Question? Just ask us!
Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
InspectAPedia ® Home
AGE of WATER HEATERS
ALTERNATIVE HOT WATER SOURCES
ANODES & DIP TUBES on WATER HEATERS
ANTI SCALD VALVES
APPLIANCE DIAGNOSIS & REPAIR
APPLIANCE EFFICIENCY RATINGS
AQUASTAT CONTROL Functions
BACKDRAFTING HEATING EQUIPMENT
BACKFLOW PREVENTER, HEATER WATER FEEDER
CARBON MONOXIDE - CO
CHECK VALVES, WATER SUPPLY
CLOGGED SUPPLY PIPING
CROSS CONNECTIONS, PLUMBING
DEBRIS in WATER SUPPLY, Water Heater
DRAIN a WATER HEATER TANK
ELECTRIC WATER HEATERS
FLOODED WATER HEATER REPAIR
FREEZE-PROOF A BUILDING
GALVANIC SCALE & METAL CORROSION
GAS BURNER FLAME & NOISE DEFECTS
GAS FIRED WATER HEATERS
GAS PIPING, VALVES, CONTROLS
HEATING COST SAVINGS METHODS
HOT WATER EXPANSION TANKS
HOT WATER SUPPLY
HOT WATER IMPROVEMENTS
HOT WATER DELIVERY SPEED UP
HOT WATER EFFICIENCY IMPROVEMENT
HOT WATER PRESSURE EXPANSION RATE
HOT WATER PRESSURE IMPROVEMENT
HOT WATER PRESSURE LOSS
HOT WATER PROBLEM DIAGNOSIS
HOT WATER QUANTITY IMPROVEMENT
HYDROGEN SULFIDE GAS
INDIRECT FIRED WATER HEATERS
MANUALS & PARTS GUIDES - HVAC
MIXING / ANTI-SCALD VALVES
NO HEAT - NO HOT WATER: HEATER DIAGNOSIS
NOISE, WATER HEATER
ODORS IN WATER
PIPING IN buildings, Clogs Leaks Types
RELIEF VALVE LEAKS
RELIEF VALVE, WATER HEATER
SCALE REMOVAL, WATER HEATERS
SEWER GAS ODORS
SOLAR HOT WATER HEATERS
TANKLESS WATER HEATERS
THERMAL EXPANSION of HOT WATER
THERMOSTATS, WATER HEATER
TIMERS for ELECTRIC WATER HEATERS
WATER HEATER ALTERNATIVES
WATER HEATER ANODES, DIP TUBES
WATER HEATER AIR INLET
WATER HEATER DEBRIS FLUSH
WATER HEATER DRAIN PROCEDURE
WATER HEATER EFFICIENCY
WATER HEATER FLUSH PROCEDURE
WATER HEATER NOISES
WATER HEATER PROBLEM DIAGNOSIS
WATER HEATER COMPARISONS, PROPERTIES
WATER HEATER SCALE
WATER HEATER SAFETY
WATER HEATERS for HOME HEATING USE?
WATER ODORS, CAUSE CURE
WATER PIPES, Clogs Leaks Types
WATER PRESSURE TOO HIGH: DANGERS
WATER PRESSURE LOSS DIAGNOSIS & REPAIR
WATER QUANTITY IMPROVEMENT
WATER SOFTENERS & CONDITIONERS
WINTERIZE A BUILDING
How to Calculate Tankless Water Heater Requirements - tankless water heater capacity sizing guidelines: here we explain how to calculate the size or capacity of tankless water heater you will need in a building.
We name all of the factors that you should consider when buying a tankless or demand or point of use type water heater, such as incoming water temperature, desired output water temperature, and the total hot water flow rate in gallons per minute.
Green links show where you are. © Copyright 2014 InspectApedia.com, All Rights Reserved.
You can calculate the approximate tankless water heater capacity required for any application as follows: Sketch of a point of use tankless water heater courtesy of Carson Dunlop Associates.
[Click to enlarge any image]
T1 = incoming water temperature
T2 = desired output hot water temperature at the plumbing fixture (say a shower)
GPM = anticipated hot water flow rate or usage rate in gallons per minute.
If your incoming water temperature to the building is 40 deg.F. and you want to provide hot water at a bath shower at 100 deg. F., and if your shower fixture runs at 3 gpm, then
T1 = 40 deg.F., T2 = 100 deg.F.
T2 - T1 = 60 deg.F. - that's the temperature rise you need.
Our photo (left) illustrates a Rheem EcoSense® tankless water heater. A wide range of sizes and capacities of demand type water heaters are available, so indeed it makes sense to look carefully at your requirements.
Your tankless water heater will need to be able to provide a 60 deg. temperature rise at 3 GPM.
Note 1: our example does not consider temperature losses in the piping between the water heater and the point of use nor the use of temperature limiting or anti-scald valves in the plumbing system, both of which reduce the actual hot water flow rate in gpm
While 100° may be a good shower temperature, this means that mixing valves at the shower will be delivering almost 100% hot water. This may make filling a bathtub slow or result in lower water pressure than anticipated. When we blend hot and cold water, we enjoy a higher flow rate and commensurately higher pressure at the shower.
Note 2: it's quickly apparent that if the hot water system is going to be asked to provide 100 deg.F. hot water to multiple fixtures simultaneously, the gpm heating rate of your tankless water heater is going to have to be big - and may be beyond the capacity of the equipment. In this case see
Gas fired tankless water heaters can usually produce more hot water faster than an electric unit. Electric tankless water heaters are simplest to install and operate, requiring only wiring and water piping, no fuel piping, no venting or chimney.
If your building includes three or four bathrooms or even just two baths, and if the occupants are likely to want to run hot showers simultaneously at multiple fixtures, the performance of a single demand water heater or tankless water heater may be marginal unless the unit is quite large.
An Example of Sizing a tankless water heater: specifying hot water flow rate & temperature rise required
- Adapted & expanded from Tankless or Demand Type Water Heaters, & from Sizing a Water Heater, - U.S. DOE.
As we explain in detail at TANKLESS WATER HEATER CAPACITIES, since there is no reservoir of hot water in a demand type water heating system, instead, tankless or demand-type water heaters are rated by the maximum temperature rise possible at a given flow rate through the device. As DOE points out in their article, to size a demand water heater, you need to determine
Continue reading at TANKLESS WATER HEATERS or select a topic from the More Reading links shown below.
Suggested citation for this web page
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs)
No FAQs have been posted for this page. Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
Questions & answers or comments about how to determine the size of tankless or demand water heater needed.
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Search the InspectApedia website
HTML Comment Box is loading comments...
Technical Reviewers & References