September 15, 2002

Abstract: 

Combo systems use a gas water heater to provide domestic water and space heating. It’s pretty slick having one equipment set-up handle these two. You can save on floor space and equipment/ducting costs; and adding space heating demands to a properly sized tank water heater more fully employs its capacity, increasing its efficiency during the heating season. These systems can be first-cost effective when the upsized water heater plus the air handler coil (and any reduction of gas supply lines and venting that can be credited to the combo system) cost less than a furnace and water heater. Find out when its appropriate to use such a system and guidelines for doing it right.

Combo systems use a gas water heater to provide domestic water and space heating (see diagrams at top right).1 It’s pretty slick having one equipment set-up handle these two. You can save on floor space and equipment/ducting costs; and adding space heating demands to a properly sized tank water heater more fully employs its capacity, increasing its efficiency during the heating season. These systems can be first-cost effective when the upsized water heater plus the air handler coil (and any reduction of gas supply lines and venting that can be credited to the combo system) cost less than a furnace and water heater.

This type of combo system is best suited for relatively modest and/or energyefficient single-family detached and multi-family dwellings with heating loads of 25 kBtu/hr or less at design temperatures. How much you will need to upsize your gas water heater for a combo system is a function of domestic hot water needs (usually correlated to square footage/number of bedrooms), the thermal performance of your building envelope, and your climate. (See chart on sizing on next page.)

This all sounds really elegant and straightforward, but sad to say it is not. BSC has found combo systems difficult to configure with existing commercially available equipment and tricky to install. Here are the (sometimes bloody) details:

a. Trades coordination If you are lucky, you have one trade contractor that handles plumbing and HVAC installations. If you are really lucky, that one contractor will be the type to persevere with the integrated plumbing and HVAC installation that combo systems require. Chances are, though, that you will need to have two really savvy trade contractors (plumbing and HVAC), two that are willing and able to work together on this system. A tall order, to say the least.

b. Installation Given that combo systems involve (1) a coil carrying hot water through an air handler that also supplies cooling, and (2) a single source of heat supplying two very different demands, there a number of installation items that have just got to be right (see installation diagram at right)

Configuration Proper performance of the tank water heater for both space and domestic water heating requires specific configuration with respect to the piping entry and exit for the space heating loop. Note that while positioning the air handler above the water heater can reduce the potential for thermo-siphoning, space limitations often preclude this arrangement.

Mixing valve To achieve a comfortable delivered air temperature (≈ 120°F), the tank water heater must be set at an elevated temperature (≈ 140°F). A mixing valve is required to temper the domestic water temperature delivery to eliminate the risk of scalding water and injury. During installation, these valves must be protected from the damage that can result from overheating.

Check valve Combo systems must have a check valve that prevents thermo-siphoning of tank water when the space conditioning system is in the cooling mode. This thermosiphoning can occur when water in the heat exchanger loop is cooled during air conditioning, the cooled water falls to the tank and is replaced by rising heated water, that cools, etc. (Both water heating and air conditioning costs go up when this happens). Trouble is, this valve is really prone to jamming from contaminants—flux, soldering debris, pipe tailings, etc.—during installation. The entire system must be completely and thoroughly flushed as part of the installation, a surprisingly difficult detail to impress upon many technicians.

Controls Combo systems require two special control features:

1. priority control that regulates flow when there is simultaneous space heating and domestic hot water demand, and;

2. cycling control that kicks on the heat exchanger line pump for one minute if the pump has not been on for 24 hours (this keeps water from standing for extended periods of time in this loop during the nonheating season)

While we certainly have the technology for this type of electronic logic, there are not many reliable controls available that accomplish this for combo systems.

c. Service life and maintenance If the water heater in a combo system goes out during the heating season, you lose both hot water and home heating. Maintenance and service checks are therefore doubly important for combo systems. While it’s reasonable that the much higher circulation rate and frequency of tank water heaters in combo systems should reduce tank degradation from two common foes, tank sediment and corrosion, scheduled maintenance is still critical for combo systems. Flushing your tank water heater and regularly replacing the anode go a long way. A great resource is: http://www.waterheaterrescue.com.

d. Operation The most efficient yearround operation of the combo system depends on the homeowner being willing and able to change the tank water heater temperature setting from 140°F (during the heating season) to 120°F (during the non-heating season) and back again. Otherwise, unnecessary heat loss occurs during the non-heating season or insufficient comfort is experienced during the heating season.

The Bottom Line: BSC has Building America production builders with the right combination of trade coordination, experienced field technicians, attention to detail, and sensitivity to the issues above for whom the benefits of combo systems—to their operations and to their customer utility bills—make it the right system. But until the manufacturers of water heaters, air handlers, and controls for combo systems demonstrate more coordination, technical and field support, and develop a greater sense of responsibility for combo system performance, BSC does not recommend combo systems for production homes. Stout of heart, stay the course; all others, take note.

For custom homes, we still can recommend combo systems, IF the following conditions are met:

  • Guaranteed proper flushing of the plumbing system before startup to ensure properly functioning check valve;
  • installation per diagram in sidebar above;
  • air handler not located above the water heater to reduce the thermosiphon potential;
  • tank water heater with large enough sizing/capacity to eliminate the need for priority control;
  • high-quality air handler/fan coil, such as the Carrier with ECM. . .

Download complete report here.

Footnotes:

  1. Although there are a lot of different types of combo systems, in our Building America production homes combo systems have meant the types of systems shown in the diagrams. Not covered in this discussion are systems such as the Lennox CompleteHeat™ or American Water Heater Polaris Comfort™ systems or boiler-based combo systems using equipment such as the Heat Transfer Products Munchkin™. These are high-efficiency (90+%), high capacity (100 Btu), premium systems designed primarily for larger and/or custom homes, not usually considered for production homes.