minimum return temp

steve_93

Buderus has no minimum return temp requirement.

harvey

return temp

What is the minimum recommended return water temp for a cast iron American boiler? How about a steel tube boiler?
And for a cast iron Euro boiler Thanks

Ron Schroeder_3

As a rule of thumb

I have had good experience when I operate atmospheric boilers, be they steel or cast iron, American or foreign interlopers, at return temperatures no lower than 130F though 135F would probably be better. Please consider the effect of the dilution air that is introduce though the draft hood, because condensation can also occur in the vent/chimney even when it is not occurring on the surface of the heat exchanger. Damage to all surfaces not intended to be exposed to condensing flue gases will certainly result.

J.C.A._3

Wow,

Loaded question?

Depends on the unit used for heating and in my estimation also, the fuel used and the way that it's used.(too many variables for a straight answer)

If you can give a more specific description of the piping and terminal units, while describing what's there now....I think the answer would be far more attainable.

Sounds nit-picky doesn't it?....but the answer lies with the system and piping...not so much the temperature of the returns. Chris

hr

The dewpoint

of the fuel plays into this also.

I believe oil fired likes a bit higher return temperatures than gas or LP. I've read 140°F most often. I believe the Viessman cast iron gas fired have a non adjustable 140° return protection aquastat, if I am not mistaken.

Also some of the thermostatic mix valves for return protection have 140° elements in them.

This B&G article gets into temperatures a bit more.

http://www.bellgossett.com/press/thermalshock.htm

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Mike T., Swampeast MO

And it also depends upon how much efficiency you're willing to loose the further your loss is away from boiler output.

Check out the current thread with the Sears Indestructo boiler.

Steve Ebels_3

It depends............as always

Here's a quote from Burnham in regards to gas boilers. "Flue gas condensation will occur when the burner is on and the water in the boiler is less than 130*F." Note that they refer to the temp in the boiler, not the actual return temp. As long as you can keep the boiler temp up where it belongs you are safe regardless of the actual return temp. There are however, a lot of other factors that influence what your return temp should be. Atomospheric or power burner, venting issues, oil or gas, flow rates through the boiler etc.

If in doubt, install some type of boiler protection. This can be anything from a pump aquastat that opens on temp drop to a boiler bypass to a sophisticated mixing valve or injection control.

The European boilers, Viessmann and Buderus have "pump logic" built into their proprietary control packages. Buderus will let their iron atomospherics run down to a temp of 104* IIRC. Viessmann is a little more cautious and recommends around 140* for gas and 125-130* for oil. The Viessmann Vitola has no low limit at all, nor does any condensing type boiler.

Larry (from OSHA)

Steve

You touched on an issue that I have pondered for a while due to my own systems design. My question is what effect if any does an induced draft type burner have on condensation and the resultant effects of condensation.

My WM boiler is serving fin baseboards and an indirect for the domestic water. The system has a basic outdoor reset control.

Your perspective is greatly appreciated.

Larry (from OSHA)

hr

You also need to be sure

that the flue temperatures are hot enough to prevent condensation. In the ENTIRE flue length.

If you start running cold boiler temperatures like that you may need AL29 stainless flue systems. I know the Weil GV and Burnham Revolutions I have installed required stainless although neither were (are) condensing boilers.

If you want to run cold return temperatures I would suggest a condensing boiler. They thrive at low return temperatures. I believe flue temperatures are a mere 10- 20 degrees above boiler operating temperatures. Gas or oil fired run 300° and up.

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Larry (from OSHA)

my flue is stainless

and the stack temps are around 300 or so after about 4 minutes of firing. But my question remains, are induced draft boilers subject to the same effects of condensation at the block that atmospheric types are? Or does the stiff wind blowing past the pins do anything to minimize it?

As far as replacing with a condenser, I have it in my plans for sometime in the future (lots of other capital expenditures ahead of that one as long as the cost of a therm stays below 2 bucks).

Larry (from OSHA)

Bill Nye_2

You don't

You don't have to spend crazy money to fix this. When the pump starts on a call for heat the return water is room temperature. Usually in the unheated basement and often 50° or less! But so what, it will warm up. Almost all baseboard systems have only a 20° ^T or less.

The water will warm up. We are not melting snow or heating swimming pools. The temperature will come up.

You can keep the temperature up in the boiler and still use cooler system temperatures. Boiler by-pass or system by-pass. You could do that with a couple of 2 or 3 dollar tees and some scrap pipe.

ESBE and Danfoss and many other manufacturers make thermostatic by-pass valves for fairly cheap money.

Boiler run time is key here too. You want it to be on long enough to warm up the chimney and flue etc. Just like your car, short trips eat up the exhaust system because it never warms up.

I have a Viessmann Vitola [so I am a little spoiled] and it is fairly high mass, so even with the low water temps it has long run times and even longer off times [burner].

Low mass boilers, as long as they are hooked up to some kind of load, are going to run for a while and tend to heat up quickly.

The trouble can start with cold start boilers and low high limit settings. The boiler/burner never has a chance to get above the dew point. I have seen some multi-zone systems where there is no reverse acting control for the pumps, lots of iron pipe and radiators,and a minimally fired or underfired burner. What a mess.

Where am I going? I just think this minimum return water thing is often misunderstood. Radiant floor has lower temps. But not all of the water returns to the boiler all the time!

You should read Dan's book Primary secondary piping. There are a lot of ways around this.

link

http://www.heatinghelp.com/shopcart/product.cfm?category=2-44

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hr

Larry

I'm not sure I follow your question. The effect of condensation on a ferrous boiler block would be rust and corrosion. I would say this would be true of induced or atmospheric cast iron boilers. Copper tube boiler subjected to condensation turn green and corrode and flake the byproducts onto the burner tubes, generally.

Ideally the burner on cycle is long enough to dry out this condensation on each and every cycle.

Noel or Glenn would be able to answer better, perhaps.

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Larry (from OSHA)

Thanks

For the most part I think you answered my question. I guess from what I've heard here before, extended low temps with atmospheric types can actually drip. I've never seen any evidence of moisture with mine. My situation with the baseboards is that they tend to allow for boiler temp rise fairly quickly. I'm curious though what the temp difference is between water side and fire side on a cast boiler. I can have a stack temp of 275-300 and water temps either side of condensing temps. (Never being schooled in this, I just wonder about these things.) Thanks for your insight.

Larry (from OSHA)

hr

Low mass emitters

should not pose a problem for extended low return temperatures. Possibly a grossly oversized boiler that banged on and off in a minute or less, might go to sleep wet.

High mass emitters, like a concrete slab on grade,can be a problem for any non condensing boiler, IF adequate return protection is not provided. It can take a few days to ramp up a concrete slab that starts from 35°

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Tundra

It sounds as if you may have an American made cast iron boiler. If so you may want to keep your delta-t around 20 - 30 degrees to prevent thermal shock. The Euro cast boilers (Viessman, Buderus, Biasi) are made from a different cast iron and are far, far less suseptible to thermal shock. I have tried some extreme tests on mine and have been very pleased.

Unknown

We aren't talking about thermal shock

Perhaps you've seen an American boiler with a tankless coil and cast iron radiators. You know the kind, the ones that have been sitting in the basement for twenty or thirty years. Every time the pump starts, the entire boiler goes from near 200° to about 80° before the lightning fast aquastat stops the pump so that it can repeat itself. Where's the thermal shock on these boilers?

COMMERCIAL boilers are prone to thermal shock, but you weren't talking about commercial boilers, with the brands that you offered.

Do you have any EVIDENCE of this thermal shock that you speak of, in residential systems?

NOW, back to the issue.

At what temperature does condensation occur in EUROPEAN boilers that are used in North America? Same oil, same air, same draft. How can it be different?

Noel

Steve Ebels_3

Larrry

The condensation issue I was refring to in an atmospheric vs forced draft is not related to the block. What I was thinking of when I posted that was flue condensation which may require a stainless steel vent of some type for the forced draft appliance.

Larry (from OSHA)

Steve

Thanks for the clarification. I sometimes think about this stuff a little too much. I'm glad that I have a stainless flue. If therms keep going up in cost I'll have a condensing boiler that much sooner to make the most of those little btu's.

Larry (from OSHA)

Steve Ebels_3

Read the fine print

Even the Thermostream series boilers (315&515) call for a minimum BOILER temperature for sustained operation. IIRC the operating temp minimum is 131* or around there. You are correct that they will tolerate low (<100*) return temps.

The factory setting on the 2107 control will allow the circ to run down to 104* but when the boiler fires the circ is shut down until minimum operating temps are acheived.

leo g_13

Noel,

as far as i have understood the spiel, the euro's use a siliconized cast iron, so yes, they still condense at the usual temp, but the condensation has very little effect on the castings. But then I start thinking of that cheap galvanized flue, and eventually talk myself into setting the control to allow 130* minimum return water.

So then I ask, why spend the extra cash, if I'm going to treat this boiler like I treat my local steel boiler, and now have happily decided to just basically cruise with the "old" fashioned boilers!

Though if the client or situation calls for a condenser, then that is what will end up going in.

Leo G

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Unknown

That's where I disagree

I think that's what they want you to believe.

It isn't true.

An analogy would be a glass of ice water on a table on a humid day, sweating to the oldies with condensation.

Am I to believe that a metal glass of ice water wouldn't condense? Maybe a "Special metal" glass of ice water wouldn't sweat?

Condensing is condensing.

Did you ever see the dew point outdoors expressed as, say, 65° F, except in the presence of special metal which cancels the dew point altogether?

Can someone explain this to me so that it makes sense?

Noel

Unknown

Leo

All the "Hoopla" regarding the European GG-20 iron may just be that...."Hoopla". Tests have proven that the European Iron actually has less Silica content than the Grey iron we use here in the US. As far as what Noel has already stated, it doesn't make any difference where the boiler is made, the condensation that is a byproduct of the combustion process is still condensation and will occur on the flue side surfaces of the iron sections when the temperature of the block is below the dew point of 138°F.

When burning Natural Gas there can be as much as one gallon of water vapor for every 100,000 btuh of input. Introducing excess air reduces the dew point to about 127°F. One thing that certainly helps reduce the likelyhood of condensation is the way that the manufacture controls the flow through the boiler such as blend pumps, sparge tubes and the like. Yes, induced draft boilers can also condense unless they are equipped with a setup like our Revolution and Revolution II boilers. New boilers such as our MPO oil-fired boiler have internal blending to warm up the return water and to equalize the temperature throughout the boiler.

Noel's statement regarding boilers equipped with tankless heaters or simply boilers equipped to maintain a limit temperature can be subjected to pretty good stesses when systems with large masses such as Radiators and large pipes slug the hot boiler with cooler return water is a reality. The European boys don't neccessarily have to worry about that because they simply don't make boilers that have tankless heaters and unless modified in the field, don't maintain temperature (Sorry George L.). In case you didn't notice, they also don't make steam boilers either so they don't have to deal with other realities such as internal corrosion from systems with too much makeup water! If the US manufacturers stopped making tankless boilers and steam boilers and walked the line along with the Euro manufacturers, there would be somewhat of another revolution in this country.

Glenn Stanton

Manager of Training

Burnham Hydronics

U.S. Boiler Co., Inc.

Bill Nye_2

Thermos bottle

Insulation. A thermos bottle won't sweat. If the inside glass is not in contact with the outside air, it won't condense.

So on a Viessmann Vitola they have this stainless steel combustion area, there is an air gap between the stainless and the cast iron rings of the boiler. This is 'sposed to let the flame temp. get up and not condense? This is the way I understand it.

My smoke pipe is going into its sixth season and no sign of premature failure. I rarely operate above 140°, except when the temp. outside drops below 20°.

Noel, in an above post, I did not mean thermal shock, I meant 7 or 8 zones wired w/o reverse acting aquastat and boiler struggling for extended periods to bring water temperature up.

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hr

That air gap, Bill

is costing you a bit of efficiency. As I recall the Biferral runs low to mid 80's while basic cast iron, including US brands run upper 80's. I think their stack temperatures also run a bit higher.

Of course the Vitoden would be the best choice if you system can carry the load at below 140° temperatures, as you know.

Think of it along the lines of a good solid extruded transfer plate compared to suspended tube. The air gap is costing you heat transfer efficiency.

Not to take anything away from that Viessmann product, time has shown it to be a "rock" and I can't recall hearing from an unhappy Bi customer

Water heaters, which are built to withstand much of their life operating below dewpoint, are built with inefficient HXers to handle that. I've heard numbers from mid 60 to mid 70 for tank type, gas fired, water heater efficiencies.

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Ron Schroeder_3

with all due respect

The Vitola is AFUE rated between 84.5 and 83.9 on NG, 86.0 to 85.7 on LP and 87.3 and 87.1 on oil. In my experience, measuring with a Quintox fresh from service I have found these values to be easily attainable. What is more, I can do so with CO values under 10PPM.

hr

My Viessmann lit

shows the Vitola 200 gas at 84% (with optional flue damper) at 290° flue.

The Vitogas 100, 85.6% with a 235° flue.

My Weil GV has a 87.2% rating.

Hense the few % I mentioned

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Ron Schroeder_3

Thanks for supporting our product.

The Vitola 200 was never offered with a vent damper. There is no factory support for such an installation. What it does need is a barometric damper. Just FYI

hr

Fred ?

footnote 3 at the bottom of the Technical Data manual for the Vitola 200.

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Weezbo

Depends... Harvey

often on a system that has been down for any lengthy time...even with extremely cold fluids ..ok i will be honest ,i have fired a boiler on more than one ocassion that had /was frozen. there is a method to it, as i tend to have a slightly diffrent perspective from time to time is this what you are seeking or the minimal operating return temps?

Ron Schroeder_3

hot rod, i promise

that I will follow this up.

Ron Schroeder_3

AFUE testing evaluates steady state performance

that is performed with 140F supply and 120F return constant temperatures. If all of the manufactureres that you cited operate their equipment at these temperatures then and only then are the numbers meaningful.

One of the quirks in the AFUE testing methods is that omitting reference to damping penalizes every manufacturer by 5% without further consideration for combustion chamber or burner orientation. Natural damping impressed by occlusive burner fans and flappers is not considered in the AFUE formulation so for test purposes dampers are fitted. Hence the documentation.

The Vitola boiler is constructed with a horizontal two pass combustion chamber that is completely surrounded by a water jacket. The flue gas collector is centered in the rear water cooled wall. Carefully metered combustion air is moved into the boiler by means of a close tolerance squirrel cage wheel. None of the natural damping that results from this construction can be plugged into the AFUE testing model.

The Vitola is designed to operate comfortably without minimum return temperature( Technical data manual). Condensing internall to the boiler is prevented by the patented bivalent construction. In fact the boiler can approach condensing so nearly as to put conventional venting at risk. VDE, EC, and OVE testing verify the Vitola to perform in the 96% range.

What kind of heating system operates in this temperature range and which other AFUE verified boiler will survive it?

My position is that the AFUE values are only good for comparing one product with another if both can be operated under test conditions.

There is no test condition for a boiler like the Vitola.

Shalom

Bill Nye_2

Thanks Fred

Thanks Fred, that is what I wanted to say but didn't know how.

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Steve Ebels_3

That points out

That points out the inadequacy of AFUE testing. I guess it does provide a general yardstick for comparison amongst equals but when you try to measure a different breed of animal like the Vitola, things can get sideways a little. My natural gas Vitola will sit there at 150-160* boiler temp cranking 86%-87% all day every day. When the water temp drops into the 120-130* range it spins the Testo into the 88%+ range. I have never found ANY trace of condensation internally in the boiler. Even in shoulder seasons where the water temp routinely runs 100-120* there is no condensation.

The Vitola is not a valid reference for safe condensing temps when compared to normal iron or steel boilers. It's a horse (or maybe an elephant) of a different color and as far as I know it's the only one of it's kind.

That being said, the flue is definitely a different matter when using a borderline condensing appliance. When I first hooked up the Vitola, I vented it directly into the clay lined chimney running through the center of my house. The chimney is always "warm", being that its surrounded by 70* house. I pulled the flue pipe one day to see what was going on in the chimney after a firing cycle at around 115-125* water temp. I was immeadiately convinced that a stainless liner was needed. It was literally raining inside the chimney.

hr

But a combustion analyzer only

measures the combustion efficiency. It doesn't tell the whole story about the heat exchange efficiency, and that's my point.

An air space, even a small one, has an R value. Seems a better measurement of HX efficiency would be to meter the fuel consumed and the amount of energy actully put into the fluid. I think this is what ME is doing with his gas metered Munchkin. You are only looking at part of the efficiency with combustion analysis.

Hell the bubble foil manufactures have been telling us for years a 1/4" thick bubble of air is R-5 to R-10

To my think if there is an air space between the flame chamber and the water jacket, you will have a bit of insulative property.

Personally if I were to design around low temperatures, with occasional 140 and above I would approch from the other end and use a condensing boiler.

I suspect this is why the Vitoden is available Be interesting to know how the sales of the two compare and if the Vitoden has cut into the biferral sales?

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thp_7

There will always be a need for steam.

After reading a lot of these postings: a) condensation does not mean jack about efficiency. b) dilution air temp. & amount of excess air mixed with the flue gas can make any unit look good or bad. 82% to 85% with a little room to wiggle will keep most units from rusting out. If your not happy with that you by a condensing unit.