How to pre-heat hot water tank inlet?

Question

I'm looking for a cheap and easy way to extend the amount of hot water available for showers in my house.

I've got a 40 gallon water heater, installed about 1 year ago, and I wish I'd purchased the 50 gallon. In the colder months, it will run out of hot water in about 10 minutes of showering. I have measured the inlet water temperature using a thermocouple pressed against the copper pipe and found that it can range from as low as 8C during the depths of winter to 22C during the warmer months.

I'd like to counter the effect of the super-cold winter water by pre-heating the water before it gets into the tank. I'd prefer a solution that doesn't require any plumbing, but will consider that if I have to. I've heard that there is something called tempering tanks that could be used to store room-temperature water.

I'm considering using a wrap-on copper pipe heating wire or tape on the incoming water line to pre-heat. Something like this or this. I could wire it to only turn on when the burner is activated. I have about 15 feet of exposed copper pipe that I could easily access and put the wrap on.

I'm also considering if I use this method if I should wrap the cold water pipe in an insulating pipe-wrap to avoid losing half the heat to the room. But I worry about overheating the heating wire itself or the pipe wrap.

Would this method work? Is there another method I should consider instead?

EDIT: All of the accessible hot water piping (about 15 feet worth) is already pipe-wrapped. The rest is in the walls, can't get to it.

Answer 1

You can install a water tank booster which typically is sold as a kit that includes a thermostatic mixing valve, temperature gauge and braided hose with fittings. What it does is allow you to set the water heater at a higher than normal temperature (say 160F) and then the mixing valve mixes cold water with the hot water and outputs a normal temperature water (say 120F). This provides 2 advantages. One is that the higher temperature of the water in the water heater prevents the occurrence of contamination such as the virus that causes Legionnaire's Disease. The second is that the mixing valve keeps the water temperature at a safe level preventing any possibility of scalding. Since the hot water is mixed with the cold water, the volume of hot water supplied by the hot water heater is less than without the mixing process thus effectively increasing the capacity of the hot water heater. These kits are sold by Amazon, Home Depot and others. An example is the Cash Acme 3/4 Inch Tank Booster Water Heater Thermostatic Valve sold by Amazon.

Answer 2

A few options:

• install a continuous water heater before the tank.
• bump up the tank thermostat setting, if not already maximum. You'll dial the temp lower at the tap and use less tank water for a shower
• in addition to bumping the tank temperature, add a mixing valve or tempering valve at the tank to avoid scalding
• recover heat from the drain using "drain water heat recovery"
• insulate all hot water piping from tank to shower

Answer 3

A typical keep-from-freezing pipe wrap that plugs into a 15A 120V receptacle can't be more than 1500W. I actually suspect they're a lot less than that. But even at 1500W, that's not nearly enough to make a substantive difference. If the water is just sitting there, it will heat up the water in the pipe. But that isn't very much - even a 1" pipe would have less than a gallon in 15 feet of pipe. So the only way this can work is if it heats the water as it passes through. Use enough electricity and you have created a tankless electric water heater!

But there's a catch. A typical tankless water heater heating up 2.5 gallons per minute uses 13,000W - that's almost 9 times as much power! Which means that to do this effectively you will need a lot of electricity running around that 15' pipe, requiring a big circuit installed to do it. Or to do it safely and effectively, get an actual tankless water heater, install the big circuit, get it plumbed into your system and you have limitless hot water. But at a cost - many people don't have 40A or more of capacity in their electric service to add tankless electric hot water.

Bottom line: Little fixes won't do it. Big fixes will - but they either mean a much bigger tank, a second tank or a lot of power and tankless.

Answer 4

Using a tempering valve and setting the tank temperature higher is the simplest method I know of. Also deals with the potential of legionella bacteria by killing them off with high temperatures.

The tempering valve connects to the tank output and a cold water line, such as the tank inlet. The water in the tank is heated to temperatures that would be unsafe for direct use due to scalding concerns, and them mixed to a safe temperature by the tempering valve on the way out to your fixtures. The mixing of cold into hotter water to make safely hot water means you get a larger volume of safely hot water than you have in your tank.

Technically the tempering valve can go anywhere along the path, but I find having one at the tank itself simpler than multiples around the house. It also costs less.