Indirect hot water tank


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Old 02-28-07, 10:11 AM
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Indirect hot water tank

Hi,
Can I have some input on a domestic hot water sysyem.

We are restoring a 1907, 8000 sq.' house that has been changed into 4 apt.'s. The house is heated by a new Weil-McLain cast iron boiler, 216,000 btu that supplies hot water to the original cast iron radiators. The system works wonderfully, no issues with heat. The boiler runs until May 1st or so each year and is asked for heat again about Oct. A plumber friend who installed the new boiler suggested using an indirect tank to heat the hot water (there are currently 2-50 gal gas-fired HWT's). He thought that besides being a better way to supply hot water it would be beneficial for the boiler to run throughout the year as it would with an indirect tank as opposed to sitting dormant during the non-heating season.

The physical and mechanical advantages for the indirect tank as I see it would be no exhaust venting, no gas piping, less maintenance and longer life (stainless steel), more space in the boiler room (1 larger tank compared to 2 smaller tanks), and probably more hot water.

Economically it would seem to make sense as well. The cost of the indirect tank is more than 2 gas HWT's, but not by that much, so I'll say for the sake of argument the initial costs are equal.

The plumber we are using at this point (a different plumber) says to just replace the 2 gas HWT's and carry on as before. I realize there are different opinions and that each have their own merits, but is there a more correct solution in this case?

The issue comes down to annual heating costs. The cost of natural gas to run 2-50 gal HWT's year round verses much cheaper hot water throughout the heating season with the indirect tank plus the cost of the boiler running during the non-heating season to supply the hot water alone. Would a condensing boiler be more appropriate for this application where the boiler must run just to supply hot water during the non-heating portion of the year?

Any experience or knowledge of this situation would be greatly appreciated.

Thank you,
Michael
 
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Old 02-28-07, 12:14 PM
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The indirect is about as efficient as the boiler - much better than a typical gas-fired water heater (unless the boiler's completely inefficient). I think they are the best choice with any newer boiler.

As for considering a modulating condensing boiler, I'd try and really examine your heating needs and consider target temperatures at varying outdoor temperatures and what the normal temperature profile is over the winter. They tend to have far more saving in the shoulder seasons in comparison than that at minimum outdoor temps.
 
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Old 02-28-07, 04:25 PM
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Response to WHO

Hi WHO,
Thank you very much for your reply to my situation. You were very clear on the indirect tank set-up, regardless of the type of boiler (as long as it has an acceptable efficiency rating) it is the best way to go.

I live in Vancouver, living in Ontario you know we have a relatively mild winter usually (although this winter has has at times been colder than you have had). Does this climate lend itself to the advantages of a modulating boiler? (which I gather is ability to modulate the amount of heat it applies to the heating requirements as compared to my cast iron boiler which can only come on at it's full rating of 216,000 btu and send a lot of heat not to the heating requirements, but up the chimney).

Two questions about an indirect tank.
1. Is there an increase in efficiency in heating a larger volume of water as
opposed to a smaller volume? Say a 80 gal tank compared to a 100 gal
tank for example.
2. Does a larger tank have smaller stand-by losses than a smaller tank?

3. OK I lied. Is there a particular manufacturer that I should look at or is
there one I should stay away from?


Thank you again,
Michael
 
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Old 02-28-07, 06:25 PM
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I'd put in 2 small modulating condensing boilers, with an indirect piped to each. I'd pipe them so that either boiler can heat everything or half. I'd also pipe the indirects so that the domestic is split in two but either one can supply their half or all of the house. With 4 apartments, redundancy's probably a good thing.

My only experience is with a Triangle Tube Prestige 110, only negative so far is that it doesn't modulate as low as it is supposed to. I would have bought one with half the capacity but that's the smallest they sell. One would probably be enough to heat your home, but like I said, I'd get 2 for redundancy. A couple of Smart 50s and that's that. You can even use their internal circ as the sole pump for central heating - on its low speed. You'd be using less than 100 watts when heating and under 15 watts when idle.
 
 

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