Resources: Combi-DHW Systems
Some hot water tanks are now approved for combination use to both make domestic hot water (DHW) and to provide heating. These appliances are the lowest-cost heating sources for most residential heating and DHW applications, thus their increasing popularity.
Benefits
- Low installation cost
- Single heating and DHW hot water appliance
- Low maintenance
Drawbacks
- Low efficiency
- Increased wear on hot water tanks (Tanks designed for low-flow, high-delta-T, not heating loads (high flow, low delta-T))
- DHW availability can be limited in cold weather
Sizing DHW Tanks for Combination Systems
In Canada, we have a recent addition to the building code that specifies that no more than half of a hot water tank’s capacity may be dedicated to heating capacity. While this code may not apply everywhere, the rationale for its existence is sound: a combination system must be able to serve both of its loads in all heating conditions. I would argue that in most cases, the hot water tank must be oversized for its DHW load, because only in very cold conditions will the heating load reach its maximum, but when it does, the user will have a lot less DHW than they are used to! Furthermore, events such as winter holidays, when the DHW load would already be strained, often coincide with cold weather. Bottom line: put in a slightly bigger tank than the user may think they need, budget permitting.
Keeping your Combi-System Safe
Combi-Systems require timers on the pumps (as they rightly should) to re-circulate water in the heating pipes such that it does not become stagnant and a breeding ground for bacteria. Because of this, EVERY LOOP of pipe must be able to be re-circulated. This necessity usually means that the system is zoned with pumps, not zone valves or actuators. While it is possible to signal the valves to open for the re-circulation period, it makes for a much more complex electrical system, and is usually avoided in favor of the simplicity of pumps.
Local Codes and Combi-Systems
It’s important to check local codes before installing a combi-DHW system. Some municipalities require heat exchangers to separate potable from heating water, and in some cases, double-wall heat exchangers may even be required.
Chemicals and Combi-Systems
It’s fairly obvious that chemicals in your potable water system are a bad idea. What about chemicals in your closed loop on the other side of the heat exchanger? Depending on what local codes require, and whether the chemicals are toxic, you might be able to add chemicals without a double-wall heat exchanger. However, it should be noted that in the absence of such codes, single-wall heat exchangers should be sufficient, given that if your heat exchanger walls fail, the direction of flow will almost always be from potable side to heating side, rather than the other way around. Why? Simply because the pressure on the potable side is usually 50 psi, whereas the heating system’s pressure usually sits around 12-18 psi. Given this operating pressure, the pressure relief valve on the heating side will be 30 psi, and will pop, producing a visible cue that something is wrong, leading (hopefully) to the replacement of the heat exchanger before any cross-contamination can occur.
Components in Combi-DHW Systems
- Combi-DHW Hot Water Tank
- Drain
- Isolation Valves
- Heat Exchanger
- Unions (for Heat Exchanger)
- Copper Pipe (Type M)
- PEX Pipe
- Fittings
- Manifold
- Wet Rotor Circulators (Pumps)
- Bronze, Stainless or Composite Circulator (potable side only)
- Flanges
- Check Valves
- Expansion Tank
- Air Separator (heating side)
- Water Pressure Regulator
- Backflow Preventer (R.P. or D.C.A.P.)
- Thermostat(s)
- Relay(s)
- Timer (typically 24 hour)
- Transformer (24V)
Diagrams
Combi-DHW system with heat exchanger
Combi-DHW system without heat exchanger
Dual-Combi tank system (2 tanks)
Combi+Storage tank system with recirc pumps
Note option for electric element in storage tank