Can tankless water heaters be used in series?
My tankless water heater works fine for water use near the unit. For areas far from the tankless unit a lot of water is wasted waiting for the long run to heat up. Can a small unit near the areas that take time to get hot water be used in series with the bigger primary unit?
Getting the smaller unit to still pass the hot water from the larger unit but with the power (propane) off seems to be the issue.
Replies
You can use a small tank.
Small tankless water heater
Thanks, DanH. I'm still wondering if the small tank heater water valve will stay open when the there is no longer power demand on it because the big tank is now supplying the heated water. I suppose the small tank set point could be set at a higher temperature than the big tank but then both tanks are drawing (propane) power.
Series WH
There are some tankless water heaters made for solar back up. They adjust to the incoming water temperature. i have a small electric tank heater in series with my larger tank heater. If you installed a small unit and insulated the heck out of it, and set the thermostat to less than the incoming water temperature, it might never need to come on. If so only for minimal times since it would only be storing preheated water.
Do keep in mind that a tankless is generally less efficient than a tank heater, except in situations (eg, a weekend cabin) where the unit is used intermittently.
Short Answer: Bad Idea
Much of the marketing for tankless heaters has disappointed me. Let me use an example, and maybe even make you smile a bit.
A guy goes out and just buys a pair of shoes. He might ponder which shoes are 'best,' but shoes are just shoes.
A woman, however, can't do that. No, each pair of shoes is looked at as part of an outfit, and the occasion for wearing the outfit matters.
Let's look at plumbing as a woman looks at shoes- as part of a system.
In traditional plumbing, ANY heater has to flush out lots of cold water before the freshly heated water arrives at a remote tap. Part of this waste is because of the distance between the tap and the heater; much of the waste, however, is caused by the design of the plumbing. In traditional plumbing, the heater feeds a big pipe, which only gets smaller near the end of the run. Before hot water can arrive, all that cold water has to be flushed out.
These days, we have "PEX" as a plumbing material. This has introduced us to the idea of using a manifold at the heater. Instead of feeding a single big pipe, the heater feeds many small pipes. Each small pipe (tube) goes straight to an individual tap. When the tap is opened, you only need to flush out a small amount of water before the hot water arrives.
There's no reason a manifold can't be used with a conventional heater. PEX is the key. With PEX, you can run the line pretty much without any fittings- the tubing bends around obstacles and around corners.
Now, let's look at water heaters.
Every water heater has a 'recovery time.' That is, the time it takes to heat water a certain amount. Here's where different approaches are used.
Let's use a shower as an example. A shower head is limited to letting 2.2 gpm of water flow. Since we typically use a mix of hot and cold water at the shower, let's assume you need 1.5 gpm of hot water. To get enougn hot water for a 15-minute shower, you need either a water heater that can heat 1.5 gpm, OR you need to have 23 gallons of hot water sitting in a tank, ready to use.
Water heaters are available in different recovery rates - that is, the amount of water they can heat in an hour. For example, for a typical 50-gal tank heater, you can get heaters with a 50 gallon per hour recovery rate, or one that will heat 100 gallons in that same time. Think of the difference between heating water with a candle, versus using a blowtorch. A tankless heater will supply that shower forever, since it heats the water as fast as the shower can use it.
If you want hot water to arrive at the tap, quickly and with minimal waste, you need to do two things. First, you need to make that pipe as direct and small as you can. Second, you need to insulate the pipe so less heat is lost along the way. A simple run of PEX will be a dramatic improvement, on both counts, than traditional copper.
If you run 1/2" PEX to the tap instead of 3/4" copper, you'll have to flush out less than a pint of cold water before the hot arrives. That's 1/3 the water wasted. For long runs, the key is to reduce the amount of water in the pipes by using a smaller, direct line.
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There's another problem with placing tankless heaters - or any kind of water heater - in series. That is, water heaters also have maximum temperatures. Once the water is hot, the heater won't heat it further. This is, ultimately, a safety factor. Placing water heaters in series at the souce will get you hot water faster, and can increase the amount of hot water available, but it won't make the water hotter past about 110 degrees.
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Now, you can instal a small 5-gallon water heater at the tap, in series with the main water heater. This will give you nearly instant hot water, All this heater will do is to heat that first quart or so of cooled water that was sitting in the (traditional) pipes.
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POWER reliability is another issue. Most tankless heaters simply won't work without power- electricity is needed for the controls. Most 'point of use' heaters are also electrically powered.
Tankless heaters in series
Thanks, Reno and all the other posters. I have a good handle on how best to proceed. I like the idea of a small volume tank at the source in series with the mainheater.
Tankless heaters in series
Thanks, Reno and all the other posters. I have a good handle on how best to proceed. I like the idea of a small volume tank at the source in series with the mainheater.