most efficiently, he was told to maintain the temperature at a 140-degree setting. This produces extremely hot water (though not technically scalding) at the faucet and he was concerned about safety issues. The decision was made to install a thermostatic mixing valve, which will provide tempered water to the household at a controlled temperature. A high-quality mixing valve runs about $200-250 and may need frequent replacement in hard water situations.

 

Being the left-brained, analytical person that I am, I asked for some numbers that would help me better understand the payback on this type of system. Using the tankless heater to begin with, here is a simple illustration. Let’s use $3000 as a base installed cost for a high-quality, commercial-grade tankless. Assuming a 3-person household, the average monthly savings is about $20+ at current energy rates. That amounts to roughly $250/year. So, based on savings alone, the payback period would be 12 years. However, keep in mind the lifespan of a tankless heater is generally 20 years. After the payback period, the homeowner will realize energy savings of about $2,000 over the remaining 8 years of life. In addition to energy savings, we must consider replacement cycles and costs.  New government standards for energy-efficient tank-style heaters (N.A.E.C.A.) have resulted in a reduction of their lifespan, from about 10 years to 5-7 years. This is due to thinner tank walls for greater efficiency. So, we need to add at least 2 hot water tank replacements, at $1,000 each, into the mix over the 20 year period – another $2,000 in savings. This means the total savings would be about $4,000 over 20 years for the tankless heater alone. Now, factor in the furnace conversion. The conversion itself is at least half the cost of a new high-efficiency furnace. In addition, with the average life of a furnace estimated to be 20-25 years, we can factor in savings realized by eliminating the need for another replacement. Only minor components (e.g. fan motor, pump, etc.) will likely ever require replacement. These replacements will be infrequent and relatively inexpensive ($100-$200 ballpark per component). So, in doing the math, the numbers do support the claim of a more economical solution when you consider the entire lifecycle. The table below summarizes this example:

 

 Summary

So there you have it. In weighing the pros and cons that must be considered, it is clear this solution offers many benefits – material re-use, better economics due to energy savings and durability, improved indoor environmental health, and the ability take advantage of solar (and possibly geothermal) in the future.