Andy Kerr - Natural Gas Water Heater

By Andy Kerr

Demand hot water heaters only heat water when you need it, thereby eliminating standby losses. In particular, the Takagi T-KD20 was chosen because:

Not only does it heat only as much water as needed, but it only heats it up to hot as needed. The on-board computer senses incoming water temperature and then modulates the gas flame (between 20,000 and 185,000 BTUs) to raise the water to the desired temperature. It comes from the factory set for 120⁰F output, a temperature often recommended by authorities. By flipping a few dip switches on a circuit board, I reduced the output to 112⁰F, which is hot enough for baths, dishes and laundry (we don't use a dishwasher, but most modern ones come with an onboard auxiliary heater to ensure hot enough water for optimal cleaning). In any case, if the water coming from the solar storage tank is 112⁰F or hotter, the water heater doesn't flame at all.
It has an energy factor of 0.84 (0.85 for liquid propane) or (roughly) 85% of the energy is the fuel is usefully used.
It has electronic ignition (no pilot light), thereby saving gas. The standby electrical power draw is 8 watts when water is not flowing and varies between 30-50 watts when heating water. A cooling fan that runs briefly after flowing hot water (even if solar preheated), not burning gas, draws around 28 watts for 10-20 seconds.
It draws its combustion air from the outside, rather than inside air, thereby maintaining better indoor air quality.

A green LED means the unit is plugged in. When the red LED lights, the unit is heating water.

In times of plentiful solar hot water, the water heater is turned off at a switched power outlet to save electricity.

Costs

Takagi T-KD20 $1,019.00

TK-TV03 Exhaust Backflow Prevent Kit $49.00

K-TV-5 Wall Ventilation Terminator $111.00

TK2 Wall Hanging Bracket, Mounting hardware, pipe, fittings, P-T value, etc. (included $0.00

Misc. gas pipe & fittings $68.50

Misc. copper pipe and fittings $29.75

Stainless steel 4” 90 flue fittings $88.00

Labor (12.75 hours @$45/hour) $573.75

Building Permit $65.00

Gross Installed Cost $2,004.00

State of Oregon Income Tax Credit (40% of equipment cost) -$340.00

Avista Utilities Credit for high-efficiency gas hot water heater) -$50.00

Net Installed Cost $1,614.00

The estimated payback or return on investment has not been determined. I've seen estimates ranging from 20-40% annual savings in energy by using a demand water heater. Complicating such an analysis is that I sell my excess photovoltaic-produced power at a premium. Previously I heated my water with electricity when the sun didn't shine. Although I use a bit more natural gas, I make more money selling electricity.

The new system was mainly installed for comfort. The previous electric hot water storage tank wasn't quite up to the job. In the winter with no sun and two people taking one shower each (after workouts) and one bath each (for therapeutic benefits) daily, we would often run out of water, especially if the dishes were done that day.

The plumbing is not as complicated as it looks in the picture. The black insulated pipes from above circulate the glycol solution to a heat exchanger in the round storage tank. Cold water comes into the top of storage tank and hot water comes out of storage tank on left to rectangular natural gas water heater on right. A pressure-temperature relief valve (vertical copper pipe at bottom) drains through floor. By manipulating five gate values above the storage tank, either the solar storage tank (left) or the natural gas heater can be isolated from the system for maintenance, etc.

On the gas hot water heater top, is the air intake (the smaller flexible silver duct) and the air exhaust (the larger flexible silver duct). An in-duct damper prevents back draft and reduces risk of freezing. The water heater also has heating elements that automatically come on to prevent freezing of the unit.

On the gas hot water heater left side is:

• gas intake (yellow flex pipe from floor)

• 120-VAC electrical input (black wire hanging loosely)

• Pressure-Temperature relief valve drain (bare copper pipe)

• Cold water input (black foam insulated pipe)

• Hot water output (black foam insulated pipe).

A belt and suspenders will more reliably keep your pants up. If the gas water heater is dysfunctional and it's not sunny, the top half of the solar storage tank can be heated to provide some hot water. The storage tank comes with a top electric element. The heat exchanger replaces the bottom element. An electronic timer (actually a relic of the previous system) can be set to come on and off during the different days of the week (heating water at 3 AM, as all storage heaters do, is like leaving your car running just in case you need it).

This exterior vent serves both to provide fresh outside air for combustion and to expel exhaust air.