Jeff Flowers on January 10, 2014 52 Comments Tankless water heaters are rated by the maximum temperature rise possible at a given flow rate. Therefore, to size a demand water heater, you need to determine the flow rate and the temperature rise you’ll need for its application (whole house or a remote application, such as just a bathroom) in your home. It is important to note that you should never try and save money by undersizing your tankless water heater. Step 1: Determine the maximum number of devices you want to run and their total flow rate. Then, add up their flow rates (gallons per minute). This is the desired flow rate you’ll want for the demand water heater. For example, let’s say you expect to simultaneously run a hot water faucet with a flow rate of 0.75 gallons per minute and a shower head with a flow rate of 2.6 gallons per minute. The flow rate through the demand water heater would need to be at least 3.26 gallons per minute. To reduce flow rates, install low-flow water fixtures. Step 2: Determine required temperature rise. To determine temperature rise, subtract the incoming water temperature from the desired output temperature. Unless you know otherwise, assume that the incoming water temperature is 50°F (By using a low temperature assumption you ensure that you will not undersize your tankless unit. If you live in a warm climate your water temperature will likely be much higher.) For most uses, you’ll want your water heated to around 105–115°. In this example, you’d need a demand water heater that produces a temperature rise of 55°. Step 3: Sizing Example: An average shower will be between 104–106° and uses 2.6 gallons of water. Assuming your water temperature is 40° coming into your home, and you want to produce enough hot water to run 2 showers at the same time, what temperature rise would you need to produce to accomplish this? Answer: You’ll need to raise the incoming water temperature from 40 degrees to 105. You’ll need to be able to heat at least 5.2 gallons of water. So you’ll need a tankless water heater that can produce at least a 60 degree rise in temperature at 5.2 gallons per minute. Flow Rates How much hot water do you need at one time? Do you need to run 2 showers at the same time or maybe a shower and a couple sinks? The chart below shows the range of water usage range and average water temperatures for various fixtures. We suggest using 2.5 gpm for a shower and 1.0 gpm for a bathroom as a reference point in determining your total simultaneous water needs. Fixture Average Flow GPM Average Temp. Tub 4.0 GPM 102°F Shower 2.5 – 3.0 GPM 104°F Washing Machine 2.0 GPM 120°F Dishwasher 1.5 GPM 110°F Kitchen Sink 1.5 GPM 110°F For example, if you are running 2 showers at the same time, you will need 5 gallons of hot water per minute from the tankless water heater. If you were running a shower and the washing machine at the same time, you would need 4.5 gallons per minute from the water heater. In either of these situations, you will want to size the unit that meets or exceeds how much hot water you need at the same time. Note: tankless water heaters are intended to heat potable (drinking) water only and the inlet water to a tankless unit should not be pre-heated. Other Sizing Notes Gas tankless water heaters are able to produce a larger temperature rise per gpm than electric models. Most demand water heaters are rated for a variety of inlet temperatures. Typically, a 70°F water temperature rise is possible at a flow rate of 5 gallons per minute through gas-fired demand water heaters and 2 gallons per minute through electric water heaters. Faster flow rates or cooler inlet temperatures can sometimes reduce the water temperature at the most distant faucet. Recommended Reading: How to Use a Washer Dryer Combo 3 Common Misconceptions About Washer Dryer Combos Wall Heaters 101: Your Complete Guide Infrared Heaters 101: Your Complete Guide
Roger Powell says December 26, 2014 at 3:04 pm Jeff – In your opinion, to serve the kitchen sink/dishwasher, a 1.5 GPM would suffice? Reply
Doug says September 6, 2015 at 10:01 am Great info! Clicked on you from google search and you told me exactly what I needed to know! Thanks! Reply
Dave says September 9, 2015 at 4:15 pm I didn’t realize gas tankless water heaters could heat the water faster than electric. Thanks for the info. Reply
Ed Galt says December 13, 2015 at 3:48 pm I have a tankless water heater for my home that is in the attic. My wife complains that it takes too long to get hot water downstairs in the kitchen for the clothes washes and dish washer. Can I mount a unit outside the kitchen and have it plumbed only to those two units? Reply
Doug Pendarvis says January 2, 2016 at 11:13 am I don’t have a whole house tankless water heater yet, but on my water heater I put in a recirculating pump on the hot water side and I have instant hot water at all my faucets all the time. Yes, you may have to do some modification to your hot water piping to make this work.Talk to a plumber for his ideas. This is the best thing I have done to get instant hot water to my furtherest faucet which is about 90 feet away. The recirculating pump does run continuously, but you could put an inline timer and have the pump come on only while you are at home. We are retired so we keep the pump on all the time. Good luck to you. Reply
Ken S. says January 9, 2017 at 9:58 pm Doug: The down side of the recirc pump is it puts all the cooled water back into your tank which now has to be heated again so your energy bill to heat the water will dramatically increase. When I pulled the plug on mine, our propane bill was cut 50%. I’ll wait the extra 30 seconds or a minute for the water to get hot in my shower. Just enough time to pour a cold one and get undressed. I also have small electric units under the master and guest bath sinks so instant hot for washing hands etc. Reply
Gary says November 9, 2017 at 9:35 am Doug, unfortunately, Ken is correct on the added energy bill, whether it is gas or electricity. The incoming water to the tank is heated to 140 Deg F, then shuts off. Depending on water usage, the energy source kicks on once the water temp reaches 100 Deg F. There is time in between the on and off sequence. Using a continuous recirculating pump system, turns the water heater on – off sequence into a race horse. It is constantly turning on and off, thereby per Ken, increasing your utility bill, either electric or gas. Suggest using a multiple timing system to energize the recirculating pumping system, for specific need for hot water during the day, thereby, decreasing some of that added utility cost. Reply
Junior says December 21, 2017 at 10:32 am If you are concerned about operating costs but still want hot water quickly the best way to do it is to install a flow switch on the inlet side of the hot water heater and set your recirc pump to run when flow is detected. This way the hot water will get to the low consumption faucets 2 or 3 times faster and the water only heats when you are using it. Reply
Fonz says November 12, 2017 at 11:09 am You can get as many tankless water you want. The value of tankless is it does not need much space. The drawback is need the energy source. If it’s easy, feasible, and economical to run gas line or power line close to use of the water it reduces loss and makes the whole system more efficient. Reply
Charles says January 19, 2016 at 1:50 am Great well researched article. It has helped me in getting the perfect sized heater for my home. Reply
David Davis says January 21, 2016 at 9:09 am I never thought of putting more than one water heater in my house. I want to get a tankless water heater, but I wasn’t sure if it would work for our house. We generally aren’t trying to run laundry, dishes, and showering all at once anyways. Putting in two units could definitely help, maybe just one for each floor. Reply
Ray says February 6, 2016 at 10:24 am The GPMs you quote are the total output of the fixtures? If so who uses all hot water when showering or taking a bath? In my experience one usually sets the knob to something comfortable, which, of course, is a mixture of hot and cold. Thus the tankless heater demand would be lower per fixture. Reply
Tom says November 14, 2017 at 7:47 pm Well done…you proved you didnt read the article. It purposefully STATES to size for simultaneous use…meaning absolute worst case. There is a follow on article linked to that explains why you should NEVER skimp on sizing….but good job Reply
w weihe says March 9, 2016 at 6:10 am This is some great information, and I appreciate your point that tankless water heaters can provide continuous hot water over a long period of time. We live in six persons household, so there are a lot of us that have to share hot water for our morning showers, and usually the last two people don’t even have any. I’ll definitely look into the option of getting a tankless water heater so we don’t run out. Thanks for the great post! Reply
chris parent says August 11, 2017 at 10:30 pm hello, did you ever get a tankless water heater for your large family? We have a family of 7 and want to know a tankless WH would work for us and if so what specs would you recommend? Reply
Linda Moreno says April 6, 2016 at 4:36 pm What sixe would I need to do a hot load only on 1 day a week and maybe a warm load another day in the week?? Reply
Janice says April 21, 2016 at 12:15 pm I have a 4.3 gpm model and it does not give me hot water to bathroom faucets or shower without running more than one faucet. Is this normal? Reply
Josh says February 6, 2017 at 4:45 pm Janice, turn the temp down on the heater. Set it to 120ish and try. keep going down as needed if that doesnt help. The higher the heat setting, the more GPM it takes to turn on most instant heaters. Reply
Sarah Smith says May 18, 2016 at 3:15 pm My families hot water heater has stopped functioning and we are trying to figure out what size to get for our new one. Thanks for the suggestion to figure out the maximum number of devices that the heater will be powering. Hopefully, we can find a good sized water heat to properly heat the water in our house. Reply
Peter says May 26, 2016 at 10:22 am hi, Thanks for the great article. I enjoyed. I am using the electric tankless water heater since last years and they are suitable for my mobile home. Your article will help the newbies to choose the right type and size of tankless water heater. Reply
Sarah says June 2, 2016 at 9:20 pm I am wondering what size tankless hot water heater I need for my kitchen sink and dishwasher ? I am really in need of help Thank you in advance Reply
Garland Stueart says June 12, 2016 at 9:32 pm Need to run up to 6 showers at once. What do I need to achieve this using tankless water heaters? This is for a Disaster Relief Shower Trailer using propane. Reply
Graeson Maddox says September 20, 2016 at 10:32 am You need to understand the GPM requirements of the water heater. The showers flow relates to the pressure. If the setup already exist the best thing to do would be to run all the showers at once for 1 minute and measure the amount of water used. I’ll let you decide how to do that. Once you understand how much flow the showers are requiring you can then select a water heater. My only guess would be a lot. If we use his 3.0 gpm case for a shower and you have 5 showers this is 15.0 gpm. You will need an extremely high KW instantaneous water heater to get any type of significant heating. Reply
Sharon Hodges says June 28, 2016 at 7:40 am Desperately need your advice! I have a 800 sq ft one person apartment that needs a tankless electric water heater. Only need hot water for shower, bath faucet and kitchen faucet, what size water heater do you recommend?? That’s for your help!!! Also I live near Savannah GA. Reply
Nancy Blueweiss says November 15, 2016 at 3:23 pm I have a similar set up but am in Boston. Did you ever get a reply to your post? thanks! Reply
teri butler says July 11, 2016 at 6:04 am Jeff, Great summary of pros and cons! My very small church is considering buying a tankless water heater. We have no showers, automatic dishwashers or laundry facilities. Basically we need to have hot water in out kitchen and bathroom sinks. We are located in the the South. Is tankless the way to go? What is the smallest size we can get and still get reliable hot water? Reply
vanessa stephens says October 20, 2016 at 10:59 am new home construction. 5 bathrooms and 5 people. I have tried to calculate out the needs. plumber rec’g one Navien unit(about 8gpm flow rate) and another rec’g two rinnai units(i think about 7.7 gpm). struggling to decide between brands as well as if need 1 or 2. people have experience they can share? Reply
Nancy Blueweiss says November 15, 2016 at 3:14 pm Jeff, thanks for this helpful information. I have a 685 sf 1 bedroom condo in Boston that I rent out. I’m considering an Eccotemp FVI-12-NG tankless. I’m concerned about whether there will be sufficient capacity for a bathtub, as one review says no. The specs say the capacity is .93-4.8 GPM, other info says 4 and 3.4 GPM, and “45 degree temp rise when operating at max capacity and 77 degree temp rise at 2.3 GPM. Will this be sufficient to fill bath with hot H20? many thanks Reply
Crissy davis says November 17, 2016 at 8:40 pm Tankless water heater is the way to go nowadays! It will save you $ and the hot water supply is endless Reply
John Bock says November 22, 2016 at 7:44 am We are installing a vat/pasteurizer to make cheese. The vat is brought to pasteurization temperature, 145 degrees, by a circulating jacket of hot water, like a bain marie. Commonly closed loop systems are employed. Vat manufacturer recommends we look for a thermostat controlled 20gpm water heater, less waiting around while the milk heats. Where do I find one? Reply
Mk Hughes says December 2, 2016 at 10:37 am Our regular water heater just gave us the gift of a wave of water pouring out and all into the garage….therefore today we must get another. In choosing a tankless….question for a dumb blonde consumer: I am a homemaker to fourboys and husband….run the dishwasher and clothes washer often at same time, have three full baths which may or may not be used at same time when getting ready for church or other events….suggestions?? Reply
Josh says February 6, 2017 at 4:53 pm all depends on the shower head flow. Typical is 3 gpm but some can go much higher. You’ll need a fairly large heater to hit 9 gpm + dishwasher and clothes washer. They are out there though but they require 199,000 btus of gas which most peoples homes aren’t really piped for. Majority of homes only have 1/2″ and 3/4″ gas piping. You could be hit with more up front cost to increase the gas service. Reply
Ashvin Patel says February 12, 2017 at 12:45 pm I WANTS TO KNOW ABOUT SIZE OF TANKLESS WATER HEATER I NEED FOR MY HOUSE. WE ARE FOUR PEOPLE IN HOUSE. TWO WASHROOM USE FOR BATHE OR SHOWER. CAN SUGGEST WHAT MADE IS BETTER & LAST LONG. IS IT TO CHANGE FROM WATER TANK WITH GAS OPERATION TO TANKLESS. IS IT COST SAVING & SPACE SAVING .CAN YOU GUIDE ME TO BUY OR NOT. Reply
Jackie says March 1, 2017 at 11:02 pm When I lived in Asia, there is a tankless water heater for each bathroom and kitchen. May be they are smaller unit then the ones in the US. Reply
David says March 19, 2017 at 11:41 pm I lived in Japan while in the military. I lived off base for a short time. The house I lived in had gas tankless hot water heaters also. One at the kitchen sink, one in the bathroom and one in the laundry room. It seemed far more efficient than heating 50+ gallons of water 24/7. This was way back in 1993, good to see the US catching up. I live in a 825 square foot house with a need for hot water for a shower (no bathtub), bathroom and kitchen sink and clothes washer. I am thinking I need only 5 gpm, any suggestions? Reply
Lukas Brockton says March 22, 2017 at 12:24 am It should be noted that many modern day Dish washers can actually heat their own water electronically. Reply
sue says April 13, 2017 at 8:47 am ok, so I want a tankless. if I never run 2 “appliances” at one time, what size can I get away with. I have a shower and tub, dishwasher 2 sinks. (I live in a small house). can someone reply. OH, and how difficult are they to DIY install?? Reply
Tom Sheehan says July 5, 2017 at 11:57 am What size tankless heater do i need for a 3 bedroom 2 bath house. I have 2 small children and we run the washer daily Reply
Kent Ward says September 12, 2017 at 6:31 pm We want to replace old water heater with gas tankless. Three bathrooms, on on each level all with showers. Dishwasher and washer on second floor, which is our main floor. We live in mountain area of east coast. Have any idea the size of one that would run our whole house Reply
Debra Robinson says October 2, 2017 at 3:22 pm I would like to know what size tankless is needed for a 2500 square foot house Reply
Pope says October 18, 2017 at 8:22 am Can I use a 110v electric tankless water heater for a shampoo bowl area for a salon and how many 4gpm would I need Reply
Pope says October 18, 2017 at 8:23 am Can I use a 110v electric tankless water heater for a shampoo bowl area for a salon and how many gpm would I need Reply
Robert says January 12, 2018 at 12:05 pm I have a heart shape jacuzzi tub and 3 showers in the home never enough hot to fill tube can i fill the tub if someone is showering and wash is being done what size unit do i need or how about 2 showers and wash Reply
Lee says January 15, 2018 at 1:15 pm Dear Jeff, I have a nail salon with 9 pedicure chairs. I currently have 80-gal AO Smith tank water heater. In the busy days, I ran out of hot water. I though to install an additional water heater and not sure that should I purchase another tank water heater or tankless one, and if tankless, what will be the right size? Thank you and best regards. Lee Reply
Vilya says January 16, 2018 at 9:25 am Hi, we presently have a gas heater in our holiday home, but the municipal water pressure is very variable. We also seem to have a bit of scale build-up in the warm water pipes (any advice on how to get rid of that would be much appreciated). Unless you shower early in the morning or late at night, chances are slim that you will get any hot water, since the gas heater is triggered by a sufficient flow of water. So cold showers and hot tempers all around. 1. Would a tankless water heater be similarly affected by the lack of water pressure? 2. Can we install the tankless heater in the same spot where the gas heater is presently i.e. connect it to the same pipes? (the gas heater would be removed) 3. If so, it can heat the bathroom and kitchen taps. Correct? Thanks for the help, Vilya Reply
Joe says January 19, 2018 at 11:35 am Jeff I’m moving into a smaller home 1 shower and dishwasher we do wash only in cold water. We live in penna what would you suggest as size for gas hot water tankless unit. Thanks Reply
Don says January 22, 2018 at 3:04 pm You never do answer the actual question of what size water heater is required for demand. You talk about flow rate but not kW. So running a shower and dishwasher at the same time would require what size heater as far as kW? Reply
eugenia care says February 1, 2018 at 10:30 am Great info! So, I have a tankless gas water heater – it is very problematic! The temp is turned up as high as it will go, and still, sometimes water will not get hot or even warm and sometimes it will! However, it seldom stays warm thru the entire shower! Sporadic heating! The unit is approaching 20yrs old – time to replace it??? Or might it be repaired? It is a 3 story bldg with a beauty salon on the 1st floor and 4 other people on the 2nd and 3rd floor. Note: even when the salon is closed, there are still issues! HELP!!! and, in this case, might it be better to replace the tankless with a regular heater? Thanks Reply
hilary says February 8, 2018 at 12:23 am Hello, your article notes that inflow water should not be preheated. With the new smart technologies, is this still the case? I have solar preheating and want to install a Takagi T-H3-DV-N to boost the solar heated water. Thanks Reply
Kathryn says February 9, 2018 at 10:02 am I have a small restaurant, no dishwasher, 3 compartment pot sink, 2 hand washing stations and 2 bathroom, what size tankless water heater do I need Reply
[…] of water flow that is around half a gallon every minute before it starts to kick in. in short, a tankless water heater requires you to run the faucet on its maximum flow even if you only need a little amount of warm […] Reply
