Combined (Indirect) Hot Water and Heating Systems

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Cross-section of a water heater and a heater.
  1. Hot water pipe (to taps)
  2. Heat trap
  3. Cold water inlet
  4. Hot tap water storage tank
  5. Heat exchanger (filled with boiler water)
  6. Stored hot tap water
  7. Pump
  8. Hydronic heating pipe (to house radiators)
  9. Boiler for house heat

Energy Source Gas Boiler Oil Boiler
Minimum Efficiency Recommended .83 EF .85 EF
Maximum Efficiency Available .90 EF .88 EF
Expected Life 30 years 30 years
Arroximate Cost To Install
(In Addition To Hydronic Boiler Costs)
$600-$900 $600-$900

If you have a boiler or a heat pump for home heating, you can use it to provide hot water in what is called a combined, or indirect, system. These systems are more efficient than separate systems because they eliminate the extra standby losses of another tank or unit.

Boilers

A boiler has almost three times the life expectancy of a standard storage water heater. The hot-water storage tanks in these systems also tend to stand up well; they are usually made of durable materials (such as stainless steel), and they are not subjected to direct combustion.

All of the components in the indirect system are very simple, so maintenance is straightforward. The main mechanical part is a pump that circulates water between the storage tank and the heat exchanger. There is no separate burner for hot water, which reduces heat loss in the home and improves safety.

Special Features

An indirect water heater can be added to an existing boiler. However, it's best to install one at the same time as you replace your heating system, since the new system can be bought with water heating in mind.

Precautions

Check with local code authorities. Some areas require a double-walled heat exchanger between the tap water and the boiler water to ensure safe tap water.

If you are building up a system from components, make sure you are working with a contractor who has design experience. It is easy to end up with water that's too hot if the system isn't sized right.

Make sure that all the components of the indirect system are well insulated, including the storage tank, the heat exchanger (if it is outside the storage tank), and all the pipes that carry water to and from the boiler and storage tank. If you are replacing your boiler as part of the installation, see chapter 8: Heating.

Sizing

You will need to work with your contractor and the manufacturer's representative to size a combined system. You can usually use a smaller storage tank for the tap water, because a space-heating boiler heats up water very quickly and needs fewer reserves.

Heat Pumps and Heat Reclaimers

Another type of combined heating and water heating system involves air conditioners and heat pumps. A heat reclaimer, or desuperheater, can move waste heat from an air conditioner, air-source heat pump, or ground-source heat pump into the water tank.

Heat reclaimers work best in homes that use the air conditioner (or the cooling function of the heat pump) a lot, so that enough waste heat is generated to justify installing the reclaimer. But heat pumps can also provide water heating in winter, at times when the home heating needs are not that great.

Savings average about 25% over electric-resistance water heaters, but they can be as high as 60% in certain applications. However, heat pumps with heat reclaimers are not rated by Energy Factors, so it is difficult to compare them with other types of water heaters.

Ventilation Heat Recovery Systems

Ventilation heat recovery systems are based on heat pump technology. They are useful in homes with central mechanical ventilation (see chapter 6: Ventilation and Moisture Control). Warm, moist air is drawn from the kitchen, bathroom, or laundry. A heat pump extracts heat and moisture from this airstream, and transfers the heat to the domestic hot water. This system provides mechanical ventilation as well as hot water. Energy use of a conventional electric water heater can be reduced by 40% if it is supplemented by a ventilation heat recovery system.

Wastewater Heat Recovery Systems

A wastewater heat recovery system reclaims some of the heat from hot water that is going down the drain. In some systems, hot wastewater is kept in a holding tank, where a heat pump can extract the heat. The heat is then re-introduced into the domestic hot water stream. Although this does save a lot of water-heating energy, the cost of these systems has generally been prohibitive.

The new generation of wastewater heat recovery systems get rid of the heat pump. Instead, the incoming cold water runs through a coil of pipe around the wastewater drain. As warm water drains away, it transfers its heat through the pipe walls into the water going to the water heater. This gives the water heater less work to do, saving up to 40% of water heater energy.

Heating the House With a Water Heater

While most combined systems start with the home's boiler or heat pump, others use the water heater to produce heat for both tap water and the home. This is an option only if your house is very well sealed and well insulated, or if you live in a mild climate. The hot water is used to heat the fan coils for your home heating system, and the heat is then distributed through ducts, as with a standard furnace.

This type of system requires a high-efficiency water heater and fan coil. It can work if you need less than 75,000 Btu/h of heating capacity for your house. (To find out, ask your heating contractor to do a heat load calculation.) See chapter 8: Heating for more information.

Excerpted with permission from No-Regrets Remodeling by Home Energy (1997)