Sizing Boiler and Indirect Hot Water

Old 11-17-08, 08:32 PM
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Sizing Boiler and Indirect Hot Water

Need some help sizing a gas boiler and indirect hot water tank (DHW). Our home is a 2,00 sq. ft. brick colonial split located in Philadelphia suburbs where the coldest winter temperature is about 5 degrees F. We currently have a 167,000 BTU oil burner with a hot water coil and 3 zones of fin baseboard heat (Taco 570s) and a single circulation pump. We also replaced all our windows with double paned glass.

Demand Requirements:

I completed two versions of heat loss: 1) Crown Boiler’s radiation method (157 linear feet x 600 = 94,200 BTUs) and 2) Slant/Fin heat loss method (68 degrees indoor temp, 5 degrees cold temp provided a 64,000 BTU demand).

Using the radiation method and adding in another 20-30% for DHW puts demand at about 110,000. The heat loss method required only about an 80 to 90,000 BTU boiler. Why the difference?

Possible Solutions:

The Peerless MI-05 (DOE Heating Capacity 115 MBH), the New Yorker CG50CNI (DOE Heating Capacity 105 MBH) or the Burnham P205-NG - P205 (DOE Heating Capacity 108 MBH) seem to cover the higher BTU requirements. Is this sizing overkill and am I setting myself up for short-cycling?

I like either the Peerless PP-40 or the Burnham or AL50SL indirect tanks. (The Peerless documentation says it requires a minimum boiler output of 141,000 BTUs to reach its first hour rating.)

I would like to take advantage of the Tekmar 260 control but that would require buying a second circulation pump since the 260 does not control a zone valve for DHW priority. I also read in another thread in this forum that a separate zone valve control is connected to the 260. Is this a requirement for the 260 to control the 3 zone valves?

Thanks for taking the time to look at this epistle.
Old 11-18-08, 05:13 AM
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The connected load (radiation) has absolutely no bearing on a boiler size at all. The amount of and type of radiation in the home means as much to sizing your boiler as the color of the car you drive. AWAYS and I repeat ALWAYS do a heat loss and decide on a boiler by the DOE output (most of the time).
Adding an indirect is where it gets tricky. WHY a 50 gallon? As in boiler sizing most indirects are also over sized which slows down recovery time and wastes fuel.
Old 11-18-08, 09:28 AM
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90k is out to lunch. 64k is more like it, and probably still an overestimate by 10-30%.

The nice thing about the 90k estimate, however, is that it shows that you have enough "extra" radiation capacity to make very good use of an outdoor reset control. (More radiation means you can run a lower water temperature, which is more efficient.)

A boiler in the 80-90k range would be fine for the space heating and a ~40gal indirect. If your installers are suggesting Burnham, ask about the MPO84 or the MPO115.

The tekmar 260 is a good control (I happen to have one. Works great.)

The preferred setup with a 260 is a dedicated circulator for the indirect. A dedicated circulator is also preferred by most (maybe all) indirect manufacturers. This is not a big deal. A circulator costs about as much as a zone valve. A good way to wire/control the system (regardless of whether you have the tekmar) is to use a zone valve control or switching relay. A common one is a Taco Zone Valve Control. It can handle the space heating zones, and also has a terminal for a circulator (or zone valve) for an indirect water heater. It is a neat, clean, simple to wire solution for connecting the boiler to the rest of the system. A Taco ZVC only costs about $100, and usually saves time in labor during the install. The ZVC also has a built-in priority circuit for the indirect, which is a good thing.

You could also go with a Taco ZVC-EXP, which allows a direct plug in of a simple outdoor reset controller (the electronic guts are actually made by tekmar), if you didn't want to go with the 260. The 260, however, is a very nice control and does a great job.
Old 11-18-08, 11:11 PM
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I would actually bet that you don't even have the emitter capacity in your home to matgch a boiler the size you talking, especially if you'll actually be thinking 160 for rating and not 180.

How many BTUs of total radiator capacity do you have in total? You have to start from at least that low.

No sense having a 200 MBH boiler hooked up to 80 MBH of rads. It'll really short cycle.

Beware and think how small not how big for sizing.
Old 11-19-08, 04:22 AM
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Original Post Update

I saw an error in my original post: The square footage of the house is "2,200 sq ft" not "2,00".

Anyways, I want to thank everyone for their responses. I will pass along the correct sizing requirements for both the boiler and the heater to my plumbing contractor. I will also pass along the zone valve and pump control information to him. This has been an education!!!

If anyone would like to add to the thread I will continue to read posts.
Old 11-24-08, 06:59 PM
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Famous last words: one last question

Actually, two questions:

I've settled on a gas boiler that will provide 86K BTUs (D.O.E.) based on a heat loss analysis of 62 to 68K BTUs.

Does anyone see a problem matching the boiler with an indirect tank that requires 99K BTUs for a first hour rating of 200 gal/hr (58-135 degeres) even though the domestic hot water demand at the busiest time of day is less than 100 gallons? (I'm obviously ok with less than 200 gal/hr.)

Also, is about a 0.7 degree/hr heat loss acceptable for an indirect?

Thanks for all your help (again).

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