Different tankless water heaters have different flow rates and can supply different rates of hot water.  Some can produce 5 or more gallons per minute which is sufficient to run two showers at the same time.  You have to choose the right size tankless water heater for your household.  The amount of hot water (flow rate) that a tankless water heater can supply depends on the incoming temperature of the water and the set temperature of the tankless water heater.  The higher the temperature rise the lower the flow rate.  So in the winter when the city water comes into the house colder than in the summer, tankless water heaters will produce a lower flow rate.

As the difference in temperature decreases between the set temperature and the incoming temperature of the water, the flow rate increases.  Some tankless water heaters can produce as much as 9 gallons/minute if the difference in temperature is as low as 40 degrees F.  This could happen for example if the tankless is set for 105F and the incoming water temperature is 65F.  A solar hot water system will preheat the water before it reaches the tankless thereby increasing the flow rate of the tankless.

Having said that, water pressure is usually the real limiting factor for how many household facets you can run at the same time with hot water.  Many households don’t have sufficient water pressure to run 2 showers and do the dishes at the same time and this is NOT as a result of the tankless water heater not supplying enough hot water but a result of the size of the water pipes coming into the house.

Cathy: A cold water “sandwich” can occur if water is quickly turned off and on again in one part of the house (like the toilet flushing in older homes while someone is taking a shower).
Aaron:

In my experience and talking to my customers, the cold water sandwich doesn’t seem to be an issue.  I think the cold water sandwich occurs not when someone flushes the toilet but when a small section of the pipe has cold water trapped in it.  For example, lets say you take a show and then someone else in the house takes a shower 20 minutes later. The tankless water heater will take a few seconds to heat up again so although there’s warm water in the pipe after the tankless water heater, some cold water will pass through the tankless water heater before it gets warmed.  This would be eliminated with a solar water preheat system because the water would be warm or hot before entering the tankless.

Cathy: There is significant water wastage while the heating unit is warming up.
Aaron:

The one disadvantage of a TWH (lets use this acronym from now on) is that when the tap is turned on it takes about 10 to 20 seconds for the TWH to trigger and get hot enough so that the water passing through is at the set temperature.  Its this extra 10 to 15 seconds on top of the usual wait time that people notice and it can waste a bit of water.  Having said that, adding a solar water heater before the TWH as a preheat eliminates the added wait time most of the time because the water coming into the TWH is already warm or hot.  So the TWH doesn’t have to work as hard to heat up.

Cathy: An electric system uses too much electricity to off-set any real environmental or cost savings. A gas system (either propane or natural) is better.
Aaron:

An electric tankless water heater needs I believe a 100amp service and uses a lot of electricity to heat the water.  They also typcially have low capacity compared with gas units and can usually only run 1 shower.  Its not something we usually recommend unless there are no other options.

Cathy: Wouldn’t the optimum use of a solar hot water heater, combined with tankless, be during the middle of the day when the sun is shining? Is there any sort of storage unit for solar-heated hot water?
Aaron:

A solar water heater has a storage vessel (tank) usually next to the tankless water heater that heats up during the day and stores the heat for when its ready to be used.  Most solar tanks are insulated well and only lose about 1 degree F/hour once the sun goes down.  So if you shower in the morning, the water in the solar tank will still be hot.

Cathy: Are the “hybrid” systems a better bet for a large family? (ie., a tankless system that includes a small storage tank).
Aaron:

Not really.  A TWH with a small storage tank is usually only used to eliminate the wait time for HW.

Cathy: The pressure is often stronger than is needed for faucets in order for a larger capacity tankless system to work.
Aaron:

TWHs have a minimum flow rate to trigger the burner so if you only have the facet on partially the TWH might not trigger.

Cathy: Tankless systems are best suited for one and two person households.
Aaron:

Not true, because 20 people could live in one house and use a small tankless water heater.  As long as they shower one after the next, they will all have HW.

Cathy: I also wondered if you could pair a tankless system with a drain water heat recovery unit or a circulating pump on a timer.
Aaron:

A drain water heat recovery (DWHR) unit will increase the hot water flow rate of a tankless water heater because it increases the incoming temperature to the tankless and therefore lowers that differential I was talking about earlier.  For example, if the DWHR unit increases the city water temp by 10 degree F and the city water was coming in at 45F, that means that its now reaching the TWH at 55F instead of 45F.  Lets say the tankless is set at 110F.  Then that means instead of having to raise the temp by 65F it only has to raise it by 55F – this increase the hot water flow rate of the unit.

A recirc pump will keep the water running to the taps hot at all times so that when you turn on the tap the water is hot.
However, this is costly to install and will add more electricity consumption and gas consumption.

With a tankless water heater you can’t run out of hot water.  There is no storage of hot water. When you turn on the tap the tankless water heater is triggered and heats the water as it passes through it.  Therefore you can have the tap on 24/7 and never run out of hot water.  With a correctly sized tankless water heater you could run two showers all day a the same time.

Cathy: What’s the biggest obstacle to installing a tankless hot water system?
Aaron:

One of the biggest obstacles to having a tankless water heater installed can be installing the venting or exhaust from the unit. Because they produce a lot of heat in order to heat the water as it passes through the TWH the exhaust from the unit can be a very high temperature. Some tankless water heaters use stainless steel 5 or 6 inch venting pipes as a result of the high temperature. This type of TWH comes with a venting kit, however, any additional venting needed can be very expensive.

However, some units, called condensing tankless water heaters, recover the lost heat that would have travelled out the exhaust. This increases the efficiency of the tankless water heater to as high as 98% efficiency. These are the most efficient water heaters on the market. Also, since the exhaust is at a lower temperature these units use smaller PVC venting. PVC venting is cheaper and can be easier to run longer distances thereby making it easier to find a spot to vent the TWH.

The biggest issue with TWH for installation is that the units aren’t typically exhausted up a chimney. They are direct vent so the exhaust is typically run out the side of the house. Locating an appropriate spot for the venting can be tricky because the building code dictates how close the vent can be to different objects. For example, a vent cannot be within 3 ft of any door or window and it must be 1 ft above grade. It also has to be 2 ft from the property line. So if you have a very narrow passage between houses it may be difficult to find a spot to run the vent. A tankless installer should be able to determine if it can be installed within code.

Thanks for all your help Aaron!

For more information visit their webiste: http://www.goldwatersolar.com/

or, you can reach Aaron at:

Phone and fax: +1-416-400-4747

Goldwater Services North
38 Algonquin Cres.,
Aurora, ON, L4G 3M5
647-520-4942