DHW coil in boiler + mixing valve question

CoalBoilerGuy

Is it possible to provide for a homes hot water service directly from a combination of a boiler with a DHW coil and a mixing valve alone, with no other tanks (direct or indirect) or tankless heating devices installed?

RobbieDo

Plate heat exchanger

I'm confused on your question, are you talking about using a plate heat exchanger for the domestic hot water off of the boiler?

CoalBoilerGuy

DHW coil in boiler + mixing valve question

No, I'm talking about using a devise such as a Taco 5003 Mixing Valve, set at an output of 125 degrees F. , and tied into both the DHW coil and the city water supply.



The Taco 5003 (and units like it from other mfg's) has a cold water inlet, a hot water inlet, and a mixed water outlet (that is temperature adjustable).  It is a mechanical valve device.  

CoalBoilerGuy

DHW coil in boiler + mixing valve question

Here is a link to Taco literature for the Taco 5000 series mixing valves. 



http://www.taco-hvac.com/uploads/FileLibrary/100-6.0_REV.pdf

RobbieDo

Mixing Valve

Yes, this will work. What is the domestic water pressure at the house? What is heating the DHW now?

CoalBoilerGuy

DHW coil in boiler + mixing valve question

Currently I have a conventional electric hot water tank.  I also have an oil boiler for the homes hydronic zone heat, but I have purchased a coal boiler to replace it, and the coal boiler has a DHW coil.  I would like to eliminate the hot water tank at the same time while I'm removing the old oil boiler and installing the coal fired boiler.  The coal boiler would then run year round, providing my DHW all summer.

RobbieDo

Not sure

How efficient that is going to be? Do you have LP or NG available?

CoalBoilerGuy

DHW coil in boiler + mixing valve question

We're drifting way off the topic, but the coal boiler is between 81.4% and 86.6% efficient based upon a detailed 1950 through 1952 study (published in 1953) that was conducted by the 'Unites States Department Of The Interior' upon its early (but direct) ancestor.   The range of efficiencies resulted strictly from differences in the anthracite coal used to fuel it.  There is no NG near where I live.  I've calculated that my annual heating costs with anthracite will be only 1/3 of those for my old oil boiler, and approaching half those for a modern high efficiency propane boiler.

RobbieDo

Look at this

I'm sure this is how you coal system is, this is just a different type of boiler. Reason I was asking efficiency is tankless electric or propane is better than coal running all day just to heat DHW. Anyway, check this out below this will tell you how to do it, anymore questions are away.



http://slantfin.com/documents/770.pdf

CoalBoilerGuy

DHW coil in boiler + mixing valve question

I agree that coal boilers are at their least efficient at idle fire, and that based upon actual reports from several coal boiler users regarding their real-world daily coal consumption during the summer months it may cost me perhaps as much as about $200 more to heat my DHW with coal during the warmer months of the year than it would with one of the more efficient DHW alternatives, but a coal boiler is subject to rust and serious internal corrosion if it isn't fired year round (sans for its once annual clean-out).  It's easier to keep it running and not corroding with the knowledge that perhaps 10% of the winter fuel savings will be sacrificed (given back) during the summer months than to pay for the damage that not running it will certainly cause over the long haul.  The bottom line is that if it makes good sense to fire the coal boiler year round, it might as well be heating the homes water while it is doing so.   I figure my total investment (boiler plus install) will be recouped in only about 3 - 4 years, even when factoring in the summer inefficiency.