Sizing The Boiler

Clif Heeney

Do I use the DOE Output or the Net I=B=R Output when determining the correct size boiler to have installed? I am going from a gravity fed hot water system to a gas fired boiler.

Duncan_2

Big cast iron radiators?

I like smaller boilers and less energy use. For those reasons, and considering your system, I'd probably opt for new controls using constant circulation with an outdoor reset control. This will your system to run at a lower, modulated temperature. As it gets colder outside, the system water temperature gets hotter.

With a large mass system like yours, this approach has the advantage of more even room temperatures than on/off controls. Less temperature swings, lag and overshoot problems.

Old gravity systems have large diameter piping which means large piping losses at high temperatures. With lower system temperatures, parasitic piping losses are less.

If I went the low temp way, I'd figure on using the DOE rating based on a twenty-year low outdoor temperature.

If you're going to stick with on/off control, Net I=B=R might be the way to go.

How much bigger of a boiler are we talking here? Ten percent isn't that much of a big deal. Do you want fast, good, or cheap? Pick two of those three.

See that pipe on the left of the main screen? Click on "Hot Tech Topics" and scroll down about a page to "Boiler Ratings Explained". It's at:

http://www.heatinghelp.com/newsletter.cfm?Id=64

Clif Heeney

My Situtation

Duncan, thanks for your pointing out the article in the Hot Tech Topics. I was interested in Dan's story about a man who regularly takes off 10% per zone up to a total of 30% as he figures that rarely are all the rooms occupied at the same time during the coldest day of the year.

I am an American missionary living in Tallinn, Estonia. My wife, baby daughter and I live in a 2800 sq. ft. multi-purpose building. We live in a small self-contained apartment on the second floor. On the first floor are two guest rooms with 16 beds, a large kitchen, WC's and a shower and a large meeting room. In the basement there is the boiler room, garage and a sauna, shower room and changing room.

Rarely, if ever, would all the rooms be used and heated at the same time on the coldest day. The meeting hall is generally only used on Sundays for about 5 hours. During that time the guest rooms would not need to be heated.

I did a heat loss calculation using the Slant/Fin Hydronic Explorer. Our building construction methods are different here, but I think that I got a valid heat loss calculation. I came up with 120,451 Btuh. If I deduct the heat loss for the boiler room, and the two guest rooms, I come up with 104,135 Btuh in heat loss.

My choices are using the Viessmann Vitogas 100 GS 1 - 29 which has a DOE output of 98,000 Btuh. Or we could go with the Vitogas GS 1 - 35 which has a DOE output of 123,000 Btuh.

I want to put in a quality system with the size of boiler that I really need. I don't mind spending the extra money for the GS 1 - 35 if I need to, but don't want to waste money on the front end as well as the next 30 years in wasted effieciency and fuel if I don't need to.

What are your thoughts?

Duncan

Fudge factor.

Clif, I don't think I would EVER knock off 30% off a calculated heat loss without a compelling reason. It may work in some instances, but that would be more luck than science in my opinion. It scares me too much to try.

Your 98,000 Btuh DOE is 20% shy of your 120,451 Btuh calculated loss. A five percent smaller boiler would concern me, but I'd definitely go with it. Ten percent smaller would worry me, but if there were mitigating circumstances, like a wood stove and eye-to-eye contact and 100% assurance from the owner that they're OK with a possible need to light the wood stove on cold days, I'd be OK with that. I wouldn't go to twenty percent less boiler, though. It's just too close, even considering extra room for error built into heat loss calculations. But my main experience is residential, not too much like the situation you're describing.

Whenever I get in a boiler size borderline dilemma, I end up carefully reviewing my heat loss calcs manually. Sometimes three or four times, just to make sure I didn't miss anything. If a computer driven heat loss calc program verifies my calculations, so much the better. I play with air infiltration numbers and tweak the numbers here and there to see what kind of range of heat loss numbers I come up with. If there is any supplemental heating like a wood stove, I'm OK with going to the small boiler.

You're right to be concerned about a thirty year expense of inefficiency, it adds up. If I were you, I would go over the heat loss with a fine tooth comb and size the boiler accordingly.

If many of the rooms are intermittently used, and need to be quickly brought from cold temperature up to comfort level (like the meeting hall), the low temperatures of outdoor reset I mentioned will not work well in this respect.

It occurs to me that widely varying loads like yours might be asking for two smaller, staged boilers, if it's in the budget.

Keep in mind, Clif, that I may be missing something very obvious from here. There's no substitute for an on site evaluation. Keep asking questions.

Duncan

Clif Heeney

Heat Loss Calculations

I've been over the heat loss calculations several times, tweaking the numbers each time, but I will go over them again. Our construction methods are somewhat different here, 24" thick solid concrete block basement walls and 8" thick hollow core concrete precast panels between the basement and first floor. The heat loss program that I used from Slant/Fin doesn't have factors for those types of construction materials in it. As a consequence I changed the factors somewhat but NOT enough, I think, to materially change the 120,000 Btuh figure for the entire building.

Don't know how to do a heat loss calculation manually so I'll rely on the program and just verify all my inputs and factors.

About the outdoor reset. I seems beneficial to me where my family lives in the apartment all the time it would save some on energy. Also it would work very well if I had some longer term guests in the house, rather than just here for one night. That is why I was planning on going with it. Normally, I know at least a day ahead of time when the guest rooms or the meeting hall will be used. BTW the heat loss for the meeting hall alone is 34,367 Btuh. Would having 24 hours notice give the boiler time enough to bring the temperature from chilly/cold to 68 - 70 degrees F while utilizing the lower temperature that comes from using the outdoor reset? I'm saying chilly/cold because during the week I would just about turn the radiators completely off unless it was cold enough to freeze the lines. Or can the outdoor reset be overridden so that the boiler will fire at the maximum temperature? In other words can you have the best of both worlds, outdoor reset control when you want it and indoor temperature thermostat when you want to use that?

Good idea Duncan about the two smaller, staged boilers! Unfortunately we don't have the money in hand just yet for one boiler, so unless we have manna coming from heaven, two will probably be out of the question financially!

FYI, the outdoor design temperature that I used was -5 F. I am also planning on using a 200 liter indirect water heater. I'm also planning on putting Danfoss thermostatic radiator valves on all the radiators. Also, I only have 513 EDR so at 170 BTU per EDR, only 87,210 Btuh out of the exisiting radiators. I don't want to size the boiler to what I have though, as I'm hoping to put in bigger radiators in the future. Do these things change your opinion on anything?


Do you have any experience with the Viessmann or Junkers boilers? Any recommendations for them or other brands of European boilers?

Thanks for you help and for your encouragement to keep asking questions. I'll do it!