Tankless water heaters have become very popular and are many times offered as upgrades on new home builds. After dealing with them for more than a few years I will share my thoughts on these devices.
The 2 big selling points are:
They are more efficient than tank water heaters mostly due to the fact that they don’t have any standby heat losses as you do when keeping a tank hot.
They provide a never ending supply of hot water that can service multiple showers simultaneously and service shower after shower without any need for a recovery time.
While both of these things are ostensibly true, there are some considerations.
There are no standby losses however the energy savings can vary based on usage patterns. Tankless water heaters are most efficient when hot water usage is concentrated over a short period of time. Studies have found that when hot water draws are small and spread out over a larger period of time this benefit starts to drop. The issue being that when the tankless water heater cools off, it takes a lot of energy to heat up the heat exchanger and get it producing hot water again. Also, as to point 2, with unlimited hot water people may be encouraged to take longer showers thus using both more water and more energy.
And while they do provide an endless supply of hot water you do need to pay attention to the heating curve of the tankless water heater. For example, one unit is rated at 75F° temp rise at 5 gpm. So if you have water coming in at 45°F you can get 120°F water out. However at 6 gpm the rise drops to 60°F so the output would only be at 105°F. So servicing multiple showers at once may be possible in the summer when the water coming in is warm, but in the winter these units can be pressed to provide fully hot water at the desired volume. I have seen installations where 2 tankless water heaters had to be plumbed in parallel and I have seen installations where a tank water heater was installed prior to the tankless water heater to preheat the water in the winter in order to provide fully hot water at the desired flow.
One misconception is that tankless water heaters are “instant” water heaters which lead some people to believe they will be gettin instant hot water at the tap. This is not the case as the hot water still needs to be distributed throughout the house to the water fixtures via the plumbing. This issue can be remedied by installing a hot water recirculation loop in the home which will provide “instant” hot water. However, this is not a function of the tankless water heater but a function of the recirculation loop.
I have had people contact me saying they just had a new house built and they paid extra for the upgrade to an “instant” water heater only to discover that it takes forever to get hot water to their fixtures and they want to know how to fix it. The builder gave no consideration for a hot water recirculation loop so at this point, because the house was built without a recirculation loop, the best solution is the Smart Thermostatic Control 32 Complete System. This is a good compromise but I am still shocked and appalled at the new construction that is going on in this country without plumbing in a dedicated recirculation loop. Though I know they have historically had issues, our “on demand” Smart Recirculation Control solves them. But I’m getting ahead of myself.
Another consideration for tankless water heaters, especially when replacing a tank in a home that is already built, are the BTU, combustible air and exhaust requirements. Tank water heaters are typically 40k BTU and, as mentioned earlier, most whole home tankless water heaters are 199k BTU. So they have 5 times the BTU requirement. This can require reworking the gas supply line and may not be possible in certain situations. This would have to be assessed by a qualified plumber.
When gas is burning it requires air for combustion where the containing room is required to be a minimum of 50 ft3 per 1000 BTUs. For a 40k BTU tank water heater this is 2000 ft3. For a 199k BTU water heater this is 10,000ft3. And while this requirement can be met by installing fresh air venting and/or louvered doors or relocation of the water heater, it is still a consideration not to be missed when switching from a tank to a tankless water heater.
The other issue I mention is the exhaust gas. The “high recovery” or “condensing” style tankless water heaters use the exhaust gases to preheat the water through a heat exchange before it hits the main gas fired heat exchange. The reason they are called condensing is because they condense water out of the exhaust gases as the incoming water cools it causing the exhaust to drain water. This requires the exhaust flue to be PVC instead of double wall galvanized steel which is what you will find when replacing a tank style water heater. And because it needs to drain, it has to run downhill so it can’t exit the house through the existing roof vent. Given these considerations the installation may end up costing more than the tankless unit itself.
For these reasons tankless water heaters are many times installed outside in areas where there is not a concern with them freezing in the winter. However, this creates an issue for the recirculation pump as it must be rated for outdoor installation or installed in a crawl space underneath the house or in a separate waterproof housing. As our controller are not rated for outdoor installation they would need to be installed in the same weatherproof location.
Another comfort consideration is that Tankless water heaters require a minimum amount of flow to turn on and to keep the burner on so as to provide hot water. If you are doing dishes or shaving and turn the water down to a trickle you may end up with a “cold water sandwich”. This is where the tankless water heater quits heating the water because its minimum flow requirement is not met. This allows cold water to run through it for a period of time and when the hot water is turned back up there is now a slug of cold water in the line working it’s way to the water fixture. While this usually isn’t a big deal, it could be very unpleasant if this occurred when one was in the shower. I have had clients tell me of similar issues when showering during the summer. Because the cold water coming in is warmer in the summer it doesn’t require as much hot water to heat it to a comfortable shower temperature. As they turned the hot water down to cool off the shower the flow of hot water dropped too low and the tankless water heater turned off and the shower went cold. This can be mitigated by turning the temp of the water heater down in the summer time, but is a consideration.
Getting back to “instant” hot water and the delivery of hot water via a recirculation loop. A recirculation loop is a ¾” plumbing line that is routed throughout the house so that is travels within 6’ of every hot water fixture and makes its way back to the water heater. A pump is installed at the end of the loop that, when running, circulates hot water through the loop and back to the water heater thus providing “instant” (or practically instant) hot water at the plumbing fixtures. Historically these pumps were installed and ran 24/7 keeping the loop hot all the time. Not only does this waste a tremendous amount of energy through radiant losses in the loop, but it is terrible for the plumbing and after time causes pin hole leaks. Even when the pump is run on a timer it is a tremendous waste of energy as the tankless water heater will be constantly turning on to maintain the temperature in the recirculation loop. A poorly insulated recirculation loop or one installed in a slab foundation can have as much as a 10F° or more temperature drop in the loop causing the tankless water heater to never turn off. The solution to this problem is to only run the recirculation pump when you actually want to use hot water and only run it it until the loop is hot.
However, there is a comfort issue with this solution which is, if the pump doesn’t turn on until there is demand for hot water, then some period of time will pass before the pump can circulate hot water out to the fixtures. Reducing the resistance in the recirculation loop and installing an adequately sized pump can mitigate this issue. As the loop starts and ends at the same location, it is only the resistance in the recirculation loop that the pump needs to overcome. Installing the recirculation loop with ¾” piping all the way around helps with this and we have written an article on pump sizing that explains all the details of choosing an adequately sized pump complete with calculations.
As I mention, the only resistance that the recirculation pump needs to overcome is that of the plumbing. But one thing that I recently learned is that the “high recovery” or “condensing” tankless water heaters contribute a huge resistance to the recirculation loop. This is quantified in a chart from the manufacturer that plots PSI drop vs gallons per minute (gpm) of flow. This is a chart for the Navien NPE-240A2.
From this chart you can see that at 5 gpm there is an 17 psi drop. To translate this to ft of head (which is how pumps are rated) 1 PSI = 2.31 ft of head. So 17 x 2.31 ft/psi = 39 ft of head just for the tankless water heater alone. If you looked at the pump recommendations article you see that 100 ft of ¾” copper piping provides 5.8 ft of head resistance at 5 gpm. So you would need a pump with 45 ft of head in order to get 5 gpm flow through this type of system. Since 100 ft of ¾” copper contains ~2.5 gallons of water, at 5 gpm it would take 30 seconds to heat up the loop. However this level of flow would require a very large pump. The largest pump we sell is the AquaMotion AM5-SF1L which has 1/12 HP and 9 ft of head 9.5, roughly 1/5 the required head to provide 5 gpm flow. As the resistance curve is not linear at 2 gpm the resistance in the water heater is 3.5 psi x 2.31 = 8.1 ft of head and the resistance in the piping is roughly 1.5 ft for a total of 9.6 ft of head. So a pump with 9 ft of head would provide ~2 gpm of flow which makes the recirculation time approx. 1 minute 15 seconds. This is an idealized calculation for 100 ft of ¾” pipe so actual results would be slightly higher because, as we discuss in the pump recommendations article, there are going to be elbows in the loop which increase the resistance as well as some of the heat will be absorbed by heating up the plumbing itself.
There are 2 options to this issue. The first one I’ll call the brute force method. It is possible to gang 2 AquaMotion AM5-SF1L together and double the head to 18 ft so that would provide roughly 3.2 gpm in the above calculated scenario. Or you could search for a pump from another manufacturer that meets these specs. The Taco 0013 has 31 ft of head at 4 GPM and the above water heater has 11 psi at 4 gpm which is 35.4 ft of head and at 4 GPM the piping would add 3.83 ft of head for a total of 29.25 ft of head. So that pump would provide flow right around 4 GPM.
The second option would be to eliminate this pressure drop from the recirculation loop by installing a 10 gallon tank water heater installed immediately after the tankless water heater and the recirculation loop should only recirculate through the tank thus eliminating the pressure drop created by the tankless water heater. With this configuration there is only 5.8 ft of head drop in a 100 ft ¾” loop so the AM5 with 9 ft of head would be able to provide 5 gpm flow and heat the loop up in 30 seconds as opposed to 2.5 minutes. Any hot water used will be replaced by hot water from the tankless water heater so there is still a never ending supply of hot water. This design also eliminates any issues with a cold water sandwich as the 10 gallon tank will act as a buffer preventing any large swings in hot water temperature. However this solution can cause an issue with long recirculation loops as the water coming back can cool off the small tank quickly. The calculation to figure this out is:
Another design that I came across recently is that instead of putting the small tank after the tankless water heater, the small tank can go at the end of the recirculation loop and the tankless can feed directly into the hot water line after the tank. This will allow the tank to heat up the loop but any water that is used, instead of it going into the tank, will go directly into the hot water line eliminating the need to pull the 10 gallons out of the small tank before the 120°F water from the tankless gets into the line. I am working on a few schematic drawings to better show this type of layout but if you have questions before I have a chance to do that please contact me and I’ll try to better describe it.
If you have made it this far in my article, congratulations! I hope you have found it informative. After seeing what people have gone through, and the expense they have incurred, and the workarounds they have had to implement, I can say most of the time I recommend against a tankless water heater. However, if you want a never ending supply of hot water I recommend the design I described in the previous paragraph whereby a 10 gallon tank water heater is installed immediately after the tankless water heater. If you are installing a tankless water heater because they are more efficient, I recommend you look into either high efficiency tank water heaters or one of the new hybrid electric heat pump water heaters. Regardless of the method of heating the water, every home should have a recirculation loop with an on demand Smart Recirculation Control system from Leridian Dynamics, Inc.
Disclaimer: The thoughts expressed are my opinion and a result of my personal experiences dealing with tankless water heaters and with customers who have them installed. I certainly don’t mean to disparage any manufacturer or product or offend anyone as your experiences may be different.