Decision on Buderus oil or gas boiler/undersizing?

Ron Schroeder

Hi David,

No real need to oversize the boiler. Remember that the lights, cooking etc put btu's into the house too.

Something that most people don't realize is that undersizing the capacity of the the indirect can INCREASE fuel usage. A larger indirect will reduce the number of runs of the burner per day. Starting or stoping a burner is when it is the least efficient. Also the smaller indirects often don't have as large of a heat exchanger and can't absorbe all of the boiler's energy causing the boiler to short cycle. Make sure that the piping and circulator for the indirect is large enough to get the rated flow to the indirect. Even though the fittings on my Buderus indirect are 1", I piped it with 1 1/4" with minimum lengths and fittings to get rated flow with a 007. Don't forget the flow checks to prevent piping standby losses in the non heating season.

D107

51mbh net too low for 47mbh load?

Trying to make decision on new boiler;narrowed it down to Buderus. There's alot of talk on the wall about not oversizing boiler, but I have noticed that there is a tendency amongst even good installers to hedge their bets on the sizing. I understand this, since it would be an installer's worst nightmare to get a not-enough-heat call back on a cold spell.

Anyhow two independent heat calcs--one from an experienced pro and one by me --a meticulously measured one done as a learning experience--both confirmed a load of 47Kbut and a gain of around 100K. (Long Island NY, split level 1956 house, monoflo, convector boxes, 2150 sq ft, 16.5KCu ft, 15deg design temp, 70 deg indoor temp)

Both the Buderus GA 124 and G124X smallest gas units have Net outputs of 51k and 53k respectively. Given an indirect Buderus L-200 53gal for a single senior citizen with occasional guests, is there any reason not to go with any one of these gas units given the phrase 'size the boiler to the heat load' or does that mean adding 10 or 20% to the heat load just for some headroom or margin of error? Is it agreed that the heat calcs systems are often overly conservative to begin with, and adding another 10-20% on top of that is overkill and results in oversizing? I've often seen comments from wallies about their own boilers, "I see I could have gone a size smaller now that it's been in awhile."

The choice for oil would be the smallest Buderus G115, which nets at 64K, which is 27% more than the load.

I should add that the Logamatic R2107 would be used to run outdoor reset and DWH priority.

Thanks,

David

RoosterBoy

hi David i went with a g115 and instead of going with a biger boiler i went with more baseboard that gave me lower water temps to work with and saved me oil.

also for just one guy a 53gal seams to big i have 2 people here and got a lt160 42 gal and never run out of hot water in fact i wish i could have gone smaller 32gal

the 2107 with bfu room sensor is the best thing i ever had and hope you do that also.

with oil and gas prices the way there going id go as small as you can. most of the time your boiler is over sized anyway. i don't know the gas models but from your load the samllest g115 would work and a 32 gal hot water tank would work just fine. that's the way id go.

also if you have gas id look at mod-cod boilers like gb142
and maybe on damand hotwater heaters

thanks
Jason

jp_2

hot water consumption

look ar your real water usage, if its real low, like a lot of bachelors, I might consider a small 20gal electric.

Brad White_81

Un-Oh- Bachelor?

David- you may want to tell him you are married...

D107

thanks; thought was to keep the tank temp at 125 deg

to avoid mixing valve like you'd need if you kept water at 140deg, though of course at that temp you'd have more hot water but of course higher standby losses. It's for my 82 hr old mom, by the way, but I'll have to take another look at mtman's post the other week:
"The capacity of the indirect domestic water heater should be based on a realistic estimate of peak demand. And again, since the indirect is usually supplied via priority you want an indirect that can instantly absorb the FULL output of the boiler....."i.e. that the heat exchanger in the indirect tank must be capable of transferring heat from the boiler just as fast as it can be produced at maximum boiler ouput..."

So as I'm understanding it, on a design day, the HX of the indirect must be capable of handling the full boiler net output (G115 = 64mbh) which it will be on DHW priority. Probably a Superstor is better than a Buderus, but I would think the column I need to look at would be 180degBoilerBTU/ size. The smallest superstor rating in that column is 84Kbtu for a twenty gallon tank. The Buderus L135 (36 gallons) rates at 73Kbtu Continuous rating inlet 50deg; outlet 140 deg. Both of these are above the 64kbtu of the G115 but hopefully close enough. Not sure if this means I'd be getting less efficiency with the larger tanks.

Thanks,

David

D107

buderus has written me \"The G115/21 has a gross output

of 74,000 Btu. Buderus indirect water heaters can easily accept the 74,000 btu’s." so i guess their heat exchangers are large enough to absorb all the boiler's energy. Though I don't know how really specific he was; probably good to measure boiler flow, output vs heat exchange capacity.

As for getting large enough indirect, i guess it's a balance between avoiding short cycling waste of energy with standby loss wasted energy of keeping excessive water heated and stored.

Thanks,

David

Paul Pollets

Small oil boilers...

I like a larger boiler size rather than the smallest. I've seen problems with setting up the smallest Buderus boilers on oil. I prefer a G115-28 with a 53g tank, or going to the 80g tank, if you have teenage daughters. The small indirect tanks (under 53g) work best when there are very small DHW loads.

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D107

sounds like you have direct family experience with hot water

load and your kids. This boiler is for my mother who lives alone--not that we're tailoring the indirect solely for her use since she may move someday.

I assume the problems you spoke of have more to do with hot water load than heat load. Or perhaps a big family taking long showers during morning setback recovery/domestic hot water priority could make for heat cycle interruption for an hour or more on a design or lower day.

Thanks,

David