[Greenbuilding] Question: stove to hot water tank

[Speireag Alden]

Sgrìobh Tim Martens:

>I'm running lines from a small wood cookstove to a 40 gallon hot 
>water tank.  I know that the tank should be above the stove height 
>and that the heated water natually rises into the tank and the 
>cooler water leaves the bottom of the tank and makes it's way back 
>to the stove.

     Not only that, but the pipe running from the stove to the tank 
must have an upward slope at all points, except at the very end, or 
performance is much impaired, and generally stopped.  In fact, one 
way to prevent thermosiphoning in a system is to put a little dogleg 
in it.  The return pipe's slope is not so critical.

>Does the loop need to be always filled with water in order to 
>function or can both cold and hot enter the tank at the top?

     The loop needs to be filled with water, yes.  That doesn't 
preclude both lines entering at the top of the tank.

     However, the cold water return line needs to extend down into the 
deeper parts of the tank, and preferably to come off of the bottom of 
the tank.  Otherwise, the hot water goes up the supply line to the 
tank, and sits in the top of the tank, and stagnates, because the 
return line is trying to draw water from the same hot section of 
stratification in the tank.

     For best results, the tank should more-or-less be a large bubble 
in the continuous loop.  The hot water goes up the loop and gradually 
fills the top of the tank, and the colder water flows out of the 
bottom of the tank.  Your T off the tank, leading to faucets, should 
be off the top of the tank, where the hot water line goes into the 
tank.

>Also, what happens when the water in the tank  begins to get too hot?

     That depends on the tank.  If you generate steam, you could have 
a pressure build-up and a cracked pipe or tank.  If conditions are 
just right, it could be a violent crack.  If the tank is plastic, it 
could melt.

     Your system needs to have a pressure and temperature relief 
valve.  Any hot water system needs one.

>Should I use a pressure relief valve to open  another loop to cool 
>the water or to allow more cold water into the system?

     The safest I can think of is to plumb the P/T valve down to a 
drain where you will see it.  If the system overheats, it will dump 
water down the drain, and, if the tank is pressurized, draw make-up 
water from a colder source.

>I know that a regular hot water tank on city water works on the 
>pressure of the cold intake. Since I'm probably going to have a non 
>pressurized D.C. pump to bring the water into the system from a tank 
>below, am I going to have to monitor and operate the system manually 
>by monitoring the temperature of the water and switching on the 
>water pump to allow cold water into the system?

     I'd arrange matters so that cold water can flow from another 
reserve tank, perhaps set a bit higher and filled manually as 
necessary.  When it comes to very hot water and steam, I like things 
to work automatically.  Eventually, you're going to be distracted at 
the wrong time, and someone will get hurt.

-Speireag.

-- 
A wise man will make haste to forgive, because he knows the true 
value of time, and will not suffer it to pass away in unnecessary 
pain.
--Samuel Johnson, lexicographer (1709-1784)