Truly tankless DHW from a coal boiler, is it possible?

Is it possible to connect a conventional electric hot water tank to the optional internal "tankless" hot water on demand coil from a coal boiler, and run the conventional hot water tank off of a zone valve, or do I need to get an expensive "indirect" hot water tank?

If this is possible, what would the basic schematic for the installation look like? How does the zone valve get its on/off signal from the conventional hot water tank?

The problem is your Boiler is closed loop and your hot water tank is open loop, and you would let air into the heating system if hooking it up directly. You would need a separate zone and a hot water storage type tank (Amtrol, Boilermate, etc..). There would be an aquastat on the hot water tank to control the zone and heating. if you have a separate hot water coil in your boiler, then yes.

lsayre wrote:Is it possible to connect a conventional electric hot water tank to the optional internal hot water on demand coil from a coal boiler, and run the conventional hot water tank off of a zone valve,

yes

and no

you will use the boiler coil as an energy source for the tank - but you will need to pump that loop with a Bronze circulator - from the supply out of the tank to the boiler and back to the bottom of the tank.


You may use a strap on aquastat wired to open on temperature rise attached to the supply out of the tank to limit.



You can also add a thermo mixing valve to take boiler temp DHW - mix it with street water - and deliver safe hot water to the taps



lots of other ideas here - some good - some not so good

Sting,
Would it be possible to get a line drawing of that first pic showing the aquastat and pump, and lines to and from the boiler?
I have a KA-2 and would like to get it set-up for DHW. Also what size is that pump? It looks really tiny.
Thanks

not much of a line drawing guy

no software

but if some of the other more tech savy guys can help out

if not -- Ill draw something on a napkin at the strip bar and post a picture of that later -- umm the drawing that is

I do it, sort of. I run a cold water feed through my DHW coil and have that hot water then feed my electric hot water tank. I don't use any pumps or circulators, I just let line pressure circulate the water. I have it valved and plumbed, so in the summer I just send the cold water feed directly into the hot water tank in a conventional fashion. Once I start my boiler, I open one valve, and close another so I send the cold water to the boiler first. You just have to remember to keep one feed to your hot water tank on all the time. Seemed pretty simple, and I've used it that way for 20 years now. The electric hot water tank almost never runs in the winter.

Does the dedicated circulator pump run continuously? What specific function does the aquastat perform? I take it there is no zone valve in this arrangement. Does the aquastat merely turn the circulator pump on and off, or does it take the place of a zone valve? I'm a bit confused.

What are the specs on the small circulator pump?

I'd like a drawing of that last pic!


I suppose you could use a HW tank ... but you would have to find a coil to stick in there ... and then keep it leak-free, in order to copy an indirect boiler for cheap. Sounds like too much work for me though. Indirects are pricey, but I'll tell ya -- you'll never run out of hot water again ... EVER.

As an added benefit, I can use mine as a dump zone. When my boiler starts getting over 240° from the HW coils in my handfired, I manually set the hi-limit of the indirect boiler as high as it will go (150°), and that not only pulls all that extra heat out of the boiler, but stores it for hours & hours inside the tank until I need it. They have really good insulation, those indirect tanks.

Rather than placing a coil inside a conventional hot water tank to make it function just like an indirect hot water tank, wouldn't the tankless hot water coil that resides inside the boiler itself be potentially capable of acting as this coil? That is what I'm thinking of here.

Additional information: I intend for the electric hot water tank to work as usual (as designed) during the warm months when my coal boiler is shut down. However, when the coal boiler is in use, I want it to either supplement the electricity or do all of the heating of my homes domestic hot water. I don't want my coal boiler running year round. That is the main reason why I don't want to replace my inexpensive conventional hot water tank with an expensive indirect hot water tank.

I think I'm thinking of 2 different types of hot water tanks. One is a gas-fired unit, which has the burner below, and many, many feet of coiled copper inside ... but if memory serves me right, there is only one way in & one way out. That wouldn't work.

The other type I'm thinking of is a storage tank, which has no less than 3 ways inside the tank .. but that's just a big open space, which will pose the problems previously addressed. I believe electric ones are setup like this as well, with 2 electrodes in the center instead of a burner on the bottom.

Not an expert on this by any stretch. My brain hurts ...

Why not just do a summer/ winter hook up. Domestic coil for winter and electric in summer. If one fails you can switch over, something starts to leak you have to fix right away. I save $50/month heating my DHW with coal. I can get 4 tons with that per year, and will get me 2/3 of the way through winter.

Yes, my tank has two electric resistance heater elements in the mid section of the tank. The tank is otherwise just a well insulated 80 gallon storage vessel (a big open space). No coils inside. Cold water in, and hot water out are both at the top. There is a drain valve at the bottom.

You would have to stuff a coil in there somehow in order for it to function like an indirect. Otherwise you'd be introducing fresh water ( oxygen rich ) into your normally closed loop boiler. Not a good thing.

Here's a cutaway of an indirect == the coil would be hooked up to your boiler as an extra zone:

I can't see how fresh water could enter the coal boiler from the hot water tank system, as it would circulate through the boiler only within a closed coil intended originally for "on demand tankless hot water". In a nutshell, I want my on demand tankless hot water coil that is inside my coal boiler (which is sealed and separate from the boiler chamber, though it is inside the main boiler chamber) to supplant or totally supply the heat requirements of my conventional hot water tank.