Indirect HW design question

JohnHenry_2

I'm designing a new boiler/indirect hot water heater system and have a question about the near boiler piping.



I'm going to be running the hot water heater at 140*F for sanitary reasons with tempering to the faucets and straight 140* water to the clothes washing machine (turns out the temp for killing legionella is the same for dust mites in bedding).



All of the I/O manuals I've looked at show the indirect returning to the boiler return. My question is: Is there any issue returning the indirect to the supply side of the low loss header?



I'm thinking that during the heating season, October to May here, it would be a shame to dump the 140* indirect return water back to the boiler rather than having the cast iron radiators getting a chance use it. I figure with outdoor reset, the radiators would probably never see > 140* anyways. This way the heating system also never really get's interrupted by a call for hot water.



Assuming proper pumping, can anyone see an issue with this?



Also, I can't seem to find a head loss curve for the TT Smart 100 indirect. Anybody have that curve?



TIA,



JH

TonyS

You would need to

run the boiler pump with the dhw pump using more electric. Other than that it seems ok .

Aaron_in_Maine

Will work

That setup is how I would pipe it.  But depending on the boiler you use you might need a seperate control for the circ.  Most wiil shut off the circ on the secondary side of the system until the indirect is satisfied.  But that is the way I piped this one.

JohnHenry_2

So,

Plumbing it with the return to the boiler return let's you run DHW calls without running the boiler pump? That would require a circulator capable of overcoming the boiler HX and the indirect HX head losses?

JohnHenry_2

Is that

a Knight boiler in the picture? One thing I was thinking about was using a low head loss fire tube unit (Knight or TT) along with a low head loss indirect (TT smart 100) and then instead of using a separate pump for the dhw use a 3-way valve that will send all of the boiler output through the indirect on a dhw call and then to the low loss header to be used by the radiators. That set-up should have a lot lower head loss than the non-fire tube Mod/Con boilers have through their HX alone.

SWEI

3-way valve

Will work just fine to divert the flow.  There's an output on the PE110 controller that does just this inside the box.



Head loss in the SMART indirects is basically negligible, so as long as the indirect is close to the boiler and has adequately sized pipes, the flow should be about the same as you have in a short primary loop on the other leg of the valve.



I'm tinkering with proportional control of the diverter valve (diverting some flow to the indirect and the rest to CH) but stick with on-off if you're using the onboard controls.



Someone else can speak to the WHN controls, but I'm pretty sure they can handle it.  Worst case would be to get a spring-return valve with a 120VAC actuator and hook it to the DHW pump output (but you have to tell the primary pump to keep running when there's a DHW call.)

JohnHenry_2

Thanks for the insight

I'm thinking full diversion would be best here anyways as the return from the indirect will almost always be hotter than the system side is being set for.



Can you see any issue running a 95 gal indirect with a 110K btu/hr boiler? The heat loss on the structure is around 90K btu/hr but this is a small apartment building with 5 units and 8 tenants. I'm thinking a 40 gal indirect might be a little small.



Also, any preference of Lochinvar Knight WHN 110 or TT Solo 110?

SWEI

indirect performance

Check out http://triangletube.com/documents/2/SMART%20Commercial%20Performance%20Data.pdf for detailed performance data.



http://rightspec.bradfordwhite.com/Sizing/Apt.aspx should help you come up with a demand estimate.



The controls on the WHN are far better than the ones on on the old PS110 were, but the new PTS110 has similar capabilities and an updated HX design with a PP bottom pan. According to a poster here a few months back, the WHN air intake directly feeds the boiler HX which may be allowing more dirt and leaves than the TT indirect combustion air design does.  The advantages of the low loss fire-tube HX are huge, and I would happily take any one them over most any other mod/con.

Jean-David Beyer

That would require a circulator capable ...

Exactly. My boiler's installation manual specifies that. The indirect is piped across the boiler supply and return connections. Then the return has the boiler circulator and the supply has a Flow Check valve. They then go to the closely spaced Ts (or you could use a low loss header). When running the indirect, the boiler circulator is off. This is important for my setup because I use 175F water to the indirect, and the house uses 120F max for one zone and 135F max for the other zone.



They specify (for my size boiler) a Taco 007 for the boiler circulator (they even supply that one to be sure I use the right size) and another Taco 007 for the indirect. The indirect has low head (tank within a tank design).

TonyS

My mistake

I assumed you were using a Prestige with the smart tank in which case there would be no need for running 2 pumps because of the low pressure drop. Ive been laying out a 16 panel photovoltaic microinverter system on my home, so ive become extremely anal about wasting electricity. But if your going to use a high head boiler and dont mind running two pumps...go ahead but dont forget to turn off your dhw priority or it will just circulate back to the boiler anyway. Top of page 21 shows how to pipe primary secondary and bypassing the boiler pump(located inside cover) during DHW recovery

Aaron_in_Maine

NTI

It's a NTI Firetube boiler same exchanger as the rest. The key is getting a contractor or supplier that is comfortable with the unit and has the parts available.

Zman

Drawing

The only issue I see with your drawing is that you are not "pumping away" I also think it will be difficult to get an accurate temp to the radiators (especially with outdoor reset)

I agree with the firetube hx comments. Personally I would pipe It as shown in the triangle tube manual posted above. I would run the boiler on outdoor reset and domestic priority. The smart 100 basically has no head loss. It is just a big vat of water. The smart has so much surface area that at a slow circ speed you will likely condense the boiler through much of the DHW cycle

Carl