Ultra Burner Condensation

Constantin

... a good vent terminal to maintain a ΔP as close to zero as possible to ensure that the combustion remains stable even on low-fire on a windy day. Based on my personal experience developing a vent/intake terminal for an appliance w/o the benefit of a blower... it's a challenge!

Unknown

condensation on the BURNER of an Ultra

Hey all,

another Ultra Puzzler. This time, our contractor client has observed condensation on the actual burner of a weil-mclain Ultra, which congeals on the bottom of the burner, prevents flame from getting through... right where the flame sensor is. This results in a lockout.

This is a *very* low load home.. minimal firing.. very low temperature.

Has anyone else run into this? We're toying with the ideas of making it run longer, increasing the length of the post-purge to clear out humidity, even (if we can get WM to sign off) modifying the burn chamber to move the flame sensor to the side of the unit.

Apparently there is a limit to "low temperature" operation heh heh...

mikea23

we have the same problem

We Installed A ultra 310 full radiant home and installed another ultra 310 for domestic water,airhandler backup heat (homeowner request), and swimming pool heat all high temp. The radiant boiler runs at 135 the system works great most of the time but when the load is light such as one or two bathrooms calling for heat the boiler locks out. We repiped the radiant boiler into a buffer tank problem solved but it was an expensive fix.
Mike A

Unknown

Hmm, yeah I'm thinking we may have found the "buffer tank required" level of heating here, maybe. Maybe we can get funky though! Has anyone else run into this?

Perry_2

The problem is likely related to the 5:1 turndown ratio...

There are many issues associated with ballancing fuel/air flow in the same duct and pipe sizes and in burner design when designing a single burner for turndown operation.

3:1 is fairly easy.

4:1 takes work and paying real close attention to several things.

5:1? Prior to researching home heating boilers I had never heard of anyone claiming 5:1 turndown on a single burner system without the ability to close of airflow ductwork sections, modify fan speed, and other variable items which are more complex systems than what exist on home heating boilers.

I think the problem you are seeing is an example of the difficulties of 5:1 turndown.

Is there any way to restrict this to a 4:1 turndown (or even 3:1 turndown). Getting it above a minimum firing that it cannot in reality handle will solve the problem - the only question is how high do you have to go.

Perry

Constantin

Is it the turndown ratio...

...or the minimum firing rate that is the problem?

You can buy a NTI Trinity today that has a 400-25kBTU/hr firing range using a Dungs valve, IIRC. A hefty 16:1 turndown sounds tasty, yet the minimum firing rate of the boiler is still pretty high for most residential purposes.

The lowest firing rates I've seen for boilers in the US are in the 12-14kBTU range. I believe the HTP T50 is the lowest one out there.

Given that mod-cons sold in the US are low-mass boilers with minimal standby losses, quick spool-up, and higher efficiency at lower firing rates than at higher ones for a given return water temp, the issue of how low a mod-con boiler can go centers perhaps more around short-cycling and control strategy than anything else.

Systems that have sufficient mass in the BOP are less likely to experience short-cycling than those that feature a low-mass boiler and BOP. However, it all comes down to the specific system, the site, and the resultant load. Buffer tanks do, and will likely continue to, have their place in the heating world, even with mod-cons that can modulate a great deal.

Unknown

I'm hoping we can avoid the buffer tank, but we might not be able to!

I'll say, it's a nice feature that Ultras actually pulse on low fire instead of starting at high fire and modulating down like some other units. But it is biting us in the butt on this one!

I'm wondering, primarily, assuming that condensation cannot form WHILE its firing, if we ran that post purge fan longer after firing whether we could in effect "dehumidify" the burn chamber with colder, drier outdoor air. Then again, maybe we'd just be promoting condensation elsewhere in the boiler where it's not built to withstand it if we cool it too much.

Otherwise, we could raise the firing differential somehow to make it run a little longer, and use a hotter supply temp.. question is how much, if too much, not a good compromise.

Or we can just add mass in the form of a buffer. groan....

Perry_2

Constantin: My opinion is yes...

The fact that companies are offering these things - and even your 16:1 does not mean that they are long term reliable.

In the power industry we threw multimillions of dollars into burner redesign, and burner replacements to get higher turndowns... The higher turndown burners did not last - and were too maintenance expensive to retain (it was possible to construct them - and they did work well initially).

This dispite the fact that we often had the ability to modulate intake air with multiducts that could be modulated from shutoff to full on giving a flexibility that home boilers do not have. We also tended to use very tough materials that are not common in home boilers either (translation: exotic and expensive materials).

Power plant - and other large industrial boilers - get high turndown ratios by using multiple burners.

So my opinion is: I'll believe it when I see them last a long time.

Perry

Constantin

Hmmm...

I've seen condensation form inside the flue passages of atmospheric water heaters, drip down, and sizzle on the burner. Atmospheric water heaters aren't exactly known for low stack temps... But if the wall is cold enough, water will form and air flows in those beasties being what they are, it's perhaps not surprising that gravity can pull the water faster down than the flue gases can evaporate it (at least initially).

Thus the issue may be the temperature of the HX surface where it meets the moist flue gases. If it's too low, the gases will condense for quite some time. Pre- and post-purge will probably do little to help. Lengthening the cycle time via a buffer tank could minimize the dripping, so I think you're on the right track.

This phenomenon may also explain why some boiler control strategies start on high-fire and then cut back their output, i.e. getting the HX to temp quickly and minimizing condensation on the burner itself.

The real issue is why the condensation is allowed by design to impinge on the burner. It reminds me a bit of the issues folk were having with using the Polaris on low-temp RFH systems... low tank temps leading to ignitor problems via condensation.

Constantin

Wise words...

... there is a bit of a scale issue going on as well though.

The cost/benefit of running a VS fan motor on a low-input boiler is a very different challenge from trying to do the same thing at 100x or 1,000x greater scale. Plus, all these residential appliances are trying to do is produce heat effectively, not drive steam-powered turbines on top of that.

And it's not as if the energy-production industry is alone in this regard. Look at how many processes and plants work fine at the lab scale... scaling them up to pilot or even industrial capacity uncovers all sorts of issues that weren't apparent before.

I agree with you that over-complicating things is in no-ones interest. I also found it interesting that NTI is the only manufacturer to use this valve, even though many other critical aspects of their design (i.e. the HX) can be found in other heaters as well. Perhaps the other manufacturers share your skepticism re: the reliability of such a boiler.

Either way, I think the bigger challenge is getting everyone to right-size the boiler to the load in the first place... i.e. how many installers perform a honest heat-loss calculation before each job? Plus, there is the dearth of boilers at very low outputs for ultra-efficient houses...

Given my experience earlier this month, it's conceivable that some installers rely on modulation to cover up the fact that a boiler was wrong-sized via the one-for-one replacement method, use-whatever-is-gathering-dust-in-the-warehouse-method, feet-of-baseboardx600-method, etc.. :-P

Unknown

which is a very big problem when your load is smaller than any available boilers can handle, dammit! heh.. gripe gripe..

Perry_2

Agree with the \"right sizing\" issue.

and with things that work in the lab not working out in the field.

I will also again state that we need some high quality smaller mod/cons (I believe for example that a Vitodens 4-16, and a 3-12, would do wonders and be good sellers). Other companies would do well with similar smaller mod/cons.

Of course, we need them to be reliable as well (or at least designed with the real intent to be reliable).

Perry

Xc8p2dC_2

could it be?

NRT, Did you try bumping up the minimun ign fan speed, so when it fires it's not dead low fire, and the added gas input may over come flame out [kinda duplicating why some start on H-F]

And in that same regard as Perry asked, I think if you bumped up fan speed higher you would be changing the turndown rate as well? I know, Floyd would be the man to get the correct/proper answer out of

mikea23

making all the adjustments and hoping it will solve the problem is great and the thery is even beter but at what cost your time is costly and your reputation is worth more then your time unless this is your home or your mother inlaws I would just put the tank in eat the cost.
Mike

marc friedman

exactly, and succinctly put ...

Floyd_29

No expert... but...

there are a few things to play with....
The Min fan speed could be increased but that would just make it short cyclye more....
I would back down the max. fan speed to make sure that it doesn't try to mod. up and then overshoot and crash out because the temp. rises to fast to let it back down fast enough. Backing down the high fire will help to control that. You can do that on the heating side without affecting the DHW side.
I also would raise the amount that the temp. can rise a bove setpoint before shutdown...it comes set at 4 or 6 degrees which is way to tight to suit me... raise it to 10-15 degrees...maybe also give it more differential so that it cools more also before it starts back up... if it is a very low load the large differentials should not be a big deal...
Also there is a parameter 42 I believe... it is the very last one that you can get to that you can shut off.... for some reason it will try to make the boiler run some crazy cycles at low temps. and low loads.....I have been able to make these boilers run much better by doing these things.... still playin' though...don't have ready access to any boilers with really low loads right now, only get to play once in a while in homes when I get to make adjustments(initial setups) or yearly service. Usually I can get them set and forgetaboutit for a year.... :-)
Should probably look up more exact parameter numbeers for you, but I just drove over 5 hours tonight in rain, fog, mist, and basic soup, so I'm whoooped.... If you want more info. let me know....

Floyd

Tony_23

Post purge, et al.

IIRC, the post purge is a circ function, not comb fan function.

I've heard of one 80 with this problem. From what I could tell conversing with the installer, I thought it sounded like cross contamination on low fire. Not much velocity on low fire to "clear the area" and vapor was entering the intake. Watch the location of venting for eddy type situations. It may be worth a try to either extend the vent or disconnect the intake (easily done by removing the cover) to see what happens. It would, in my opinion, be better to extend the vent than to increase low fire and create short cycling in the process.

rich pickering

Tony has a point

I'm finding that anything fired at about 40,000 or less, the velocity of the combustion gas is not very high and recirc maybe/is a problem.

Direct vent furnace or boiler.

Perry_2

These are one of the problems with high turndown units

One of the problems with high turndown units is that in order to support the maximum boiler capability you must design the boiler air and vent circuit for the maximum firing rate.

You inherently create a problem maintaining velocities at low fire becasue of this.

So unless you can damper in and out extra ducting capacity to change the velocitiy profiles for high and low fire rates you have a "difficult situation" to say the least.

It really is not a scale problem. It is just a problem of matching appropriate air intake flow, burner flow, and vent duct flow in a unit with a wide range of operation.

Perry

Uni R_2

Rich

Do you think that 2" might be a better venting size for these smaller modcons with very low loads?

sebast

Well put Perry, incomplete combustion is taking place at low fire. Check your CO and CO2.

Rich

S Ebels

Perry

Looks like you've picked up on the reason Viessmann limits the Vitodens to about 3:1. There's a method to their "madness" and they generally do things for the right reasons.

Perry_2

The Vitodens range varies with size - and other issues.

Steve: In general I think you are correct - I believe that Viessmann understands the problems - or at least better than some of the other companies.

Turndown ratios for the Vitodens 200 (Based on input):
6-24 @ 3.64:1;
8-32 @ 3.35:1;
11-44 @ 3.12:1;
15-60 @ 2.88:1

I note that the Vitodens 100 in Europe - with a lower cost burner has a lower turndown ratio for the smaller sizes than the Vitodens 200 (about 3:1), which is what you would expect of a lower cost burner design. I also note that there was a recent thread by people who are upset with this as the Vitodens 100 - a cheaper boiler than the Vitodens 200 is going to be released to the US market in the foreseeable future. Hey, you get what you pay for... did they really think they were going to get the same boiler for less money?

From a practical standpoint: If you are willing to spend the money I think about 4:1 is about the maximum realistically attainable for a long term reliable burner - in any size (the same issues apply to burners that need a 1/4" diameter gas supply as ones that use a 2 ft diameter gas supply to each burner). I do find it interesting that as the boiler size went up that Viessmann did not want to spend more money on the burner design to get better turndowns. I suspect that as you reach the larger loads that either the heat loads are well understood and fairly steady or that you can gang two smaller boilers to get a much larger turndown ratio (6:1 or better) where high turndown ratios are needed.

I note that flow velocities are only one of the challanges involved in turndown burners and boilers. Another problem deals with contaminated fuel. All real world residential and industrial fuel supplies are contaminated from the laboratory ideal. Those contaminants can wreck longterm havoc in turndown situations (in either the burner and/or the boiler).

Maintaining optimal air and fuel ratios are only the start of the issues when designing a long term reliable and effective turndown burner. Anyone can maintain air and fuel ratios (including the company selling a 16:1 turndown burner). Solving the other problems....

Perry

Guy_6

Venting

One aspect of venting the Mod/Con boilers that I always stress is to try and vent with the exhaust ABOVE the intake-preferably 12 inches above. The warm plume will always want to rise and disperse.
The condensing ON the burner could in fact be condensing from WITHIN the burner- re-circ, poor roof drainage allowing heavy moisture in the intake air,other vents near the intake, or even high moisture in the gas supply (rare, but maybe).

Constantin

There is also the cost benefit...

... buffer tanks on radiant systems can be very inexpensive indeed. I wonder if Radiant Engineering will continue its line of excellent products to perhaps include a well-insulated 20-30 gallon tank with LLH tappings, a microbubbler vent, a drain in the future.

Such a buffer tank is a inexpensive and reliable way to overcome the quandaries posed by microzoning... probably much better than spending endless resources to turn down the input rating into ranges where flame stability, emissions performance, etc. can no longer be guaranteed.

Thanks for your insights re: turndown ratios and plant reliability. I also note with interest how the 300 Vitodens series is supposed to deal with pretty much any gaseous fuel, using a lambda sensor to adjust the gas train independently from the air flow to achieve proper combustion. No setup for methane, propane, etc. necessary - the unit does it out of the box. That feature could be interesting for folk living in areas with a very variable gas supply.

Constantin

Depends on a number of factors, no?

My experience was with a vertical terminal also.

Our ideal distance between intake and exhaust was a lot less than the 12" you describe to achieve ideal combustion properties, regardless of wind direction, velocity, and surrounding structure. On the other hand, this was for a product not intended to be used above the mason-dixon line.

The challenge of how to handle condensate and how to prevent it from fouling the terminal via ice formation is a formidable one, I suspect.

Tony_23

I tried the other day, to post this, but couldn't

Darn security code thing got me cause I was distracted and took too long

Anyway, my low fire and high fire CO2 comes in .4-.5 % different. Usually 8.2 and 8.6 %, low to high. CO stays under 25.

I try to vent on the normally windswept side of the building, or vertical, and use a concentric assembly. If I have to vent on the leeward side or in a protected area, I try to keep the vent and intake 2-3 ft apart vertically.

I've got a couple with long runs in cold environments, and a couple with 7-8 ft of pipe outside the building to avoid walkways and with the use of ODR and proper sizing, nothing gets frozen. The colder it is outside, the more it runs, the warmer the vent stays. Works for me

Unknown

father in law's, actually. and they know exactly what's going on.. frankly, this is the smallest load we have ever seen an Ultra installed on, and I hadn't recommended it initially since the load is so long, but they wanted to be "green for green's sake" on this one. and it's heating fine except for an occasional lockout, so it's not a dire circumstance. We can play on this one.

Unknown

Guys, thanks so much for all the feedback. Lots to think about!