condensing boiler efficiency with high return temperatures?

Wayco Wayne

a lot of the older houses were built with gravity systems so the radiators are grossly oversized. (300%) One of my favorite tricks is to install a condensing/modulating boiler with re-set on these systems and use the large heat emmiters to their fullest advantage. The customers have reported much better comfort and savings of up to 50% on their gas usage. It's easier than trying to convince folks to enlarge their heat emmiters on other systems so they can have some flexibility on boiler water temperature. It's common to try and get by with the least amount of anything. It doesn't work that way with heat emmiters. Of course improving the building envelope is the best investment anyone can make and will lower the needed water temps obluiqely. WW

<A HREF="http://www.heatinghelp.com/getListed.cfm?id=255&Step=30">To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"</A>

Shawn_8

condensing boiler efficiency with high return temperatures?

Does anyone have some official information on actual efficiencies of condensing boilers at higher return temperatures, such as on a baseboard heating system?

While it sounds great to offer a boiler replacement with a 98% efficiency to a high temp baseboard system we all know that cannot happen. It would be nice to offer them real numbers to show their savings. Modulating the firing rate and using outdoor reset will definitely increase overall savings not to mention help increase efficiency, but a chart to show the customer actual numbers would be a great help.

Anyone?

Henry_9

Efficiency on all

condensing boilers is based upon multiple factors: Input, return water temperature, % of excess air in combustion.

However and far more importantly, if your boiler rapidly cycles, it will never get the "advertised" efficiency.

The moral of the story, do the math and install the right piece for the job. The right piece may be good ole cast iron.

Respectfully,

Henry Nichols

Viessmann Midwest

Chuckles_3

It depends on the boiler.

Plumdog_2

Efficiency

IT DEPENDS! HA! nobody can measure it. Show me some side by side scientific testing done in a controlled enviroment.

Tom Anderson_2

See the November 2004 issue of HPAC Engineering magazine, Page 30.

This is an excellent article about real world efficiencies of boiler efficiency vs. return water temperature vs. modulating burner percent firing.

What I found most interesting are the efficiency improvements resulting from modulating burner, regardless of condensing/non-condensing and return water temp.

:-)

Unknown

condensing boilers

Most condensing boilers or non-condensing boilers, will condense when return water temperatures fall below 130 degrees F.

Because of the variable outside temperatures, and thus heat loss, the so-called "sweet spot" (water temperatures below 130F) will be reached sporadically.
Reset does work on baseboard systems in moderate weather and will keep your boiler condensing if your zone or zones are big enough to load the boiler.

The addition of an indirect fired water heater will load the boiler nicely and improve return on investment lasting 3 times as long as a conventional water heater.

All the best,
MA

Constantin

Sporadically?

Morgan, your statement may apply to baseboard systems that have been sized for 180°F water supply on design days. However, with the proper amount of emitter surface, insulation, etc. just about any home can be heated even on design days with water temperatures below 130°F.

For example, I never expect my in-floor radiant heat to reach non-condensing temperatures, because my maximum Manual-J heatloss is 12BTU/(ft² x hr) on a design day. Even when it's -15°F out, HVAC-Calc reports a heat loss of but 15BTU/(ft² x hr).

Mike T. reduced his fuel consumption by 44% on a HDD corrected basis by simply switching to a 94% AFUE Vitodens from a standard 80% AFUE CI boiler. Such gains cannot be achieved by 'sporadic' spurts of condensing operation but by the interaction of a efficient boiler, control system, and lots of emitter surface. The previous heating system simply couldn't take advantage of the emitters the way the Vitodens can, because the previous boiler had to maintain 140°F+ hot water to prevent flue gas condensation.

Lastly, Honeywell, the DoE, and Tekmar have shown 10%+ efficiency gains from the application of OR systems, even to baseboard heating. Do a search here, it will open your eyes.

ALH_3

Yup

I have questioned the "real" value of condensation for a while now. I wonder how much is really condensing directly on the heat exchanger. How much condenses in the flue? How much is that condensate really worth? The volume of water running out of the condensate drain is not necessarily releasing all those BTU's into the heat exchanger.

The condensation is more of a side-effect of modulation than it is the goal. When you turn that burner down, the boiler will condense, so in the past everyone just ran it hot to prevent condensation. When you think about it, that's a pretty unintelligent solution when we have materials available to us that are resistant to corrosion.

It's all marketing. The emphasis is wrong, but it's the emphasis that sells the best. The water running out of the drain is a lot more tangible than knowing the burner modulates. I won't argue because the effect is positive either way.

-Andrew

Unknown

You couldn't be more correct. However, the question related to baseboard systems I believe.

MA

Constantin

Yes, but...

... like I stated above, even baseboard systems can be run with low water temperatures in many houses most of the time. After all, many heating systems were sized to accomodate uninsulated walls and attics, high rates of infiltration, etc. Fix the envelope, and the formerly right-sized system becomes oversized and a very efficient source of heat.

In new or retrofit construction, simply select enough panel rad or baseboard on the basis of 140°F hot water for design day conditions, and the house is ready for a condensing boiler, even if the consumer is not. Baseboard is cheap, so one would be silly not to use it.

dconnors

Condensing Heaters.....

Low return temperatures provide a means to lower the stack temperature. Typically condensing boilers run a stack temp about 30-40 degrees warmer than the return temp. Exhausting higher temperatures is of course a waste of heat up the chimney and wasted dollars. Baseboard is rated on flow and temp to give you btu's per foot. Lower temps just means you have to add more footage to get the same output. Problem is on the length of run. Typically you dont want more than a 20 degree drop on baseboard so you have to design accordingly. So, when choosing a new condensing boiler; you also have to choose new and improved designs as well. Just my thoughts.

Unknown

right again my friend

There are many interesting ways to add load to a condensing boiler and I like them all, but some better than others. Fin-tube can eat up a lot of wall, something all women charish for reasons beyong my meager comprehension.

Jed_2

And in most

Northern climates, and assuming an existing, aged floor plan, even with the added assumption of "oversizing",at 140°F SWT, you probably don't have the wall space to "add" the required footage.

Jed

S Ebels

The 98% number doesn't mean as much as you think

Viessmann's commercial sized condensing boiler, the Vertomat shows a 10% drop in efficiency when rated at 100* to 180* water temp. You wind up at about 87% when you hit the higher temps. Only a 10% gain for the low temps.

However!

When a school system in Indiana converted their 80% boilers to the 98% (at low temp) Vertomats, they had an across the board decrease in their fuel consumption of 36% IIRC. The article was in PM Engineer last year.

The rating doesn't tell nearly the whole story.

Floyd_10

The condenser that I'm familiar with...

will virtually always run flue temps. at or below the return water temps. The exception to this is only when firing at or near high fire.

From my experience.... if you can size the boiler so that it can be fired at midrange modulation or less than the eff. will be better than if the boiler is sized to run at near high fire at design temps.
With 5-1 turn down range this really is not all the difficult to achieve. The best part is that you can limit the boiler for half fire for the space heating and let go full bore for the DHW.... it will back down from high fire very quickly in the DHW mode, as the recovery if it is piped correctly will be very rapid.

If you haven't played with the latest and greatest condensers yet... spend a few bucks and get one installed where you can watch it and play... the knowledge you will gain will blow you away....

Floyd