How would you do it?

Glenn Sossin_2

Typically, I would pipe both boilers together in a primary seconday loop. Several advantages:

1.) Redundancy. If one boiler has a problem, the second boiler is there to provide a backup function to keep heating the house or provide the hot water.

2.) Higher DWH recovery: Depending on the heat exchanger product, you will have double the recovery rate for DHW. Usually, I would consider using a turbomax and 2 120 gallon hot water storage tanks. Let the body sprays run.

3.) Capacity: With two boilers, you might now have the potential for some snowmelt.

4.Efficiency. The Ultra uses the MCBA board that can accept the AM4 ribbon cable. This allows you to transfer the modulation of the boilers to this sequencing control. On a call for heat, it will step up each boiler as neccessary to satisfy the heat load. 2 boilers firing at 50% are more efficient than one boiler at 100%

adambuild

?

We have a very tight, 7100/sf home in North Jersey that we're doing all radiant. Heatloss is 90M/BTU/HR. We're going to use an Ultra 105 and client wants second boiler for DHW, about 100 gallons (2 indirects). Would you use a second boiler just for DHW? Would it be better to piggyback 2 Ultras for heat and DHW (if so, how would you pipe/control?)? Regardless, how would you pipe/control 2 indirects?

Thanks in advance, Adam

hot_rod

Unless they define

how much hot water they need or want, you are just guessing.

Think I would use a condensing water heater if they want separate systems. Polaris, AO Smith, Bradford White, Voyager and others build high capacity and high volume condensing water heaters.

I'd be leary of one boiler with DHW priority if they need, or want a lot of DHW.

Classic case of the potential DHW load being larger than the heating load

hot rod

Paul Pollets

My 2Cents

I'd be using a Viessmann Vitodens WB2-11-44 (with LoW Loss Header) which modulates between 49-153K Btu's and their 120g V300 indirect. The boiler can recover the tank in 15 min or less, depending upon pump selection to the indirect coil.

It would not be the greatest design to have a separate boiler dedicated to the DHW tank(s). Some like to twin smaller indirects together for capacity, rather than using 1 large tank. If the large Viessmann tank gets thru the mech rm door, it has a lifetime warranty, and shouldn't ever have to be removed.

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ALH_4

Tanks

You might even use a couple 80-gallon tanks, and size the boiler to the heat loss. It would only take a half hour to heat up 160 gallons of 100°F water to 140°F with the Ultra 105.

I would pipe the indirects parallel reverse return. Incidentally, I worked on a retrofit job that had three Crown MS-26 gallon indirect tanks in a crawlspace. Even without being piped reverse return, the tanks were all within 1°F of each other.

Kal Row

One little change….

You will be using 4 pumps – 2 for boiler and 2 for DHW
Pipe them together for heat – and have the indirects in parallel, but the pumps separately off each boiler combining only on the discharge side of the pumps toward the indirects (flow checked of course)

Set one tank 10f-20f higher than then other – and have the aquastats go to the 2 boilers DHW inputs separately, this way you stage the DHW pumps and boilers – ie when you draw hot water, only the first pump would come on and demand it’s boiler, so that ultra would stop it’s space heat pump, start it’s DHW pump, and ramp up to 180 for DHW production – but the water from it would go to both tanks of course, and if you draw hot water at a speed enough to drop the temp down another 20f then the other pump would come on and throw it’s ULTRA into DHW mode so you now have both making hot water and full flow rate – of course

Such a setup gives you heat and hot water redundancy and pump energy saving by virtue of the staging effect – and gives you hot water and heat together most of the time


I assume you are using the ULTRA-PLUS indirect – if not please do, as they are the best for low mass boilers – low head loss and lots of surface area – you can get a pair of 60’s under the Williamson label for less (wile-McLain by another name), the indirects are actually PHASE-III made by the triangle tube people http://www.triangletube.com/Residential/ProductIWH.htm


The only caveat with them, is to fill the domestic side first cause the boiler tanks is on the outside and will crush damage the inside tank if you pressurize it first