The case for low mass oil boilers

Weezbo

condensing unit...while condensing is wonderful technology, Rocky is asking a kind of 'in your experience with properly installed low mass oil fired equipment what are the draw backs if any that you find to be of noteworthy remark.'Type question *~/:)

like 1 do the burners need to run hotter to run cleaner ,if so what benifit is that in efficentcy...

2 if they indeed do need to burn hotter, hence increasing the stack, how does that function when the slug of water swamps the boiler?

ie short cycling to maintain boiler temp vs constant burn dropping into condensing Are the boilers fine with that and if so are the stacks in L vent not liking it...

would you go power vent rather than atmospheric? and is the reliability of power vents such that they in conjunction with low mass do not create situational difficulties of thier own.

Rocky can handle real world wrench twisting observation , say we have installed ten low mass oil and lp boilers you have probably installed 100's yet when it comes to oil fired quality iron, the tables are turned. and when it comes to servicing you will likely have seen thousnds of low mass boilers in gas and oil where we have twisted many a late hour wrench on CI oil fired package boilers and high mass.. given that very few if any installations are identical there must be a host of "minor Technicalities" that have been noted over the years...heres an example when you have a high mass boiler and the power goes out for 5 hours ,generally, there is a window of opportunity to bang BTU's into the place, pull all the furniture away from the corners of rooms while someone else runs about thawing pipes and someone else deals with the burner and boiler room....in radiant high mass systems should the boiler go down, one may have 16 hours or more to get in and get the place back up,with iron you step the heat up in increments and shut the boiler off while maintaining circulation in the boiler and near boiler piping...especially in severe cold.

where is a low mass boiler at in 30 below or colder in such an instance are they "History"?
if so, what are the manufacturers snivel clauses that are quoted? because no matter what goes wrong You touched it last and you should realize.....:) this has never happened to anyone else....you must be the First person who has ever happened to do That .... thats the kind of info hes asking . instead of Oh Dammbit Gumby! Rockys more of the 'lets make sure its done right the first go round' kinda human being..*~/:)

I am also interested in tech observations of the insides of low mass oil and gas fired boilers...it is a real dissapointment to me for example that the Viessmann condensing oil boiler is not even going to be available in Canada,i thought for sure i could smuggle one in as a Sprinter part :)er i digress... so far the Ultras and the munchkins seem to be hanging in here as far as condensing units go ,thing is ,thier vents are designed for various low temp output from the"stack" with the occassional 140 degree temp difference of indoor and outdoor air i am certain we will be in the Experiential Realm sooner than later.This reminds me...a guy has a moniter FCX oil appliance that he lashed up out on the end of Rosie Creek.

Rocky_2

Why would I not use low mass always?

Just wondering. Have been installing a few of the System 2000 boilers lately. Got to wondering. Why would I ever use anything else other than low mass? What advantages do high mass cast iron boilers have over low mass? I really can't think of any. Low mass purges almost all the way to room temp so standby loss is negligible; they heat up right now for quick response to DHW or baseboard calls, radiant can be accomplished via a holding tank that has no stack for stack losses, are cold start, appear to be fairly durable as I have serviced dozens here that have been in service for over 15 years, and with the heat load we see here, thats about equivalent to 25 elsewhere. Am willing to entertain any thoughts as to why I should ever bid anything other than low mass. What says ye?
Warm regards from chilly (currently -8 and going to -25 tonight) Fairbanks,
Rocky

Joe Brix

I agree with you

Just need to protect the boiler from condensation. A good buffer tank, or a reverse indirect would work also.

John Siegenthaler spelled out a great design:

http://www.pmmag.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2379,78589,00.html

Rocky_2

Would really like a few more responses

Can envision me sitting in front of a customer trying to discuss the relative merits of low mass and high mass boilers. Any other things on the pros or cons of low mass versus high mass that I can use to enlighten potential customers? C'mon Wallies, lay it on me.
Warm regards from chilly (-24) Fairbanks,
Rocky

S Ebels

High temp applications

As in 200+ degrees. I use CI boilers a lot in applications where the need for extremely hot water is required. Such as using the boiler to fire an indirect up to 180* for sterilization purposed. These are mainly encountered in commercial/agricultural jobs in my neck of the woods. A lot of the condensers are not rated for water temps required for applications such as that.

As for a basic residential system, there is practically no job that a condenser can't do more efficiently than even the best CI unit.

Joe Brix

On larger oil fired installs

Over 200K, You might have an easier time with a sectional cast iron boiler and assemble in place. Sectional boilers give you more sizing opotions. Low mass oil fired are usually steel (System2000, Burnham LE, Laars Max) so there's usually only one or two size models with a few firing ranges for each model. The over-fired models usually do not provide the best AFUE numbers. I would think the steel models are easier to cold start as there are no section nipples or gaskets to strain from the cold/hot cycles.

Low mass 3 pass CI boilers like Biasi and Pennsotti give you the best of both worlds but you'd still need the buffer tank for radiant and converted gravity systems.

Constantin

Interesting Reading, as usual, Weezbo

My personal take on the matter is that low-mass boilers make a lot of sense whenever you can control the input rating via staging or modulation. If the range of modulation is wide enough and the penalties for "short-cycling" at the lower end are not too high, then a low-mass system will provide very energy-efficient operation with little danger of flashing to steam.

On the other hand, if you have a oil-fired boiler, then most US residential installs will not be able to take advantage of even rudimentary modulation/staging. Under those circumstances, the "thermal battery" that a high-mass boiler provides can be your ticket to maximizing the time between burner firing, maximizing run-time, and hence maximizing the seasonal efficiency of the unit. However, it will have to be well-insulated, tolerant of wide temperature swings, and ideally feature 3 passes before exiting the boiler.

Rocky_2

So.......

Really the only advantage a high mass cast iron oil boiler has over a low mass oil boiler in the common residential market, is if you have a small micro load. The inherent thermal reserve of the high mass boiler or, thermal "battery" ,as Constantin says, will allow the load to be satisfied without the need to fire the burner. So the potential for short cycling with a low mass boiler is a very real possibility. As far as I can see, this is about the only real drawback I can see. Since we don't have modulating oil burners here....yet.....a good look at the existing radiation would be time well spent. Ie: big zones or small zones, ie: long burn cycles or short burn cycles.
But with the low-mass' ability to purge to room temperature after a heat call, even short cycling isn't as much of an efficiency issue as it would be with a high mass boiler with lots of heat left trapped in the boiler to waft on out the stack. Is this not correct?
I appreciate all your feedback,
Warm regards from chilly (currently -28 below and going to -34 tonight) Fairbanks,
Rocky

Joe Brix

At -34 degrees

I don't think you ever seee a micro-load! I guess you can stare at a boiler temp gauge and watch it drop like a stone after firing with air that cold. As said, the buffer tank makes for a better "battery" to save your BTU's for micro-loads then a higher mass boiler. Sure it's taking up floor space but the great thing about the reverse indirect is you get buffer tank and DHW producer in the same box.

The only issue I see with Siegenthaler's design might be how to implement priority for DHW. Need a way to stop the heating circ's when a large DHW demand comes in. Probably need an aquastat to break on a low temp limit (say 130°) to allow the boiler to just handle the DHW load.

Ron Schroeder

Piped properly with a buffer tank, a low mass boiler will beat a high mass one almost every time. Without a buffer tank, a low mass boiler will ony get long enough run times to prevent short cycling if the load of the emitters (not the structure) is near the capacity of the burner. With multiple zones and the frequency that only one or two zones may be on at the same time, a low mass boiler without a buffer tank will be short cycling most of the heating season (even in Alaska).

Ron

Ron Schroeder

Hi Rocky,

What is the average winter temperature and the design temperature in Fairbanks?

Are boilers oversized in Alaska like they usually are down here in the lower 48?

Ron

Kevin__Flynn

Low Mass vs. High Mass

An interesting thing about the heat loss of a structure is that it is calculated, as we all know, so that it will heat the structure at the design temperature (average low temp) for the area that the structure is located in.

Granted, in Alaska that design temp is certainly much lower than it is in New England where I am. However, I'm guessing that it occurs somewhat infrequently when compared to the entire heating season.

This means that for the majority of the heating season any single boiler with a fixed firing rate will be oversized for most of the year.

I am all for the concept of low mass boilers when there are multiples, idealy 4 or more. Or they have modulating burners. I agree that low mass boilers will have nearly zero stand-by loss considering there isn't much to lose heat from.

When however, there is only one boiler with a fixed firing rate it is no secret that it will short cycle for a large portion of the year. This is assuming that the boiler wasn't oversized to begin with.

Two very important things happen when a boiler short cycles. One - greater wear and tear on burner/boiler components (controls, igniters, blower motors, etc...)not to mention undue stress on the heat exchanger, decreasing their lifespan and adding to repair costs. Two and even more important - when efficiency testing is done, specifically AFUE, the appliance is tested under full and constant load. This creates a nice warm chimney and combustion chamber, establishing a good draft. the lack of this will lead to poor combustion and possibly sooting, reducing the efficiency of the appliance.

Now, I agree with Ron, the addition of a buffer tank will solve these issues but, if I'm going to essentially add mass to a low mass boiler, why not just buy a boiler with more mass. You might say, because the buffer tank is well insulated the stand-by loss is less. However, almost every European boiler manufacturer as well as some American manufactures are manufacturing boilers with 3" plus of insulation around every water backed surface, nearly eliminating stand-by loss.


Kevin Flynn

Guy_6

Mass

My take on this, and correct me if I am wrong, is that sizing is the answer. If the boiler can be properly matched to the system and load, a low mass boiler will be fine. A higher mass boiler is much more forgiving towards fluctuations in load, and therefore be less likely to short cycle.
My first thoughts here though were a low mass boiler with a managment control like the Beckett Heat Manager to enhance run times SHOULD work beautifully.

Rocky

Ron,

Design temp here in Fairbanks is -47. We have an average of 13,800 degree days here per year. Bad thing is, when the weather system comes in that brings these types of cold, it is usually a massive high pressure ridge that covers most of Siberia, Alaska, and Northern Canada. Once it sets up shop, it doesn't like to leave. These systems produce daytime highs of -40, and nighttime lows of -50 for days and days and days on end. The coldest I have personally seen here in the Fairbanks/North Pole area is -64. Dropped off into a little depression out of town about 14 miles and my service van's tranny started squealing. That is a sure sign you've hit a cold spot. Got my digital thermometer out...-64. You start to get all kinds of wierd sounds from your service rigs when you hit these cold spots. And don't even think about hitting the brakes. You hit them and they lock up and don't release because the fluid is so thick. Plus your tires are so hard they are more like skis than tires. Needless to say, me and Weezbo run pretty hard during these times. If you think he is interesting to talk to on these posts, get him at the parts counter after about 6 days of no sleep. WOW! Times like these you are glad most contractors have "oversized" their systems! Ha!
Warm regards from (not as chilly as it could be) Fairbanks,
Rocky

Joe Brix

Kevin,

I think all boilers have 3" of fiberglass wrap now a days.
Great for stopping heat loss into the boiler room, but that flue pipe is open to draft 24x7 (Talk'n oil fired here with no flue dampers.) Sizing a boiler properly is important, but we must size to design and that happens for about a week's worth of days during the whole heating season.

The conditon of stairs and room to manuver in most older homes would mean breaking down a high mass boiler to get it in. (Bad enough we have to break an old one up to get it out.) I think bringing in a 300 lb boiler with a 150lb buffer tank makes more sense then breaking down and rebuilding a high mass boiler.

Now once you're needs get over 200K BTU's, you can probably start looking at low/high firing burners. Never seen one but is an Energy Kinetics EK-3 @ 650 lbs more efficent then a larger commercial size boiler? At this point we can probably start the discussion on when single or multiple boilers should be used!