Sucky Mod Con Efficiency.... (ME)

Perry_5

Instrument accuracy is important no matter which method

Doug:

Instrument accuracy is important no matter which method is used. Truth be told - a lot of times the instruments are not as accurate as people think - and there can be a lot of error in the results. That is why I suggested some simple checks above.

Concerning the rise in flue gas with declining efficiency. That is not always true as it depends on combustion air/gas flow as well. With the mod/con boilers often the combustion side flow is varied. If you have decreasing efficiency and increasing combustion side flow you may not see an increase (or much of an increase) in flue temperature.

I don't know enough about the boiler in question to be able to say how much the combustion side flow could be changing - and how it may relate to flue temperature.

Edit to add: We are essentially at area "design" temperature last night (-7F); my modcon is running steady but even close to being maxed out. Of course, it was known to be oversized - but it was the smallest Vitodens I could get. I figure I can easily handle -30, which used to occur every now and then in this area.

Perry

Doug_7

Instrument accuracy is Important

Perry - Yes - we agree - Instrument accuracy is essential no matter which method is used. My point is simply that the two boiler efficiency methods should agree or some basic measurement is wrong.

Yes - The combustion side flow is varied on Mod/con boilers. But can it vary enough to account for a 61% efficiency at a normal flue temperature ? Check the flue oxygen and temperature to see.

We agree - Check the measurements. I will be very interested in the results.

Edit to Add: A check of the math shows that combustion side flow cannot increase enough to account for a 61% efficiency at a normal flue temperature. Not even close.

Doug

Dave Stroman

I have a few comments about this style of heat exchanger.

Like Mark said, there are several brands of boilers that use this same one. I am not knocking them necessarily. Have installed many of them, mostly the Trinity brand. Here are before and after photos of one I just cleaned. It has not been touched in the 6 years since I installed it. This early model had some problems which NTI has addressed and corrected in their newer models. The biggest pain was the HX was positioned where you had to remove the side panel of the case to access the burner door. I finally get the thing apart, breaking the ignitor in the process, and proceeded to spray it down. A few "turds" were laying in the bottom which were easy to clean out. There was significant build up on the coils which took some time to get off. With the use of a plastic card, I was able to get most of the crud out from between the coils, but in reality, it is impossible to get it 100% clean and the stainless coils seemed a little etched. The refractory on the burner door started to crumble and a few of the other gaskets needed to be replaced. With the use of some high temp sealant, we got the thing back together and fired up.

I carry every replacable part for these boilers on my truck. After cleaning this boiler, I learned that I will not attempt to clean this or any other brands without having at least all the gaskets and an ignitor on hand. I also will not start a job like this late in the afternoon on a Friday, which is what I did this time. Had I not had all the parts I really don't think these people would have had heat this weekend.

There has been a lot of talk about how long these type of boilers will last. I suppose that if you kept after the cleaning like you should, once every year or two, they might get 10 or 15 years. But disassembling them so often is going to be hard on them and replacement of things like burners doors will probably be necessary. I had one screw that I had to drill out because it had become seized. I had a new one in my kit so no big deal. But had I not, where would I have found a stainless steel one? It was inside the combustion chamber and I would have been tempted to use what ever I had just to get the thing running.

So I have a prediction. These boilers will probably be replaced due to lack of attention, or too much attention. I think the former will be more likely. Anyway, cleaning them should not be done by the faint of heart and not in the middle of heating season.

I am now going to put my attention on boilers that are not so hard to clean or do not require any cleaning. The Prestige and the rAy are two that come to mind. And you know, there is a lot to be said for a basic, plain jane cast iron boiler. And as these mod/con boilers come to the end of their life, many happy cast iron owners will be living comfortably.

Dave Stroman

Matt_67

Gas input

Mark - has the boiler input changed significantly? It would seem to me like a fouled heat exchanger would lead to less air flow at the same blower RPM and thus a lower firing rate.

Mark Eatherton

I'm sure it has Matt...

and I will attempt to verify it when I get ready to service it.

ME

Mark Eatherton

Mine runs every day...

Doing DHW at a minimum.

ME

D107

Anyone ever use mod-cons in mod-only mode? Less wear and tear?

Dave posted some valuable info; certainly on my mind as I contemplate a new boiler. I was wondering if anyone had an idea of how much of the corrosion and wear and tear is due to the condensing vs the modulating. If you had CI rads and kept the outdoor reset curve to keep water supply temps no lower than 130 (with a control to ensure that the boiler compensated for low return temps--say 104 deg like some of the Buderus models do)--might the scenario change?

Wall posts seem to indicate that most seem to feel the greater savings is achieved by the modulation side. I wonder by how much. If I knew a mod-con would last me a lot longer in mod-only mode and would still save me 15-20% over a CI with a good control, I could go for that. (not so good for radiant heat applications obviously.)

Thanks,

David

Perry_5

There is a huge energy gain with condensation

David:

There is a huge energy savings by having an condensing boiler. This comes from two aspects of the design.

1) First, you recover the latent heat of condensation.

2) The boiler efficiency curve is relatively flat over a large firing range - and can even be negative (i.e. the boiler is more efficient at partial loads than at near full load).

A conventional "non-condensing" boiler has a rather rounded efficiency curve compared to the firing rate. It is decidedly less efficient at partial loads (or modulating firing rates).

Concerning Dave's comments:

Not all mod/con boilers are hard to service or hard to clean. Admittedly, the boiler HX design specifically in question in this thread may not be well designed from a service (or even long life perspective) - but it is also a lot cheaper than some of the ones that are better designed (look at the Vitodens). A Vitodens 200 can be completely taken apart and put back together in about 15 minutes flat - without any problems with potential damage to things. Someday some of the O-rings may need changing.

In addition - I am not really convinced that the Triangle Tube Prestige boiler HX is cleanable either (it claims to be self cleaning - but I do not see how that really works with the little combustion "turds" you get that would fall into their crunched tubes (and I have decades of experience cleaning HX tubes). The Vitodens is just as self cleaning for the fine stuff and the "turds" will not fall into the tubes where you cannot get to them.

The other problem is that many modern "cast iron" boilers are no longer designed and built the way they used to be - and may also have fairly limited lives. The days of even a Cast Iron boiler lasting 40+ years is dead (or so I have been told by many experts on this forum).

The real problem is that a number of "cheap" boilers have come on the market - and win market share because they are "low cost" upfront. I don't care if its a mod/con or a "cast iron" boiler.... The cheap ones probably will not last - and you face the real possibility of needing to replace them in 10 - 15 years (if not sooner if not properly installed and maintained).

As an engineer with some experience in HXs and how they fail I personally have the following recomendations:

1) Stay away from Aluminum HXs. They are literally designed to corrode away and can have other problems as well.

2) Stay away from 304 SS as it is very sensitive to stress corrosion cracking from mild levels of chlorine and other things.

3) 316 Ti is a reasonable material - and more SSC resistant - but I would put a carbon filter to kill any chlorine on the boiler fill (no reason not to protect it). It could also have problems in areas where the inlet air has a lot of chlorine (near swimming pools or a laundry). Viessmann led the way with this material - and seems to not have had any failures - but I'm a cautious type with all 300 series SS.

4) 439 SS is still subject to chloride pitting although it is highly resistant to chloride stress corrosion cracking and has some better mechanical properties. But there are tradeoffs as well.

5) If it were up to me I'd use a material called AL 29-4C or SeaCure. I've attached the brochure for AL 29-4C which has a lot of information comparing materials in it. It was essentially designed for this kind of environment - but it is not cheap (and SeaCure is more $ yet).

So, I would not rule out a mod/con based on some theory that it will automatically have a short life. I believe all boilers that are at the low price point for their style will have a short life due to corners cut and material not used in order to produce a cheap boiler.

Of course, the real problem is that a homeowner does not really know which boilers are at the low, medium, and high price points due to how heating contractors write their bids.

And the real trick is to find a heating contractor who is also a heating professional. There are a lot of hacks out there making a living as heating contractors who do not know and do not understand how to do things right - or how things really work. They don't care to learn either.

Probably some of the best in the industry on on this site; but not all are as some are still learning too. The forums can be valuable.

Hope this helped.

Perry

(Homeowner & engineer: Oh I installed the Vitodens 200 a couple years ago and am convinced it is designed for a long life).

Paul_58

Sanity Check

Mark, Isn't your system designed to run full out at design conditions? If so, and your efficiency has decreased from 95% to 60%, well, bundle up.

S Ebels_2

C'mon Mark!!!........

Man up and tear that sucker down. The suspense is killing me. Besides, it would be a golden opportunity to press management for a new boiler if the current model in use looks a little suspect after the overhaul.

[Deleted User]

I ain't a skeered of you mister...:-)

It's tha beautiful WOMAN I sleep with that has me worried...

She could KICK MY A$$ :-) Especialy if its COLD in the house. But when momma ain't happy, ain't NO ONE happy.Hence, more reason to "Man up" as you say. I even got so bold as to leave the toilet seat UP in the basement bathroom. (Did I mention she NEVER uses that bathroom???)

Any way, I Got 'er done, and the numbers did increase significantly. This thread is worn out, so I will start another thread, otherwise people might get confused.

Later podnah, I gots some numbers to crunch...

ME (Manly man among men :-))

Steve Ebels_3

Toilet seat issues

You can solve that problem by using the sink instead. She'll never complain about the seat thing again.

[Deleted User]

Ya mean...

The Red Neck urinal/bidet/sink/tooth washer?

Ugh... This conversation is getting out of control. Now, what were we talking about???

Oh yeah. Effishuncee :-)

ME

Jim Davis_3

Merry Christmas All!!.

Mark, is this a closed system?? Sounds like a good test for a Clearwave!! Must be taking on a lot of water.

Every time I see that efficiency-temperature chart I want to #@?#@!!!

[Deleted User]

Shirley, you jest....

My names not Shirley, and I'm not jesting.

No Jim, it's a closed looper. It was just the scud on the heat exchanger that was causing the issues.

Merry Christmas to you too.

ME