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Tankless water heater diagram

jkfoote | Posted in Energy, Heating & Insulation on April 14, 2011 10:04am

I’m using Michael Chandler’s great article on tankless water heaters (https://www.finehomebuilding.com/PDF/Free/021192082.pdf) to guide my installation.  But there’s a puzzle in his final diagram (fix for problem #2) that I can’t figure out:  Why is there a connection from the cold water supply line to the small point of use tank?

According to the diagram cold water can flow into this tank from either the recirc return line or from the cold water supply line.  I think the recirc return line is a red herring – in a demand system the recirc line runs very infrequently and typically only moves a small volume of water.  Granted, the recirc pump needs to return water to the cold water line to save water, but I don’t see that it has anything to do with the fix for problem #2.  

Cold water that enters the storage tank from the supply line will not be heated by the tank since the heating element is not wired up,  so the only thing it can do is cool the hot water in the tank.  But isn’t that the point of the mixing valve?

Is it for ‘local recirc’ when the big pump runs?  Suppose there are no open faucets downstream but the hot water in the tank cools.  The big pump starts up and creates flow through the tankless water heater.  Where does the flow go – out of the tank via the tank’s cold water inlet line and back to the tankless water heater?  In that case the only confusing thing is the direction of the arrows at the bottom of the drop tube – they should just point the other way.  

If I’ve got this right then it looks important to minimize the length of the cold water leg between the tank inlet T and the big pump inlet T since that leg is going to get hot and could surprise a downstream cold water user.

Hints appreciated!

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  1. jigs_n_fixtures@icloud.com | Apr 14, 2011 07:38pm | #1

    The recirculation line is shown so you know how to plumb the system; it probably could have been omitted.  But, the system will work well as shown.

    If you want to understand what is happening, try printing several copies of the diagram and penciling flow arrows under different conditions.  Whether the water flows into the tank from the cold feed, or through the heater is dependent on the temperature in the tank.  But the arrows are correct in all but one condition.  

    Condition one, the tank is up to temperature at say 145-degrees, at the start of a demand cycle for hot water:  There has to be cold water entering the tank, to push the flow to the hot side piping.  The mixing valve will draw cold water to temper the hot water leaving the tank to 125-degrees.  The recirculation line should be static. 

    Condition two, mid demand cycle:  The tank is below 140-degrees, and the pump for the heater is running, feeding cold water through the heater and into the tank, from there into the hot water piping.  There should not be any cold water feeding into the tank.  If the water exiting the tank is still above 125-degrees, the mixing valve is still pulling cold to cool the hot water down.  The recirculation line is still static.

    Condition three, still demand cycle:  The tank is back up to 145-degrees, and the feed pump for the heater shuts down.  Cold water feeds into the tank, and hot water from the top goes out to the system.  The tempering valve is still drawing cold water.   The recirculation line is still static.  If this condition occurs often, the tank is probably too big, and the efficiency is down because you are short cycling the heater.

    Condition four, idle, with the tank temperature at 145-degrees but dropping towards the 140-degree call temperature:  The temperature is above the call point so everything is static.  But, as the temperature in the tank drops to 140-degrees, the heater feed pump will kick on.  In this case, and only this case, the water to feed the heating system will be drawn "backwards" through the cold water feed at the bottom of the tank, and replaced by hot from the top of the tank.  Again this will decrease total system efficiency.

    Condition five, when the recirculation pump is called, while demand hasn't begun:  The water is drawn from the top of the tank, and pulled into the hot water piping, and the water in the lines that has cooled is pushed back into the tank until the recirculation system senses the water is back up to temperature, and shuts off.  Some of the water is probably being recycled through the tempering valve to cool the feed from the tank. 

    I'm attaching a copy of the diagram, so people who don't have online access to the article can understand the discussion.  I don't think it violates the copyright. 

    1. jkfoote | Apr 14, 2011 09:26pm | #2

      Spectacular reply!

      That's the most complete answer to a forum post that I've ever seen!   Great stuff, makes perfect sense.  Thanks for the quick reply.

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