When is a boiler too big?

Cloud Hidden

I design houses in a style that is particularly efficient in its energy requirements. Heating load calcs for them are practically always < 12000 btu/h per 1000 sf. One in NC w/ a 14° design temp is operating perfectly at 8000 btu/h per 1000 sf. Plenty of other comparable examples. That background is to set the stage for a problem I regularly see with new designs. I have a design for a new residence in Pueblo, CO. 2500 sf. I did a heat loss calc and came up in the range of 30,000 give or take. My local HVAC contractor friend calced it as a favor to me, and separately came up with the same thing....he understands our method of construction and has built one himself.

The client is trying to locate a local contractor--we're a couple thousand miles apart--and is getting recommendations for gas boilers between 75,000 and 120,000 btu. I'm getting beyond my ability to explain things to him and advise him, so I limit what I say, but still want to understand this better myself. Client is just confused by such a wide range of recommendations. That said, it seems to me that these are too big boilers, and will short cycle, etc.

So, if a load calc comes up with 30,000 btu/h loss, what other factors play into sizing the boiler? Are there reasons to size it more than 30K? How much excess capacity is a good thing?

Thank you for any insights!

jim sokolovic

Is the boiler sized for space heating only...

or is it going to handle the domestic water requirements (tankless coil or indirect water heater) also?

Cloud Hidden

Space heating only

Only space heating. RFH in slab w/ no coverings (polished, stained concrete on-grade slab).

jim sokolovic

The factors would then be...

Altitude deration of the input specified on the boiler at sea level (4% deration for each 1000 feet above 2000 feet over sea level), steady-state efficiency of the boiler, and piping and pickup losses to the system. In general, you shouldn't need over 60,000 BTUH for this job.

hot_rod

Consider

a modulating boiler. Viessmann, Munchin, Weil McLains Ultra are a few brands. These boilers are able to match the output to the load.

As you know the load is always a moving target, and a bit of an oversize for below design days is easily handled by a modulating boiler. Plenty of excellent control options for DHW and high temperature loads.

hot rod

Rudy

Agreed

If the load is 30,000 btu/hr for space heat ONLY...

That is the OUTPUT you need to heat the home.

To figure the input, DIVIDE this by the efficiency of the boiler, say 82%

30K/0.82= 36,585 btu/hr INPUT needed.

To derate for altitude as Jim points out, Pueblo proper is 4,600 feet. Let's just figure 5,000 feet altitude, that's within 10% of 4,600. Figuring four percent derating for every 1,000 above 2,000... You're 3,000 feet above the 2,000 foot mark, that's three 1,000 foot increments, or 3 x 4% = 12% deration.

100% minus 12% is 88%. DIVIDE by 88%: 36,585/0.88= 41,574.

So you need a boiler with at least 40,000 btu/hour INPUT. The other 1,574 btu/hr needed can come from lights, people, electrical appliances, and fudge factor slop. This has always worked for me, I've installed and worked on many boilers above 8,000 feet, some as high as 10,000 feet, and I've seen the effects of improper derating firsthand. I live about an hour and fifteen minutes north of Pueblo.

I absolutely agree with hotrod on a modulating boiler for this application. Your total input will rarely be needed, only on the coldest day of the year. For well insulated homes that lose heat at a MUCH slower rate than the boiler is putting heat in, the lower burner flames of a modulating boiler are desireable. I typically ignore pickup losses for a boiler within a well insulated building envelope, also assuming well insulated boiler piping.


Jim, what's the story on Slant/Fin's modulating boiler?

Richard Miller_2

Welcome CloudHidden

To all on this thread...

I know this gentleman and he is a first class kind of guy. He asks his questions from a position of intelligence... I am glad his question is getting the respect it (and he) deserve.

Richard Miller_2

Duncan is your guy!

I e-mailed you about him.

Cloud Hidden

Thanks!

Youse guys are good! This is a huge help. Thanks.

That close to Pueblo...want the client contact? or if not, know someone who does? Regardless, I'll share your collective thoughts with him. Again, thanks.

Jim

jim sokolovic

Hopefully the story compares to...

"The Tortoise (us) and the Hare (them)". We don't have a modulating boiler available yet, but there has been development work geared toward that type of product.
Please keep in mind that I am just one of the guys in the lab, not someone who interacts much with the people in the field much, and am interacting here with you all as an effort to learn, more than to represent this company.

J.C.A._3

Don't know if it's on the shelf yet.....

But Heat Transfer Products/Munchkin has a wall hung unit that , if I remember correctly, modulates from 30-80K . Jeff, are you out there?

Personally, I would use the boiler for H.W. if possible. A running unit has far fewer problems than a start/stop unit. Biggest advantage....only 1 unit to service if there is a problem. JMHO. Chris

john wood

Small wall hung

Munchkin now has the T50M which is a wall hung, top vented boiler that will modulate down to 10,000BTU and will do indirect at 80,000BTU and outdoor reset to boot (if a vision contractor). Ask your dealer.

Mad Dog

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Rudy

Question...

John what do you mean by:

"...will do indirect at 80,000BTU and outdoor reset to boot (if a vision contractor)."

Vision contractor? Thanks.

john wood

Vision

All the munchkins have the ability to do a two temp with outdoor reset, but you have to attend one of the training sessions to get certified to use it. Ask your wholesaler or the rep for your area.

paul lessard_2

when is a boiler too big?

When It don't fit through the door!

bill clinton_3

boiler size

An often overlooked factor in the boiler sizing game is the issue of thermal mass. Good construction lowers the calculated heat loss, but does not lower thermal mass--often raises it instead. So, if you engage in turning the system on and off or deep setbacks, you need extra capacity to raise temperature of that mass in a reasonable time.

The accepted practice is to size to the design heat load because most boilers drastically lose efficiency as their capacity exceeds the load. The boilers recommended by the guys above can be safely oversized with little if any energy penalty, but some attention must be given that the lowest loads encountered be not much less than the lowest point the boiler can modulate to.

I personally like Buderus and Viessman for their ability to minimize loss of efficiency when they are oversized. Gives you flexibility with good efficiency and outstanding, proved reliability.

Bill