oversizing modulating boiler

Unknown

Those are some really interesting observations Alex. I guess then the last piece of the puzzle is to what degree is that efficiency gain from oversizing offset by a higher minimum modulation? Tough one to answer, I think. You raise some really good points to think about!

john_58

Is there any harm in slightly oversizing a modulating boiler in a replacement hotwater heat system (CI rads). If the new heater is modulating 20-100%, what is the harm, other than your pocket book?

Supply House Rick

Why...

Are you oversizing a boiler?
Rick

Scooter Libby

A right-sized boiler hardly ever runs at 100% (design days are rare). It spends almost all its time below 50%, and quite a lot of time in spring or fall below 20%. So if you oversize it, you increase the number of days in which the boiler has to run in "bang bang" mode.

Of course, in those spring/fall days the gas consumption is not high and so you don't lose a lot of money from the drop in efficiency...so it's not a big deal. It just doesn't seem to make sense to buy a modulating boiler and throw away a large chunk of the modulation range.

Mike T., Swampeast MO

Problem you might have is finding a modulating boiler small enough for smaller or very well insulated homes with low infiltration. In that case, politely pester manufacturers to produce smaller models!

Otherwise no need whatsoever to oversize. As has been said by others, "Why buy a modulating boiler just to waste much of its' range?"

My 1903, 3400 sf home with cast iron rads, 47 large windows, 14 doors to the outside, an utterly wicked north exposure and 8° design temp is easily heated by a Vitodens 6-24. The I=B=R rating of that boiler is 70 mbh which is 85% of my Manual J heat loss.

Despite what some might consider "undersizing" it had no problem heating the place in 0° weather and for at least 25% of the heating season loss is still below the minimum modulation!

Brian

Oversizing

I have the same question but for a different reason.I been using the GB142-24, which I am very happy with, but I find that domestic hot water production suffers.Especially if you have a large family or large tub.So do I jump up to a GB142-30 for another $500.00 or go to 75 gal. indirect from a 50 gal. for another $250.00 but will have higher stand-by loss.

Thanks
Dobber

Wayco Wayne_2

I over sized my

first Munchkin install 3 years ago and had nothing but trouble. Replaced it with a properly sized one a year ago and have had nary a call. There are ways to expand your doestic hot water use. Put on an antiscald mixing valve and turn the tempperature up high. Add a storage tank and a circulator. Shower with someone you know well..... One of my favorites. I'm with Mike. Size closely.

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Constantin

I'd go with the larger tank,

... and, if possible, retrofit a GFX Heat exchanger. For a nominal amount of money, this HX can stretch the hot water supply for showers or other continuously running loads. Besides, you can always wrap your indirect with more insulation to minimize standby losses. Don't forget to insulate below either!

Mike T., Swampeast MO

I'd suggest increasing the size of the indirect. The surface area of tanks does not increase anywhere near as fast as the volume, so stand-by loss doesn't increase as rapidly as if you increase the temperature of a smaller tank. I know that they're insanely expensive, but the Viessmann indirects allow extremely rapid recovery. With the DHW on priority you can nearly assume that the indirect will instantly absorb the full output of the boiler. Always a balance--use those tanks and they can be smaller with lower standby loss but the cost is significantly higher.

Trevor Baptista

I understand oversizing a boiler. But what happens if you oversize your entire heating system. Would this cause lower efficeincy? So another words, it takes a considerably short amount of time to satisfy a heat call from the t-stat. However, the T-stat is programed such that it kicks on at 68* and off at 72* at the outdoor design temperature it takes around 45-60 min before the T-stats next call for heat after is shuts off. And it takes about 10-15 minutes to satisfy the T-stat call. It is pretty wild!

Unknown

always size the boiler for heating.

Then size indirect so the correct boiler can meet peak demand.

Unknown

solution; scale your reset curve downward? In a perfect world, under design conditions the boiler would be running constantly, providing the exact water temperature you actually need. Oversizing your system drops the water temperature you need. Good stuff, unless you're using a conventional boiler that can only go so low for water temperatures.

Alex Giacomuzzi

I Don't Think So......

I do not think some oversizing to a modulating boiler is a bad thing. If you asked me the same question regarding CI --- I would not share the same opinion. In fact -- I see upsizing to a modulating unit as a positive rather than a negative. I will try to explain my thoughts.

Given that you have standing radiators which were designed to run with 170 degree water and I am assuming this may have been a gravity system at one point in time or maybe still is. I believe every application can have its own best unique solution but there are several other workable solutions for example. I do not know if you have or do not have excess radiation, but I can guarantee you that you will not be heating with 120 F. supply water in January. Your temps will be well above that value.

Regarding oversizing a condensing / modulating boiler, when ever I look at a "boiler efficiency chart" that some manufacturers post, there is a distinct relationship between boiler load and efficiency and between boiler supply and return temps and efficiency. Everyone just talks about return water temperatures. It is more than just that. The modulation capability of these boilers is what gives them the extra energy savings, not just the return water temp. The higher the boiler load, the lower the efficiency. The same for the supply and return temps..... the higher they are the lower the efficiency.
This is $$$ we are talking here.

If you spec a "just right sized" boiler --- versus specifying a slightly oversized boiler --- both will work. Will both operate as efficiently --- probably not, but unless you had an exact example to compare against, or two identical homes with one of each installed it would be hard to truly compare. Now it is true.... a slightly oversized modulating boiler will have the same turn down ratio as a just right sized boiler, but it will not be able to go as low in its fire range....that is btuh output. We are probably talking of between 12K and 20K BTUH. in the difference. Not a lot of range.

In time, suppliers will probably get better at supplying a larger turn down range and this discussion disappears. For now --- it is valid. There is another piece that I have some discomfort with. That is...Running these very high efficiency heat exchangers full out for elongated periods of time. Since I can get better efficiencies under less load (with a slightly oversized boiler), and efficiency (gas & $$ saved) is one of the main reasons for buying a condensing / modulating boiler in the first place. I would upsize slightly and reap the best rewards of the design.... of these much more compact units.

Regards Alex

Mike T., Swampeast MO

whenever I look at a "boiler efficiency chart" that some manufacturers post, there is a distinct relationship between boiler load and efficiency and between boiler supply and return temps and efficiency

Can you give a link to some of those charts?

Are you certain that the decreased efficiency you're seeing has anything to do with modulation level? Perhaps everything to do with supply/return temps?

Mark Hunt

Dobber


The GB allows you to de-rate the heat side for those instances when domestic water production requires a higher output than the heating side.

De-rating the heat side will only allow the boiler to fire to a maximum percentage of it's rating on a heat call but allow it to go to full linear bore on a call for domestic hot water.

So the GB can be adjusted to the demand.

Have fun!

Mark H

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Alex Giacomuzzi

Vitodens 200 for Example....

Hi Mike:
I will link you to the Vitodens 200 site.
Open up the Flyer, the Brochure or the Technical Data.
I believe they all have the chart that I am drawing my conclusions from. Tell me what you think??
Regards Alex

Try: http://www.viessmann-us.com/web/canada/ca_publish.nsf/Content/Vitodens200_ca_english

Mike T., Swampeast MO

I'm honestly not sure, but I highly suspect that graph shows efficiency on three different reset curves with only the maximum temperatures stated.

Constantin

It depends, doesn't it?

I think this question can only be answered in degrees. For me, it hinges in good part on the physics of the problem. For example, short cycling with a low-mass boiler ought not to impact efficiency as much as with a high mass one. In fact, as Alex points out, the efficiency may increase as the firing-rate to HX-surface area ratio decreases, even if the boiler is operating on-off.

It may also depend on the turn-down ratio of the gas valve. A 16:1 turndown like the valve offered by the Ti400 allows a pretty impressive range. If someone were to perfect a 16:1 burner/HX which starts at 8kBTU/hr, then you'd have a burner that can cover the heating needs for most residences and which can also produce a lot of hot water.

My guess is that achieving flame stability at very low output is very problematic. I still vividly remember the issues I had to battle in order to get a zero ΔP between the combustion air inlet and the exhaust of the outdoor gas water heater I co-developed. I imagine that has a large part to do with the problem, i.e. the inability of the manufacturer to control the combustion inlet and flue outlet geometry, etc.

john_58

Thanks everyone.

Wow, Thank you everyone for info. I am having the gas supplier/wholeseller do all the math, installations and PAYMENT plan. My current system is a 260000btu mid eff boiler for old converted gravity hw (not inc.domestic) system. 7000sq ft +basement house. My current system does not get the water above 140F on the coldest days. The contractor wants to use a weil/maclain heater - modulating and condensing.

Alex Giacomuzzi

Reset Curves...

Mike
To the best of my knowledge, reset curves plot an outdoor temperature against a boiler supply temp. As one changes, so does the other --- that is the resetting function. The curves presented on the Vitodens Boiler Efficiency Chart display 3 constant temperatures separately. A Supply and a return each listed. The middle curve has a traditional 20 degree delta T. The other two are near that same value. The curves appear to be for 3 typical heating applications: lowest temp for a radiant high mass system(basements), middle temp for a radiant lower mass or some homes with a an abundance of standing CI radiators, etc., and the highest temps for..... CI baseboard, or PEX tubing hanging between floor joists.. etc. This is a sales brochure and I think this is application information.

I do not see reset referenced at all--- anywhere. The chart does refer to Combustion Efficiency and Partial Load in %. It would really help if we all had some definition of terms used on some of these boilers--- definitions that is. I am going to try another approach and this data is also contained in the Vitodens Installation Manual.

I am going to make some assumptions based on this same chart and see if they can be verified via other data contained within the manual. I assume Load or Partial Load is achieved by burner modulation. The chart goes from 100% down to 30% which gets quite close to the published turndown data (25MBH to 91MBH for example). I think the turndown is the Partial Load. This then would say the following: (Data in the Manual)

A supply temp of 167 F (return of 140 F) has a combustion efficiency of 97% when loaded to 30% (of burner output) and the boiler goes down to an efficiency of 85.7% when loaded to 100% (of burner output).

Another example of the chart says for a supply temp of 140 F (return of 120 F) the burner has a combustion efficiency of 97.5% when loaded to 30% (of burner output) and goes down to an efficiency of 89.4% when loaded to 100% (of burner output).

There is a third example, but these two explain it quite well. In interpreting these curves, it appears that efficiency decreases with both load on the burner (modulation) and the temperature of the supply & return water (condensate recovery).

Looking a bit deeper into some of the specs on this boiler there is some great data. Lets look at flue gas temperature for example..


With a return water temp of 86 F (radiant concrete floor application) ...and the burner running at "rated full load" (same as the chart information) the flue gas temp. is 113 F. For this same return water temp and the burner running at a "Partial Load" (something less than full load)-- the flue gas temp is 95 F. If the efficiency was constant across the full modulation range, the flue temp would also be the same. Would you agree with that??
But it is not the same, it gets higher with added load and those are BTU's are moving up the stack. This says there is an efficiency loss between a partially loaded burner and a fully loaded burner. The same as the application chart.

It appears this whole relationship is a direct function of the heat exchanger surface area versus the BTUH rating on the boiler. Another very interesting comparison is between 3 models of Vitodens.... 8-32, 11-44, & 15-60.
The surface area of these 3 are 9.91SF, 20.91SF, & 20.91SF. Compare the flue gas temps of the 11-44 to the other two. See the manual.... Surface area plays a big part in the efficiency numbers. Burner Modulation --- while reducing the BTU input to the HX it decreases the btuh per SF of HX area which again improves combustion efficiency.

I am just trying to understand this same information that is available to everyone. Sometimes it is not easy to separate marketing from actual operation......
I welcome your thoughts.. Ciao.

Regards Alex

Brian (Tankless)

I agree, Bob

This turned into a very interesting thread, thanks for your observations, Alex, you confirmed a few theories I've always had about efficiency in general, and how to improve it.

I'm just a mere a maint' electrician but have always been interested in heat exchange and how to achieve it as efficiently as possible. And basically, that is what this business is all about: getting as much heat as possible into the water and as little as possible up the flue.

One other way, as well as having lots of temp sensors in and around boilers, including the exhaust, would be to place a CO & oxygen sensor in the exhaust directly after the heat exchanger. Feed this info to the processor, and modulate the flame & blower speed accordingly.

Thanks again, great thread.

Brian.

P.S. I use a lot of what I learn here to help make my job easier. The Engine management, and data-logging systems on our huge EMD diesel gensets are incredibly complex, but make engine troubleshooting much easier.

Thanks.

Mike T., Swampeast MO

The Reasons I Believe Reset is Included in that Chart

1) Look at the title, "Vitodens 200 Boiler Efficiency". This for for the entire class, not a model.

2) Unless you resort to subversion and operate the boiler contrary to its intended operation the Vitodens 200 is ALWAYS under the control of outdoor reset.

3) Note how the efficiency "origin" (at 30% load) is similar for all three lines. I don't believe it's possible to achieve 97.3% combustion efficiency at 167° supply and 140° period.

4) Note how the lowest efficiency stated, about 85.7% @ 167° supply and 140° return, corresponds very closely with the theoretical maximum efficiency of a non-condensing boiler.

5) I wired my Vitodens 200 6-24 to datalogging sensors for flue temp, supply temp (measured about 3' from the boiler) and return temp. Search for posts named "Vitodens Operation" here at The Wall. You'll find numerous graphs of actual operation. Pay careful attention to the flue temperatures and also pay careful attention to the dashed red line which is very near the actual "boiler temp" reported by the boiler.

If efficiency declines with increasing output I think it would be fair to say that flue gas temp should increase faster than supply temp. This is not what I see happening. As load increases (outside temp falls) and boiler output increases, my graphs show "target [boiler] temperature" rising faster than flue temp.

Mike T., Swampeast MO

But what happens if you oversize your entire heating system?

If you mean oversizing of the heating emitters the only thing that happens is that you now need a lower supply temperature. If you have a condensing (and particularly condensing/modulating) boiler with "oversized" emitters, your boiler will thrive on supplying those lower temperatures...

Alex Giacomuzzi

Reset, Efficiencies & Flue Gas Temp.

Hi Mike:

Sorry for not getting back to you sooner. Work and home projects.... normal for most of us.

Thanks for referring me to some of your previous posts under "Vitodens Operation". They are great --- I did not catch them when they were originally posted... You have truly adopted a passion for your system.

I will try to respond in kind for the numbers you have listed above:

1. I understand this represents the whole Vitodens 200 series... WB2 6-24C up to WB2 15-60. I was nor was the chart being specific to one model. So --- I am not sure what your point is here?

2. I am not into subversion, I am only into the interpretation of mfg's data. Reset is a built in feature offered on these models. Is it reflected in that chart??
You say yes and I say it may not be. My rationality is you can maintain a constant supply temp which the chart displays, vary the modulation on the boiler, and measure the combustion efficiency while holding the reset at a fixed point. You are assuming a real live world with varying outside temps outside and a varying modulation adjusting to those outside temps. I understand. Both interpretations are possible.

3. You do not believe 97.3% combustion efficiency at 167 deg. F.... Ok....This is where facts and marketing may go astray, but maybe not too far astray. You notice that this occurs at the lowest modulation level in the chart. Secondly it occurs for all three temp ranges plotted within 1 degree. That is the chart data. Glancing over to "The benefits at a glance:" section... Viessmann states "Highest efficiency with full modulation"
"Combustion efficiency up to 98%". Full modulation means the lowest rate (approx) that the burner will run. It does not give us a water temp, but the chart tell us 98% occurs at 104 F supply at 30% base load.

I can garner no further info from Viessmann, so I had to jump ship and go over to Buderus the GB 142 series. These are the only two mfgs. that publish any data...
If we go up to 122F supply they top out at 97%. This is all the published data I can find... It represents good information and I will try to tie it in below.

4. This is a very important point regarding the lowest efficiency around 85.7% for a supply of 167F and return of 140F. "corresponds very closely with the theoretical maximum efficiency of a non condensing boiler" ---BINGO.... You are absolutely correct. The more we load these technical wonders the lower the efficiency goes. That is precisely my point.

Viessmann does not give us any other numbers other than the chart, but our German neighbors at Buderus tell us exactly the same...."efficiency rating rax. capacity for heating curve 176/140 F is 88%. Interesting..

5. Mike... my hat is off to you with the data and information that you have provided to this site. It is great. Honestly ---- it took me awhile to read thru several of your very detailed posts...

I did as you requested. I paid special attention to the charts that I found (I apologize if I did not view the appropriate one), and here are my thoughts AND questions?

You state--"if efficiency declines with increasing output I think it would be fair to say that flue gas temp should increase faster than supply temp." You make a very good point. I do not have a system to test this on, so I can only rely on published information. Again -- I only have the same two sources.

I am going to take some liberty with your sentence(I will beg for foregiveness later) and say --- if efficiency declines with increasing output (I am interpreting this as higher % of load meaning more modulation NOT higher supply water temp for example) then flue gas temp should increase. Lets forget supply temp for a moment. Lets look at this..

Vitodens.... Flue gas at partial load--95F (86F return)
Flue gas at full load ---113F (86F return)
Flue gas at ???? load ---158F (140 return)
I think the last one would be fully loaded, but it is not stated.

Buderus..Flue gas @ partial load- 91F (86F ret/122F sup)
Flue gas @ Full load - 113F (86F ret/122F sup)
Flue gas @ partial load-135F (140F ret/176 sup)
Flue gas @ Full load ---150F (140F ret/176 sup)

Note --- I pick data from the smallest boiler each make...
Values can change from model to model...

Now does this agree with your statement? Temperature in the flue gas increased with LOAD. The higher the BTUH input level, the less efficient the unit is operating. Energy is lost as we increase load. This means the efficiency decreases just like the chart state and other line data support. I know this is contrary to may statements that have been made, but let the facts speak for themselves. This is why I feel a slightly oversized condensing / modulating boiler is the right choice for this boiler design. Undoubtedly both work.... one just works more efficiently.

The higher the supply temp, the higher the flue gas temp.
The flue gas range appears to narrow as supply temp increases, but I can not honestly say (at least in looking at this data) that flue gas temp increases faster that supply temp. This data says it is just the opposite --- for what ever reason, I do not know.

Reset (be it indoor or outdoor) and condensing / modulating boilers are very complementary to one another and bring some great features to many hydronic systems.

DATA---for example It gets confusing when one refers to modulation which is actually how you adjust load. Is high modulation running at 30% or is high modulation running @ 100%. Terminology will get you every time.

I have enjoyed your comments which have allowed me to dig a bit deeper than I normally might have. Keep up the great posts.

Regards Alex

Mike T., Swampeast MO

Very busy for the next few days Alex. Will reply fully when I have time.

Reason I mentioned the title pertaining to an entire class was that you mentioned a difference in "load-based" efficiency in different models in the series based on HX surface area.

Don't forget that Viessmann and Buderus are both German companies and that TRVs are required by law in Germany. When they talk about "efficiency" I sincerely believe they are talking about efficiency in the real world of changing weather with both the system temperatures and boiler modulation changing to meet the need as closely (and efficiently) as possible. Other companies are still stuck on AFUE which has little if anything to do with real world conditions.

Terminology will get you every time. I couldn't agree more--especially when language translation and completely different heating system mindsets are involved.

...modulation which is actually how you adjust load. Sorry, can't agree. Modulation is how you adjust the source to meet the load. The load comes from outside and inside the structure--not the heating appliance itself... See what I mean about a completely different heating mindset?

Alex Giacomuzzi

Enjoyable and Interesting

Mike

I would like to discuss in more detail your thoughts on HX surface area. I have my assumptions out there on this thread to compare against.. I think we are close.

I wrote above "load or partial load is achieved by burner modulation" and recenttly as you point out "modulation which is how you adjust load" This is "terminology along with not including enough adjectives". The second sentence and I agree with you should be -- modulation which is how you adjust (burner output -- btuh) load. We are on the same wavelength. I was not referring to the system load, I was referring to loading or unloading the btuh to the burner strictly.

I forgot to ask you a question on your tracking data. Bare with me a minute... The supply target temp appears to almost mirror opposite of the outside temp. This shows me the resetting capacity of the boiler control. The supply water temp appears to have an offset from the target temp and is relatively linear compared to the target temp. Why is that? I would have expected slightly more temperature change on the supply temp. Can it be due to the mass of your system and your house or??? Help me here.

Regards Alex

Mike T., Swampeast MO

Last Question First

Regarding the "target temperature" and "supply temperature":

The supply target temp appears to almost mirror opposite of the outside temp.

That's because it is. The reset ratio is 0.8 so for every 1° drop in outside temp, the "target temperature" rises 0.8°. The "target temperature" is calculated based on outdoor temperature and the reset curve in the boiler. I'm a poor boy and lack the mucho $$$ to buy the Viessmann interface plus custom computer interface to actually allow my computer to "know" the "supply target" reported by the Vitodens. I checked the boiler many times during periods of true and sustained modulation and my "target temperature" was extremely close if not exactly both the "supply target" and "boiler temperature" reported by the Vitodens.

The reset curve was carefully calculated to ensure that my flow rates would be within the limits of the Vitodens without the low-loss header and was designed for high and constant delta-t. Of course in real-world operation delta-t varies considerably--especially at lower loads.

The supply water temp appears to have an offset from the target temp and is relatively linear compared to the target temp. Why is that?

The Vitodens has an immersion type temp sensor very close to the HX. Again, I cannot interface directly between the Vitodens and my computer. My "supply temperature" is measured on the surface of 1" black iron pipe about 3' away from the boiler. My sensor is installed in a dab of heat conductive paste and has good insulation surrounding.

To be completely honest I cannot explain the large difference. I certainly expected some, but not that amount. While "relatively linear", I noticed that the difference actually increases with increasing load and higher burner output. To me this is even more inexplicable. My ideas regarding the how and why of this got me in a bit of trouble at a physics furum. I "see" a physical connection between objects separated by our concept of space as observed by our concept of time. Physicists don't--at least not regarding objects that they can actually see...

Can it be due to the mass of your system and your house or???

My only answer is ???. Mass and particularly volume of the system are quite high compared to the space it is heating. The heating mains certainly seem to be acting like a buffer. Remember that I have "wild" radiant floors that ensure flow through the mains at all times.

Mike T., Swampeast MO

HX Surface Area

This is NOT a Viessmann commercial! I have zero ties and zero benefit. I just happen to enjoy trying to understand the least understood method of heat transfer—radiation. The radiant burner of the Vitodens happens to intrigue me to the highest possible extent.

----------------------------------------------

Viessmann calls the design of the Vitodens "uncompromising". Considering the fuel savings I achieve, I tend to agree.

I suspect that the surface area of the heat exhanger (and water jacket) is linked to the surface area of the radiant burner. I simply cannot believe that an "uncompromising" design would sacrifice full-output efficiency for the sake of a few extra feet of coiling in the radial heat exchanger.

---------------------------------------------------

With other condensing or condensing/modulating boilers I can only rely on photos and illustrations for the "guts" of the HX process. As a general class all seem fully capable of extracting nearly the maximum amount of heat (as limited by supply/return temperatures) at maximum output from gasseous fuel.

Mike T., Swampeast MO

Reset IS Included in the Vitodens 200 Efficiency Chart!

This according to source high in Viessmann US management. Expect to see a change in the labeling of that chart!

According to same source there is a slight decrease in efficiency compared to say 50% output and 100% output with otherwise identical conditions. To quote the same source, "This does NOT mean one should oversize." [emphasis preserved]