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Combo Systems

Combi – Space Heat/Water Heat


There are at least two major different ways to approach the combining of water heating and hot water heating:

  1. Use a space heating boiler with a storage tank and controls for domestic hot water
  2. Use a heavy duty water heater with controls and distribution for space heating

A gas water heater is used in most installations because it is less expensive to operate; in the future, this may not always be the case in all locations.

Option 1. Boiler as Water Heater

In this option, a house with a boiler for space heating is equipped with an extra storage tank for year round water heating. This avoids the need for a second appliance, but because of the storage tank and various controls, there’s no savings in space or equipment cost. A boiler is more energy efficient than a typical water heater, and if a LOT of hot water is used, this system is an energy efficient way to produce a lot of hot water.

Principal Advantage: High Efficiency/reduced cost water heating


Option 2. Water Heater as Boiler

In this option, a heavy duty version of what appears to be standard residential water heater is used to provide hot water AND seasonally, space heat the house. This system is applicable to small houses, room additions and houses with very low heat requirements.

Principal Advantage: Lower equipment first cost for small space heating loads



Boiler as Water Heater

When a boiler is used as a water heater, a storage tank, heat exchanger, circulation pump and controls are added to the boiler. For normal loads, the boiler will be the same BTU size as what would have been used for space heating alone; the storage tank is used as a buffer for intermittent and peak loads. Several manufacturers offer packaged units.

Water Heater as Boiler

Although any water heater could work as a space heat boiler, most codes will required that the water heater be labeled and certified by the manufacturer to be acceptable for such use. General requirements are that the water heater have a larger BTU input rating to be able to handle simultaneous space and water heating, and have an additional set of water pipe fittings (see image to right that has fitting both on the top and side of the tank). One set of fittings is used for normal hot water piping, and the other set is used for space heat piping. Some codes in certain applications may require the use of a heat exchanger so that potable (drinking) hot water does not mix with water used in the space heating equipment.

The distribution system can be a radiant floor, a fan coil, baseboard or any combination of these. See Radiant Hydronic Heat


The following is a sample schematic from the Bio-Radiant Energy web site:


Space Cooling Combo Units

The Bio-Radiant web site offers information on combo units that include a space cooling system. This is a new concept that may have some benefit in a few limited applications. It appears to be a complex system and likely has some major first cost disadvantages.


More Information


Radiant Hydronic Heat

Water Heater



Combo_AlliedAllied Engineering Company

Div of E-Z-RECT Manf LTD.
94 Riverside Drive
North Vancouver, BC V7H 2M6
Telephone: 604-929-1214

Heats Your Home Efficiently. Uses a powerful Mini-Gas® hot water (hydronic) boiler, CSA rated at 84.6% efficiency. Provides an Abundant Source of Domestic Hot Water. The indirect fired stainless steel heat exchanger generates up to fives times the amount of hot water compared to a direct-fired water heater. Attractive Compact Design. A small 24″ x 24″ footprint saves valuable floor space. Ideal for closet installations.

Go to the Allied web site at


WaterHtr_AOSmithTankA.O. Smith Water Products Co., Inc

World Headquarters
11270 West Park Place
Milwaukee, WI 53224
Telephone: 414-359-4000  

Go to the AO Smith web site at




Combo_HydroHeatBio-Radiant Energy, Inc.

16928 Woodinville-Redmond Road N.E.
Woodinville, WA 98072
Telephone: 425-487-6272

Hydro-Air™ Heating and Cooling Systems utilize small, efficient, hydronic fan coil wall units installed in the rooms of a home or an apartment to distribute heating energy from a standard hot water tank or boiler for both heating the living space and heating water for domestic use. Water circulates through piping installed in floors, walls, and ceilings, distributing energy to the heating fan units; then returns to the hot water heater or a condensing unit when cooling. Line voltage thermostats provide independent temperature control of each area zoned by controlling the fan in each unit. Hydro-Air™ delivers fully zoned, hydronic heating and cooling without the need for any of the ducting associated with traditional forced-air systems. The Hydro-Air™ system is developed as a complete system.

Go to the Bio-Radiant Energy web site at


725 Talamore Dr.
Ambler, PA 19002
Telephone: 215-641-9400

Go to the Bradford-White web site at

Bradford-Whites new and greatly expanded web site now also includes a site called ‘Right Spec’ which is a comprehensive site for water heater sizing, spec sheets, piping diagrams, and brand name cross-reference guide. The site may require log-in, but it is instant and free.

To go to the Right-Spec web site



If floor space is critical or individual metering is needed, then the Minuteman II is the solution for all your heating needs. Best suited for condominium, apartment installations, or where a low heat load exists, Burnham’s Minuteman II combines a cast iron boiler for home heating while providing domestic hot water all in one compact unit. The gas-fired unit is available in four sizes and is easily installed in a closet or utility room.

Go to the Burnham web site at



Combo_AquThermFirst Operations, LP

8273 Moberly Lane
PO Box 270969
Dallas, TX 75227
Telephone: 214-388-5751

When a First Co. Aqua Therm® “Combo Heater” (air handler) is connected to an adequately sized natural gas, LP, or oil fired water heater, the water heater becomes a dual function appliance that can provide comfortable, efficient space heating as well as hot water for domestic use.

Go to the First Operations web site at




Laars MiniComboLaars

The MiniCombo (MCid) is a combination hydronic boiler and domestic water heater. The appliance combines the advantages and efficiencies of the JVi hydronic boiler (up to 85.4% AFUE) with extremely high first hour recovery rates of a 30 US gallon (113 L) stainless steel indirect storage tank. The unit comes completely assembled and wired from Teledyne Laars complete with a circulation pump for the indirect tank. The MCid is available in gas input sizes from 50 MBH to 125 MBH (14.7 kW to 36.6 kW) to suit hydronic and domestic loads of the residential market.

Go to the Laars web site at


Rheem gas water heaterRheem and Ruud

Rheem is headquartered in New York. Rheem has manufacturing facilities in Montgomery, Alabama; Fort Smith, Arkansas; Milledgeville, Georgia; and Oxnard, California. Rheem has two Canadian locations in Hamilton, Ontario and Mississauga, Ontario, and one Australian location in Mulgrave, Victoria. Rheem also has a manufacturing facility located in Nuevo Laredo, Mexico. All manufacturing facilities are ISO 9000 certified. In 1987, Rheem became a wholly owned subsidiary of Paloma Industries of Nagoya, Japan, the world’s largest producer of gas appliances.


Go to the Rheem web site at

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