Townhouse Combination Heating & Domestic Hot Water Systems

as typically installed in Seattle Washington


If you need assistance with a town house hot water heater call Evan on the phone (206) 793-8303.  


Many townhouses have been built in the Seattle area that have combination hot water and heating systems installed. Traditionally, a hot water boiler was used to heat the radiators throughout the house. Both the boiler and the radiators were heavy cast iron. Cast iron was heavy, long-lived and reliable - but it was not cheap. In the 1990's the builders wanted a lower cost so they combined the domestic hot water and space heating into one device - a gas water heater. It was a simple idea - rob the hot water from the hot water heater and circulate it upstairs to heat the house.

Many people have trouble with these water heater heating systems. Hopefully, if you understand what they are (and are not), you can have an easier time living with them.


The water heater should be changed to a hot water boiler or water heater that is designed for continuous duty and rated for the maximum heat load.

At least:

The domestic water should be isolated from the space heating portion of the system to prevent high bacteria counts from contaminating the potable water supply.


The system should be evaluated for alternative options.



The systems we see usually have these characteristics:

  1. A residential domestic forced-draft gas water heater is used.

  2. The space heating water is shared with the domestic hot water supply.

  3. The system was installed by unskilled workers

  4. The system was not intelligently designed or engineered for functionality

  5. The water heater is undersized for the heat load making it difficult to heat the house in the winter and making cool showers.

  6. There are limitations in replacing with more suitable equipment - both structurally and economically (gas pipe size and flue type).

  7. This type of system was typically installed by speculation builders with the cost of the system being the sole consideration - not functionality, longevity, safety or reliability. They went cheap upfront. The cost to run them (60% fuel efficient) and service them is higher than it could be.

These water heaters were never designed to heat houses.



The systems we see in Seattle Townhouses usually look like this and have these characteristics:

They are light-duty intermittent-use domestic water heaters. They were engineered to heat water for the shower, sink and laundry. They are built to heat water on an intermittent basis and turn off until the next use.

When used to heat the house they can run 24 hours a day continuously during the winter. The machine gets overstressed. They tend to be undersized for the function. Most town houses have a supplemental heat source - a fireplace insert in the living room. We have seen metal fatigue, burner destruction, flue baffles crumbled onto the burners, pilot wiring fried, inducer motor housing melted, plastic flue pipes cooked until they crack.

Needless to say, as service mechanics we do not think that these machines are reliable or predictably safe to operate when used as combination heating/domestic water makers.

They are expensive to fix and most plumbers do not know how to diagnose the problem as most problems are electrical control problems associated with the inducer fan, pressure switches, aqua-stat sensor, or electronic valve.

The life expectancy of these units is less than if the water heater was used for domestic water only. Many of these units are replaced in as little as 5-6 years.

This one (in the picture) is put together a little better than some but it is still underpowered and it needs a heat exchanger to separate the radiant water from the domestic water.

Bacteria Risks: When there is no need to heat the radiators, the circulation pump turns off, leaving water in the radiant piping and radiators to sit and become stagnant, sometimes for months. When the heating season starts, all that stagnant water gets pumped into the domestic tank, contaminating your shower, lavatory and kitchen faucet water with high levels of bacteria.

This picture shows a couple of zone valves which shows some attempt at real temperature control of the radiant loops, which many installations lack.

Many of these installations position the equipment in an inside utility space (not on an outside wall) so there are serious restrictions with retrofitting a larger or different type of heater and/or flue pipe or increasing the size of the gas pipe.

Installing a tank-less water heater as a replacement requires a bigger gas line and a different size or type of flue pipe.

If replasced with a tankless type hot water heater, it should be rated for continuous duty. In other words, a high efficiency hot water boiler should ideally be used.

Contact Evan Conklin if you would like an expert to evaluate your options with this type of system. We have licensed plumbers on staff that are familiar with both the radiant and the domestic aspects of these combination heating/domestic hot water systems.

Bradford-White equipment is most commonly used in Seattle town house systems.



Give us a call if you need a consultation of your options for repairing, retrofitting or replacing this type of heating system.