Domestic Hot Water (DHW) is potable water that is heated above 110 F (as opposed to pool heating or space heating) and is suitable for bathing, drinking, and washing clothes and dishes.
Massachusetts State Plumbing code specifies that “the maximum temperature of the domestic hot water in residential buildings shall not exceed 130 F” and that showers have a temperature control mixing valve that limits the maximum temperature of the hot water supplied to individual shower controls to 112 F.
There are many ways to make domestic hot water; solar collectors, using a boiler with an indirect water heater, with a stand alone water heater (gas or electric), with a heat pump water heater (the new air source heat pump units), a propane fired tankless wall hung unit, and within a boiler itself (tankless coil). Each of these approaches has benefits and drawbacks – basically the two guideline are how much domestic hot water is needed and at what cost/efficiency. Interestingly, with new net-zero homes the domestic hot water load becomes much larger than the heating load which leads to interesting opportunities to reduce the size of the mechanical plant.
Heat pump water heaters:
The concept of using a heat pump to heat domestic hot water has been around since the 1940s but was not realistic until a few years ago. The new heat pump water heaters work quite well in heating water while dehumidifying the space they are in. Most of them have a heat pump and small fan on the top of the tank and two conventional water heater elements in the tank that serve as a back up in the event there is an issue with the heat pump itself. The New AeroTherm™ Series Heat Pump Water Heater from Bradford White is highly efficient providing year-round energy and money savings. It offers advanced technology and low cost operation. It features four operating modes – Heat Pump Mode, Hybrid Mode, Electric Mode, and Vacation Mode. These are available in 50 and 80 gallon sizes.
Electric tank type water heaters:
Federal regulations will end the sale of 80 gallon tank type electric water heaters to encourage the sale of 80 gallon heat pump electric water heaters. Therefore, only 50 gallon tank type electric water heaters will be available. There may be some cases in which 80 gallon commercial water heaters will be available but they are usually more expensive than residential models. Water heaters on the Vineyard usually last 5 to 8 years – as water quality is so variable across the island it is sometimes hard to predict.
Propane tank type water heaters:
There are many options for venting of propane tank type water heaters – high efficiency, atmospheric venting, power vent, direct power vent, and direct vent – all different approaches to getting combustion air into the water heater. These come in 50 and 80 gallon sizes. Many of these models can vent vertically or horizontally and are also approved for direct-vent, closed combustion applications.
Propane tankless water heaters:
Tankless water heaters heat up potable water in a single pass through their heat exchanger; the incoming cold water at 55 F is heated up to as much as 130 F. They are very efficient and don’t have a tank (hence the term tankless). They have similar venting issues as tank types but usually need to be located on outside walls for ease of venting. They are sized based on the expected gallons per minute needed of domestic hot water – this means that plumbing fixtures that need more flow (such as Roman tub fill valves and rainshower shower heads) will need larger or multiple tankless water heaters to meet the peak flow requirements. Sometimes each bathroom will need its own tankless water heater due to the flow needed.
Boiler-based indirect water heater:
By connecting a domestic hot water indirect tank to a boiler, an enormous amount of domestic hot water can be created and delivered to meet practically any plumbing fixture flow flow requirements. It is called an “indirect” as the potable water inside the tank is heated indirectly by a hot water coil inside the tank which in turn is heated by the boiler. The only limitation to the amount of domestic hot water is the size of the boiler. Many times, multiple tanks can be ganged together. These tanks are also valuable if solar hot water is to be integrated into the domestic hot water system.
Our goal is to be Martha’s Vineyard’s premier plumbing and HVAC shop – we appreciate the opportunity to service, install, and rescue your domestic hot water system!