Plumbing: Tackling tankless
By Jason Fleming
With a long history of use in Europe and Asia, tankless water heaters have been making significant inroads in North America over the past several years. As more and more plumbers are installing this technology, the need for proper installation guidelines is becoming greater than ever.
Installers accustomed to tank-only installs need to understand some of the things that
make tankless water heater installations different from storage-type water heaters, and
how these can impact the selection, installation and maintenance of these units.
Let it breathe
Like any combustion appliance, gas-fired tankless units need a sufficient amount of incoming air for the combustion process to operate properly.
The size of the openings for combustion air — from indoors or outdoors — is based upon the tankless unit’s BTUH input.
If the heater draws combustion air from the surrounding indoors, one square inch of combustion-air opening is needed for every 1,000 BTUH of the appliance. If the heater draws air from the outside, the openings required are at a rate of one square inch per 4,000 BTUH.
Tankless water heaters will typically use higher gas quantities than their tank counterparts to meet a sudden demand for hot water without storage, so the installer must ensure that the building has sufficient gas pressure to meet all the building’s gas needs.
Once the gas meter’s capacity is confirmed, the gas line needs to be adequately sized to supply enough gas to the water heater. A typical tankless water heater will need a 3/4” gas line, but if there is sufficient pressure and run length is short enough, it may be possible to use a half-inch line, which is used by most existing storage tank-type water heaters.
But be sure to account for all the appliances using this line, not just the water heater, and verify gas requirements and sizing with local licensing rules and codes.
Neutralizing the condensate for drainage
Special considerations need to be taken when installing high-efficiency condensing tankless water heaters.
A byproduct of capturing and reusing latent heat from the combustion process is an acidic condensate that must be handled appropriately before it moves to the drainage system.
Check local building codes for safe disposal procedures. A neutralizer may be required to protect the plumbing
Picking the right size
The type of tankless water heater chosen for any given application will depend on incoming water temperatures and building use patterns.
The temperature rise between the desired hot-water temperature and the incoming ground water temperature will indicate how hard the water heater must work to provide hot water for the application.
Incoming ground water temperature can be measured directly or estimated by a knowledgeable professional, but for sizing purposes, the typical worst-case scenario (the middle of winter) should be assumed, to ensure adequate hot-water availability year-round.
To maintain the proper flow to meet the simultaneous DHW needs of the entire building, it’s important to calculate the amount of hot water typically used at any given time, or the peak amount that could be needed.
To do this, count the quantity of fixtures of each type (dishwasher, shower heads, etc.) and then multiply each of these numbers by the flow rate of the respective fixture.
Finally, calculate the sum of all these flow rates as if all the appliances were operating simultaneously. The tankless water heater must be able to deliver this maximum flow rate at the desired temperature rise.
For example, if a building has three fixtures: one shower with a flow rate of 2.5 gpm; a washing machine, at 2 gpm; and a handwashing sink, at 0.75 gpm, the total demand to meet is 5.25 gpm.
Setting the temp.
For residential applications, the hot water set point temperature should only be slightly above the highest temperature needed for domestic uses. Shower temperatures are usually between 104°F and 106°F, so a tankless water heater should rarely be set above 115° to 120°F.
Scalding is more likely to occur when temperatures exceed 125°F, so it is generally not recommended to set the water heater higher than this.
Since tankless water heaters do not store any hot water, they are not susceptible to bacterial growth, and they continuously flush any internal water with every use, so there is no sanitary reason to maintain a high set point temperature.
Water quality is an important consideration for all water equipment, and especially water heaters. Of particular note is water hardness, which is a measure of the minerals, mainly magnesium and calcium, it contains.
When heated, these minerals can precipitate out to form limescale. This coats pipes and acts as an insulator, reducing heat-transfer efficiency and increasing thermal stress, which can ultimately reduce the water heater’s lifespan. As such, scheduled maintenance should include descaling of the water heater