DHW stands for domestic hot water, the water used at taps, showers, and appliances, as distinct from the water that circulates through a home's heating system.
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Quick answer
DHW is short for domestic hot water — the hot water that comes out of your taps, shower, and appliances for everyday household use. It's a term technicians use to distinguish this from the separate water loop used in hydronic space heating, which circulates through radiators or in-floor tubing and never mixes with what you actually wash dishes or shower in. Some systems, like combi boilers and indirect tanks, produce both DHW and space heating from the same gas-fired source, while others keep them fully separate.
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It's easy to assume all the hot water moving through a home's mechanical system is the same thing, but DHW and space heating water serve genuinely different purposes and, in most systems, never physically mix. DHW is potable — safe to drink, shower in, and use in appliances — and it's what you're using every time you turn on a hot tap. Space heating water, by contrast, circulates in a closed loop through radiators, baseboards, or in-floor tubing purely to transfer heat into the rooms of your home, and it's never meant to come out of a household tap.
Keeping these separate matters for both safety and system design. A boiler that heats space-heating water directly doesn't automatically produce DHW — that requires either a separate storage tank, an indirect tank using a heat exchanger coil, or a dedicated combi boiler design that can switch between the two functions.
Most homes get their DHW one of a few ways. A standalone gas storage tank heats and stores water dedicated entirely to household use, with no connection to space heating at all. A tankless unit heats DHW on demand as it flows through, again independent of any space heating system. In homes with a hydronic boiler for radiant or radiator heat, DHW is commonly produced either through an indirect tank — a separate insulated tank with a coil heated by the boiler's water loop, keeping the two streams physically separate — or through a combi boiler, which is a single unit designed to prioritize DHW production on demand while also handling space heating.
| System Type | How DHW Is Produced |
|---|---|
| Standalone gas storage tank | Dedicated tank, unrelated to any space heating |
| Tankless water heater | On-demand heating, unrelated to space heating |
| Boiler + indirect tank | Boiler heats a coil inside a separate DHW storage tank |
| Combi boiler | Single unit switches priority to produce DHW on demand |
Whatever the system type, DHW capacity needs to be matched to your household's actual usage pattern — number of bathrooms, typical simultaneous demand (showers running while a dishwasher or laundry machine is going), and household size. A combi boiler or indirect tank sized generously for space heating doesn't automatically mean generous DHW capacity, since the two functions can compete for the boiler's output during periods of simultaneous demand, particularly in colder months when the heating system is also working harder.
This tradeoff is worth discussing directly with a technician if you're considering a combi boiler for a larger household — some combi models handle simultaneous heavy DHW and space heating demand better than others, and matching the right model to your household avoids a common source of complaints about inconsistent hot water in homes that rely on combined systems.
Inconsistent DHW temperature, reduced flow, or a noticeable delay before hot water arrives can stem from several different sources depending on your system type: sediment buildup in a storage tank, scale in a tankless heat exchanger, an indirect tank's coil losing efficiency, or a combi boiler's DHW priority logic not switching cleanly under simultaneous demand. Because the underlying causes differ so much by system type, describing your exact symptoms and system setup to a technician leads to a much faster, more accurate diagnosis than a generic troubleshooting approach.
Homeowners renovating a bathroom or adding a laundry room sometimes assume their existing DHW system will automatically keep up with the added fixture count. Whether that's true depends entirely on the specific system's remaining capacity — an indirect tank or combi boiler that was already close to its limit for the original household size may need reassessment once additional simultaneous demand is added, which is worth raising with a technician during renovation planning rather than after the fixtures are installed.
It's also useful to know that DHW recirculation systems, sometimes installed to reduce the wait for hot water at distant fixtures, add their own considerations around standby heat loss and, on some system types, slightly increased demand on the DHW source itself — a detail worth discussing if faster hot water delivery at a far bathroom is something you're considering.
Finally, when comparing system types for a new build or major renovation, it's worth asking specifically how each option's DHW output changes under simultaneous space-heating demand during a cold snap, since that's the scenario where differences between combi boilers, indirect tanks, and standalone dedicated water heaters tend to show up most clearly.
One more useful distinction for homeowners comparing quotes: some contractors describe DHW capacity in terms of temperature rise at a given flow rate, similar to how tankless units are rated, while others describe it in terms of stored volume and recovery, similar to a storage tank — asking your technician to translate whichever figure they're quoting into terms you can compare against your actual household routine avoids confusion between fundamentally different measurement approaches.
Safety note
Gas-fired water heating equipment involves combustion, venting, and pressure, and any inspection or repair that touches the gas line, burner, or venting must be done by a Technical Safety BC licensed gas fitter. If you ever smell gas near a water heater or boiler, leave the home immediately and call FortisBC at 1-800-663-9911.
In a properly functioning system, no — DHW and space heating water are kept physically separate, whether through a standalone tank, an indirect tank's heat exchanger coil, or a combi boiler's internal design, specifically because DHW needs to remain potable.
Some combi boilers prioritize DHW but still have a finite output, so heavy simultaneous demand for both space heating and hot water can affect performance depending on the specific model's design and sizing for your household.
Not quite — an indirect tank doesn't have its own burner; it's heated by hot water circulating from a separate boiler through an internal coil, whereas a standard gas storage water heater has its own dedicated burner.
It depends on your household's bathroom count, typical simultaneous usage, and system type, so a technician should size DHW capacity to your actual usage pattern rather than relying on a generic rule of thumb.
Often yes, typically by adding an indirect tank, but whether it's practical depends on your existing boiler's output capacity and mechanical room space, which a technician can assess.
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