120 gallon indirect

Jamie_6

The load on a 9700 square foot radiant home is only 117,000 btu's, but the home requires between an 80 & 120 gallon indirect! With this large of a water demand what would you guys do about (heating) the hot-water? Would Priority do the job or should it have its own heat source?

Jamie

Tom Meyer

Recovery rate and load \"dump\"

First question is how DHW was sized for 80-120 gallons? Before any solution, the recovery rate and load "dump" has to be evaluated.

How often are they going to need that DHW? How long can they wait for recovery or is it a "dump" load?

Senior Designer/Trainer
Precision Hydronics Corporation
www.precisionhydronics.com

Boilerpro_3

Why a 120 gallon

TWas wondering why this size indirect? Storage capacity is related to input available and peak draw. You'll need to look at sizing info typically for commerical jobs to help get a handle on this. If you are filling a big tub, remember that every minute that tub is filling is another minute that the unit can make more hot water.... An example. Weil McLain Gold Plus 60 (Triangle tube). With a 150,000 boiler output (about what you would want for this applciation with pick puup load) You get 320 Gallons in the first hour at 115F. Subtract the tank size...60 gallons.... and then divide 260 by 60 minutes and you get 4 1/3 gpm. This is not quite accurate, but it gives you and idea of the capacity of the unit. A more accurate way to look at it is that every gallon of water heated from 50F to 115F takes 540 btu of heating capacity (8.3 lbs per gallon x 65F x 1btu per gallon per degree F). Take 150,000 btu/hr boiler output divided by 540 equals 278 gph or 4.7 gpm. You will lose some capacity with a cold start boiler, especially depending on the mass and water content of the boiler. If it takes 10 minutes to fill that big tub, you potentially have another 47 gallons of water to work with.

Just some of my thoughts on sizing for water heating.


Boilerpro

Boonierat

120 Ga. indirect water heater

In this application the DHW load RULES. That is to say that the amount of heat to satisfy the water heater is greater (probably far greater) than the load for the building. if you look at what the heater manufacturer REQUIRES in BTU's/Hr. it could be so much higher as to cause the boiler to short cycle when in the heating mode. Some 120 gallon heaters require up to 300k to meet their maximum rated output. So what'dya do. I ask our contractors to discuss the issue with the consumer. If they need the storage for JUST peak demand use, then you can afford to 'short' size the boiler based on the water heaters requirement. In essence the boiler can be sized to do the heating load and zones placed on priority. When the DHW zone comes on it'll get ALL the heat available. Disclaimer here is that the boiler will NEVER be able to push the water heater to it's maximum capability. Most consumers understand this. HOWEVER if the consumer has a need for domestic hot water equivalent to the maximum capability of the indirect water heater flow rate then you have no choice but to size the boiler based on the requirement of the heater. The consumer HAS TO BE informed that during heat only cycles 'short-cycling' could occur. Makes for a less efficient system.
I see this a lot in the ski country here in New Hampshire. Winter condominiums with heat loads in the 40 to 50 K range equipped with 75/100 gallon whirlpools. Check out the btu/hr. ratings for a 60 gallon indirect water heater; it's mind numbing. There are times when a seperate water heater is more economical.
Sorry to get so wordy.
Nels

Paul Pollets

Sizing larger tanks

I'd be sure to give the tank at least 200K btu's for recovery on priority zoning. If adequate recovery times are desired. I look to recover the tank in 20 min from 50deg. inlet to 120. Your rep should be able to guide you through the recovery spec and let you know min. input for realistic recovery.

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TJ

Ergomax or Artesian

Jamie,

You might take a look at either an Ergomax or Dunkirk's Artesian, reverse indirects. A reverse indirect works opposite of a traditional indirect water heater - instead of having domestic water in the tank and boiler water in the coil, a reverse indirect has the boiler water in the tank and the domestic water flowing though the coils. It works just like an instantaneous hot water heater - only without the hot and cold bursts of water . With a reverse indirect, your only limited by the input BTU's of the boiler. The incoming domestic cold water never mixes with the outgoing hot water. An added benefit of a reverse indirect, is that the tank doesn't lime up and decrease efficientcy like traditional indirects.

Bob Bona_4

have

this problem arise almost every day. The best solution would be to use multiple boilers, with a staging control to fire both upon the indirect's load.

2 125K boilers would be ideal here, with a tekmar 262.

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molly

storage tank maybe?

Why not use a 40 gal indirect with a 120 storage tank with the dhw on priority. Also Taco makes a nice plug in priority protection card that plugs into their multi zone control box.