Install a Combination Boiler/Water Heater

If your home has a gas hot water heater and gas-fired hot water heating, you might be a prime candidate for changing both of these units out for a combination boiler/water heater. Why would you want to do this? Because that combination unit could save you a considerable amount of money on your monthly energy costs.

Typically, the boiler installed for hot water heating is a larger unit than necessary. This used to be common practice, before energy costs became such a huge concern. It was done in order to ensure that the system would always provide enough heat to keep the home warm, dealing with less than perfect insulation and sealing of the home against drafts.

But many of these homes still have these oversized units, which don’t adjust downwards in their energy consumption for anything. Rather, every time they turn on, they operate at full blast, burning the most fuel they can.

In addition to this, both these boilers and hot water heaters come with large tanks, where already heated water is stored. This requires that both units cycle on and off, even when there is no demand, just to keep the water from cooling. That’s another energy drain, increasing the cost of using these units.

A combination boiler/water heater only has a one gallon tank, and that’s for the boiler side. Hot water is only heated on demand, eliminating all the energy formerly wasted to keep water hot and shortening the amount of time it takes to get hot water from the faucet. The boiler side of the unit is also more efficient, sized to provide exactly the amount of heat needed, rather than oversized. In addition, they come with a temperature sensor, which is installed outside, so that they can work in a low-energy mode, in the spring and fall, when full heating is not required.

Hot water is created instantly using a heat exchanger, eliminating the need to store all that heated water and losing the energy it takes to keep that water warm. The heat exchanger turns on automatically, whenever there is a demand for hot water.

water heater, basement
Water heater, Brian Cantoni

Plumbing for the Combination Unit

The biggest part of installing the combination unit is plumbing everything in to the new location, which is usually a different one than is currently used by the boiler and hot water heater. These units hang on the wall, so it’s common to set up a sheet of plywood, where all the necessary plumbing is run into and secured, so as to make the installation easier.

The necessary connections include:

  • Inlet and outlet connections for the hot water heating system
  • Water supply inlet for the hot water heater
  • Hot water outlet to the home’s faucets
  • Gas in connection for fuel
  • Exhaust flue
  • Air inlet for burning fuel
  • Electrical power for control
  • Thermostat
  • Outside temperature sensor

As you can see from this list, several of these connections need to be tied into the home’s existing plumbing. The tricky part, is that you don’t want to disable the existing system, until the new one is totally connected. So for a time, you will have double connections. To facilitate this, you will want to put valves on all the plumbing and gas connections, so as to avoid losing services during the process.

If you currently have a gas boiler, you will also have a chimney for the exhaust gases. However, the new unit runs efficiently enough, so that there is very little exhaust gas. This can be ejected from the home through a 3” PVC pipe, through the home’s siding, rather than having to use a chimney. So the new unit does not have to be installed where the chimney is currently located.

Installing & Connecting the Unit

With the plumbing in place, the unit is ready to install. It hangs on the wall with a French cheat, making the actual installation easy. The trickier part is connecting all the plumbing.

With the unit hung in place, the next step is to install the exhaust flue gas and inlet air for combustion. This requires putting a hole in the basement ceiling (if it has a ceiling installed) and cutting two holes, side by side, through the siding with a hold saw. These air inlet and outlet are merely 3” PVC pipe, which is very easy to install. Normally an elbow is placed on the air intake, pointed down, to keep rainfall from entering it. It is necessary that the air intake be above the snow line, to meet code, ensuring that there is always a ready supply of air and that it is not blocked in the wintertime.

The exhaust tube is normally installed right next to the air intake. This can’t be within 12 inches of any window, per the building code, and at least 12 inches from the air intake, so that the exhaust isn’t recycled back into the intake. This tube also has an elbow on it, pointing horizontally to keep the rain from entering it. The ends of both tubes need to be covered with wire mesh screening to keep small animals out of them.

The connections for hot water heating and the home’s hot water heating system are all copper connections, which can be soldered; however, flared compression fittings are often used, in order to make it possible to remove and replace the unit easily. A small pressure tank is installed to the supply and return lines for the hot water heating system, to prevent pressure fluctuations in the system.

Most of these pressure fluctuations come when the circulating pump is stated. This also needs to be installed outside the system; and is usually upstream of the pressure tanks. This pump is controlled by the combination unit, so that it comes on along with the heater itself.

The touchiest part of the plumbing is installing the gas lines, which must be installed or at least checked by a licensed plumber. Failure to get this step done by someone who is properly certified could void your homeowner’s insurance, in the case of a fire. So it’s worth paying a licensed plumber to do the final connections and check it, even if you do the bulk of the work yourself.

The process of burning fuel to heat the water creates condensation, which needs to be dealt with. This is normally accomplished by the installation of a condensation ejection pump, much like that which is used for HVAC air handling units. Before passing through the pump, this condensate passes through an inline condensate neutralizer, because it can be acidic. The output of that pump is connected to the home’s wastewater system for drainage.

The final connections are electrical. Some electrical power is required, for the system’s controls. This is accomplished by installing a standard duplex outlet, so that the unit’s power cord can be plugged in. The outside temperature sensor I mentioned earlier needs to be mounted on the north side of the building and connected to the control circuitry in the combo unit. Finally, the home’s thermostat needs to be connected.

Always be sure to double check all plumbing and electrical connections before turning the unit on. You want to ensure that both the water piping and gas piping are leak free. Open all the valves, visually checking to ensure that they are all open, then you can turn the unit on.

The last step in the process is to remove your old hot water heater and boiler, freeing up room in your basement, which you can use for other things.

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