Mod/Con boiler efficiency vs. supply temps

bmwpowere36m3

I was reading John Siegenthaler's Modern Hydronic book and came across this interesting chart:


Its showing mod/con boiler efficiencies at various input % and return water temps. It got me thinking about boiler output ratings...

Is the boiler's DOE (or gross output) rating assumed to be at 100% input?

For example the ratings on:
Lochinvar WHN55, 55 input and 51 DOE (MBH)
Triangle Tube PT60, 60 input and 54 DOE (MBH)

Those ratings seem to indicate efficiencies (at DOE) of ~90 to 93%. Looking at the chart and assuming 100% input would indicate those DOEs are possible, but the return water temps (to boiler) would need to be ~90* or less.

This sets up my following question, say I want to supply an indirect water heater. SWT is 180* and RWT is 150* (design). What is the maximum BTUs the boiler can provide? Based on the chart we would expect ~85% efficiency. If our boiler has a max input of 55 MBH, then can we only expect to see ~47 MBH?

Gordy

Whats your flow rate? Temperature is only part of the equation.

Gordy

All ratings for boilers are at 100% modulation. There has to be a baseline for comparision.

Snowmelt

After a while it just gets too much, I attended the seminar from caleffi about air seperation yesterday. I enjoyed listening to it. But it just gets too complicated for the average residential boiler. Sometimes your better off understanding the theory but just dumb it down for the everyday customer that wants to know how there boiler works.

Gordy

There is always the fine print. "efficiencies up to X" falls under certain parameters.

Gordy

I guess im wondering what it is your searching for in an answer.

Is it can the boiler produce 180* water?

Is it at 180* water what efficiency is the boiler running at?

I think your getting wrapped up in the ratings envelope, and forgetting about everything else that happens.

icesailor

It doesn't matter how efficient a boiler is. If the boiler is running wide open, and it won't heat the structure to the design temperature, Efficiency becomes useless. As a lot of folks have been discovering this last month.

Gordy

Variable speed pumping to maintain a set differential temperature (ΔT) between two (2) sensors allows for the following:
- Automatic adjustment of the pump's performance to match the low of the system or zone
- Eliminate velocity noise in the zone valve systems
- Conserve energy
Since ΔT is directly releated to flow rate, the pump's speed continually adjusts to the required BTU per hour. In almost all
applications the design of the system was based on being able to maintain a certain ΔT and figured by using the universal
hydronics equation of BTU/hr = GPM x 500 x ΔT. Given that, any time there is a change to the heat load (i.e., warmer day
or greater heat loss from a structure) then the GPM should change to match the required BTU/hr. This is achieved when the
variable speed circulators automatically and continually adjust their GPM output (by varying speed) to match the required
BTU/hr output of the system, no matter the changes in heat load, while always maintaining the designed ΔT between a supply
and return sensor.

bmwpowere36m3

Gordy said:

Whats your flow rate? Temperature is only part of the equation.

Does it matter in terms of whether a boiler can supply 180* at its DOE rating?

Whatever it takes according to the hydronics formula (BTU=gpm*500*deltat)

bmwpowere36m3

Gordy said:

There is always the fine print. "efficiencies up to X" falls under certain parameters.

From manual, DOE rating:

"The ratings are based on standard test procedures prescribed
by the United States Department of Energy"

and

"Ratings have been confirmed by the Hydronics Section of
AHRI. Model 399 has a thermal efficiency rating."

bmwpowere36m3

Gordy said:

I guess im wondering what it is your searching for in an answer.

Is it can the boiler produce 180* water?

Is it at 180* water what efficiency is the boiler running at?

I think your getting wrapped up in the ratings envelope, and forgetting about everything else that happens.

No.

No. But that can be determined (approx.) from the chart if you know the return temp.

Not really. Its a simple question, which I'd like to discuss and understand.

I'm not searching for a particular answer to a specific problem, I made up a scenario to illustrate a point. Right from my first post:

"This sets up my following question, say I want to supply an indirect water heater. SWT is 180* and RWT is 150* (design). What is the maximum BTUs the boiler can provide? Based on the chart we would expect ~85% efficiency. If our boiler has a max input of 55 MBH, then can we only expect to see ~47 MBH? "

bmwpowere36m3

icesailor said:

It doesn't matter how efficient a boiler is. If the boiler is running wide open, and it won't heat the structure to the design temperature, Efficiency becomes useless. As a lot of folks have been discovering this last month.

I appreciate your input, but man... your like a one-track record. Same thing over and over and over, "your house will be cold, your boiler is undersized, etc..."

Did you even read my initial post. I'm interested in specific answers to boiler efficiencies, DOE ratings and supply temps.

Not whether my house will be cold. Because I know my house is cold.... due to the fact its lacking a boiler, among other things

bmwpowere36m3

Snowmelt said:

After a while it just gets too much, I attended the seminar from caleffi about air seperation yesterday. I enjoyed listening to it. But it just gets too complicated for the average residential boiler. Sometimes your better off understanding the theory but just dumb it down for the everyday customer that wants to know how there boiler works.

I'd like to think of myself as semi-educated and always interested in learning. My question is purely academic...

Gordy

If you had not noticed I'm trying to get you to figure out the answer.

Look at the manual and use the hydronic formula to calculate what output they expect at each delta in the table.

Then you will understand the output the manufactor is expecting to achieve

Jody_S

As an academic question the answer is to look at your flue losses at your prescribed conditions. If the return water temp is 150 F, then you are well above the dew point for natural gas so all the latent heat is going out the vent and then depending on how well the heat exchanger is designed will determine how much sensible heat is extracted from the combustion gasses. Some of that is going out the vent as well. 15% losses is not unreasonable accounting for all factors.

How/why the efficiency changes during the recovery cycle, well, that's a different question.........

bmwpowere36m3

Gordy said:

If you had not noticed I'm trying to get you to figure out the answer.



Look at the manual and use the hydronic formula to calculate what output they expect at each delta in the table.



Then you will understand the output the manufactor is expecting to achieve

Manual makes NO reference to boiler output vs. supply temps. Other than stating that the max set point temp is 180* and max offset is 20*, so theoretically 200*.

It makes no mention of being able to produce 180* water at DOE rating. Unless that is somehow implied in the rating [DOE].

The table references different operating deltas. No mention of absolute temps. At 20* delta it implies ~50k btus. However is that 180/160*, 140/120*, 100/80*.... etc.

bmwpowere36m3

Jody_S said:

As an academic question the answer is to look at your flue losses at your prescribed conditions. If the return water temp is 150 F, then you are well above the dew point for natural gas so all the latent heat is going out the vent and then depending on how well the heat exchanger is designed will determine how much sensible heat is extracted from the combustion gasses. Some of that is going out the vent as well. 15% losses is not unreasonable accounting for all factors.

How/why the efficiency changes during the recovery cycle, well, that's a different question.........

I think that pretty much answers my question... And it's basically what I'm saying. Mod/Con DOE rating (I used two different mod/cons) is only achievable with very, very low return water temps. The same holds true for their AFUE ratings, in addition to modulating near the low-end.

So if your sending out 180* water, you ain't getting low temps back in a properly designed system and thus your boiler is not operating as efficiently and it won't make its rated DOE.

The Lochs DOE rating implies its 93% efficient... which directly contradicts the chart if you operate at higher return water temps. Physics right, if you aint condensing... its going out the vent.

Gordy

why would you want to own a Mod/con and use 180 other than a domestic Load for short periods. might as well use a Ci boiler.

bmwpowere36m3

Gordy said:

why would you want to own a Mod/con and use 180 other than a domestic Load for short periods. might as well use a Ci boiler.

Because you can… I'm not trying to argue the usage of the boiler or emitter selection and I agree with your statement. However even running it a 160/140* or 150/130* is about the same at 100% modulation, as far as efficiency is concerned. Are you saying that a mod/con should only be used with radiant? I don't think you are.

I'm just trying to understand the ratings and efficiencies and what they might mean. In my mind the DOE rating is most important at design day (or whatever the coldest temps will be) and likely you'll also need to run the highest water temps to satisfy those heat losses.

What seems a little odd to me, is that the DOE rating on mod/cons seems to reflect the lowest supply temps. And that is a little misleading, since you might not achieve those BTU levels when you actually need them.

When I look at CI boilers the DOE ratings reflect efficiencies in the eighties, which is probably possible at any water temp. However mod/cons DOE rating is closer to 90+% and that it not possible at higher water temps. To get the advertised 90% at the rated DOE you would need to return water at less than 90* at 100% input. How many systems out there can do that on design day, which presumably would be when you operate at 100% input.


Maybe I'm all wrong… anyway, like I said a purely academic discussion not reflecting any specific installation or system design.

Gordy

your only focusing on design day. Think about the rest of the season. You probably start heating when highs are consistently in the low 60s, and lows in the low 50s. you can easi;y see low return temps.

A CI boiler gives low 80s efficiency all day long, and all season long. But not always if it is oversized, and short cycles.

bmwpowere36m3

Not so much focused on design day, but understanding boiler output at 100% input. I get and agree with the reasoning wrt sizing the boiler, it's function and efficiency the "majority" of the heating season.

Gordy

Hatterasguy,

That was pulled out of the Loch WHN manual. I would assume that ODR all though not mentioned is the strategy. Same with Taco.

So I,don't disagree with your thoughts.

icesailor

@Bmwpowere36m3:

"" I appreciate your input, but man... your like a one-track record. Same thing over and over and over, "your house will be cold, your boiler is undersized, etc..."

Did you even read my initial post. I'm interested in specific answers to boiler efficiencies, DOE ratings and supply temps.

Not whether my house will be cold. Because I know my house is cold.... due to the fact its lacking a boiler, among other things

I'm not a one track record but over and over, Why is everyone considering the DOW number? The IBR number is the one that should be considered because there is never a discussion about what it is and what it represents. The IBR courses aside, many here are trying to design systems to a standard of a speck of fly schitt on a wall. If you don't use the IBR numbers, to design to the fly schitt, you need to measure the length and size of each and every loop, including each and every fitting in the system, to determine the flow and loss through resistance of the entire system, added to the radiation, to determine the size correct size of the boiler. I haven't seen a single person mention that. The DOE number is higher because it will be assumed that all these calculations will be done. IBR did those calculations and in THEIR number/rating, there is an allowance of 1.15% for "piping and pick-up, Every boiler technical rating sheet that you see will have an asterisk (*) after the IBR rating. Look under the legend for such things are. You will find it.

For example, take a Veissmann Vitodens 100 boiler. It has a DOE rating of 37 to 118 MBH (Btu's), which means that it will modulate through 37,000 to 118,000 BTU's. The IBR rating says that with "Normal" piping, the maximum BTU's available to the system through the piping with "normal" piping etc. is 94,000 BTU's. That's with a SWT of 140 degrees and a RWT of 120 degrees. So, you have all kinds of off the wall piping schemes that rob energy from the system, but the boiler hasn't been corrected for these factors.

Few understand this concept it seems. So, few of us learned that as long as the maximum heat loss of a structure didn't exceed the IBR number, and you didn't have "unusual" piping, that boiler was guaranteed to deliver the IBR rating. In the Vitodens case, 94,000 BTU's. The DOE number is what the boiler will deliver out of the SW discharge at the specified temperature under test conditions. In a factory or laboratory. Like the MPG's on automobiles that never come close to the rated MPG averages. The DOE number is an arbitrary number to sell boilers. The IBR number is a real number. Because it is so time consuming to do any boiler sizing with piping and pick up, that no one can possibly do it. No one ever did it. So IBR came up with the rating to do it for you.

They started throwing massive amounts of resistance into systems and there were no allowances for this new resistance. So they came up with multi-speed pumps to overcome the resistances. Then, variable speed pumps. Then massive pumps to overcome the high resistance of undersized systems.

If you have a building loosing 90,000 BTU's/H at zero degrees outside, and you used the Vitodens, and designed for 140 degree SWT, 120 Degree RWT, etc, the building would be warm, theoretically. If the OAT went up to 35 degrees, the boiler is twice the size needed. But if it then drops below Zero to say 5 below, the boiler is now too small. The only way to get more heat is to have it zoned, turn down unused zones, and let the water temperature go up in the used spaces.

One thing about Steam. No one is going through this BS about sizing. You size to the load. The piping is already there. As I see it, on all the steam boiler ratings, the boilers are rated at the DOE number for BTU's delivered when the steam is blowing out of the top of the boiler, but the EDR ratings are IBR ratings, delivered steam to the system. Take any steam boiler rating, take the BTU output number, and divide that EDR number. The EDR number will be smaller. Because the piping and pick up allowance is made. Every Steamhead here says to size by the connected (EDR) load. While Wetheads walk in to the same building, measure it up and size to BTU's. If it wasn't so over steamed, it wouldn't steam up and get warm.

Its now 71 degrees outside. My heat strips came on for the first time Thursday night. Ran maybe 20 minutes total. Because I added another 12+" of blown in insulation, the house never goes below 70 degrees. I'm warm. It doesn't matter to me how warm anyone is. None of my own houses that I built were ever cold. No house I ever did for a customer was ever cold. And they weren't over radiated, over boiler'ed, or over pumped.

OBTW, I guess you have a M3 BMW.

My 2001 BMW 325IX wagon was advertised to get 24 MPG on the highway. With our stuff and a cat, it got 28 MPG on the highway (Rt 95) driving back and forth from Massachusetts to Florida, and still did after 153,000 miles. Not known yet what the new 2014 X1 with X-Drive will do.

Gordy

So @icesailor DOE is used to wipe our hind quarters. That drops a mod con to 80%

Gordy

Hatterasguy said:

Thanks.

I had several long discussions with John Barba which went nowhere. He clings to the position that the DT circulator will vary its speed based upon outdoor temperature with no mention or requirement for outdoor reset.

You just put in a DT circulator on an existing system and it magically adjusts its speed based upon the OAT. I believe Taco knows the facts but are marketing the product in an "alternate" way.

I'm still thinking on this.

Zman

Taco sells delta t circs....
What do you expect them to market?

Gordy

True

Gordy

Was he thinking the 00 VR circulator? Or just strictly delta T?

Gordy

In a delayed way no ODR delta T does react to OAT.

Gordy

I guess see it as OAT decreases heat loss increase so does needed btu ouput. The pump is constantly adjusting the DT to compensate for the rate at which an emitter discharges its btus.

I mean really how many systems have say 180 right out of the gate in a cold start boiler setup. So water temp climbs until the tstat is satisfied. Maybe you will hit 180 maybe not when the tstat is satisfied.

Gordy

Here is a thought. Can a system have to much emitter? Bar out costs of installing it.

Zman

That being said, there many good applications for delta t ecm's.
Getting the optimal flow rate on mod/con boilers, injection mixing for odr and many more.
I think applications like zoned in floor radiant are served better with delta p ECM. With it's new acquisition, I expect taco to enter the delta p market and change tunes a bit.
Carl

Gordy

Hatterasguy said:

The emitter doesn't know if the ambient is 0°F or 40°F. It gives BTU's at a constant rate based upon SWT and indoor temp. That's it.

We aren't talking about varying SWT's.

The fact that you're mulling this over shows how complicated this subject is.

Not directly it does not know. Indirectly it's driving to reach setpoint based on outdoor temps. OAT is sucking the btus out of the structure, and the system is trying to reach equilibrium.

But you are talking about a static SWT. In that case it would have to be a hot start boiler no? So 180 or what ever hits the emitter right out of the gate. In that case the DT circ would still react but at a much less of a,rate of change.... If any. Of course we would need to put parameters on the conditions such as set backs.

Gordy

Really this discussion is hard to come to a finite conclusion unless there are constants to a particular system. If there is a hot start boiler then proper return temp protection need be in place and the return sensor located to not see that cold surge of water. But then there would be no point, or advantage to using a DT circ.

bmwpowere36m3

Hatterasguy said:

The emitter doesn't know if the ambient is 0°F or 40°F. It gives BTU's at a constant rate based upon SWT and indoor temp. That's it.

We aren't talking about varying SWT's.

The fact that you're mulling this over shows how complicated this subject is.

But the indoor temp is not a constant, no? When there's a call for heat, likely the room temp will be a few degrees below set point.

So the emitter BTU output likely will vary as it heats the room and slowly approaches set-point due to the room/supply temp delta changing.

However, if the system can constantly supply water at the exact temp need only to cover heat loss… than the room/supply delta wont change (unless the SWT changes).

bmwpowere36m3

I agree in that there are MANY variables and that makes understanding the "issue" more difficult. Just like at work, when I'm presented with an engineering problem…. usually there are many variables. You have to start by simplying the problem (if possible) and making some assumptions. Then you try and solve the problem. Sometimes it takes many iterations and different assumptions. In the end, usually, you physically test our results and verify they match your calculations. Sometimes they do, sometimes not. If they don't, then its back to the drawing board

Gordy

I hope set point does not vary by a few degrees that's not comfort in my book

Zman

For the sake of the conversation, I think you have to assume the room temp is essentially a constant. Anyone trying to achieve significant setbacks should consider adding indoor feedback and associated control logic.

hot_rod

The part of delta t distribution that I don't follow is a programed ∆T based on the ∆T you designed for. I have yet to find a manufacturer of boilers or heat emitters that claim or show their performance is best or most efficient at a certain ∆T?

In fact most manufacturers show output differences at a selection of ∆Ts. Certainly the same ∆T choice isn't used for radiant, panel rads, fin tube of air coils.

If I design and program a system for a 16 ∆T, will it be more or less efficient than a competitors 20 ∆T?
If you add a ∆T circ to an existing system, how would you know what that system was designed to?

In another thread there was a reference to an ASHRAE study claiming flow modulation is the best way to regulate heat output. In the very early days of tekmar this was discussed and tekmar went the route of regulating heat output by adjusting temperature. They understood the flow modulation is very non lineal and hard to control accurately when you drop into velocities below 2 fps, and the implication of the reynolds number in heat transfer. So I'd bet there are ASHRAE studies and claim that temperature modulation is the better way.

I don't question that eliminating boiler short cycles increases efficient, but that is not unique to ∆T pumps. Mod cons modulate based on temperature. At some point every boiler is oversized if it cannot modulate down to almost 0 output. that is where buffer and high mass (volume) boiler types make sense.


In a finely tuned system temperature modulation is based on not only outdoor, but also indoor feedback, critical in large load- changing occupancy buildings like churches or public spaces. Another point tekmar drove home at seminars. It'd
s not just the outdoor conditions that dictate comfort and fuel savings and boiler or systems efficiency.
And now we have t-stats that track and adjust to weather station data and get a jump on the incoming outdoor temperature drop.

I like the reverse ∆T function for boiler protection, and a few other applications, much better than slow responding, pressure drop inducing valves for protection for example. I understand that reverse T is limited to 32-100 degrees on ∆T circs? Not so practical with out a added resistor to "fool" the control?

One misunderstanding is that the hydronic formula is the golden rule and always results in predictable output, which is not at all true when you run various examples and get down to low flow rates.
Could a flow rate of .2 GPM at an 800 degree delta T transfer 80,000 BTU/ hr? has anyone confirmed that is possible? The formula shows 500 (flow rate) X ∆T = energy transferred.

So when then does the hydronic formula start to predict wonky numbers that are not obtainable in real life? This is where the models, simulation, and actual results get more complicated, as flow drops turbulent conditions or transition to laminar start to over rule the golden formula.

Also not long ago there were ads claiming ∆P circs were for Euro systems and not all that smart. What market does the Viridian address then?

Jason_13

To address the DOE testing that was erroneously answered. As an overview the boiler is tested at 120f return and 140f supply with the proper flow rate through the boiler. The DOE is a tested and accurate number according to the test procedures. The Net output is nothing more than a standard deduction. So all heat losses should be sized off the DOE not the net number.

We know as the water temperature changes so does the resistance to flow. Higher temperature water will move easier than cooler water. Also to make things more confusing as the water changes velocity the resistance to flow changes again. Faster moving water will supply more heat from the radiation than slower moving water. Most thing happening is a hydronics system is a moving target and you just need to get to a good medium point. I guess the point is we can never design perfect situations unless we keep water temp and flow exactly where we design. The blessing is this is not as much rocket science as many want to believe. Hydronics is so forgiving the systems will work efficiently designing with lower water temperature and velocities to eliminate air and avoid noises.

The only problem I see is is many systems are starting to waste dollars to save pennies. Do I want to charge a homeowner $300 - $400 and they save an extra $50 a year. As a standard if we could just size boilers properly and properly adjust ODR we would see a big improvement in overall fuel savings. How much is the home owner saving by boilers being 100% oversized due to being sized off connected load or matching what boiler size that was there before. And you see each zone with a variable speed circulator and the ODR was still set as the manufacturer sent it.

hot_rod

Jason said:

To address the DOE testing that was erroneously answered. As an overview the boiler is tested at 120f return and 140f supply with the proper flow rate through the boiler. The DOE is a tested and accurate number according to the test procedures. The Net output is nothing more than a standard deduction. So all heat losses should be sized off the DOE not the net number.

We know as the water temperature changes so does the resistance to flow. Higher temperature water will move easier than cooler water. Also to make things more confusing as the water changes velocity the resistance to flow changes again. Faster moving water will supply more heat from the radiation than slower moving water. Most thing happening is a hydronics system is a moving target and you just need to get to a good medium point. I guess the point is we can never design perfect situations unless we keep water temp and flow exactly where we design. The blessing is this is not as much rocket science as many want to believe. Hydronics is so forgiving the systems will work efficiently designing with lower water temperature and velocities to eliminate air and avoid noises.

The only problem I see is is many systems are starting to waste dollars to save pennies. Do I want to charge a homeowner $300 - $400 and they save an extra $50 a year. As a standard if we could just size boilers properly and properly adjust ODR we would see a big improvement in overall fuel savings. How much is the home owner saving by boilers being 100% oversized due to being sized off connected load or matching what boiler size that was there before. And you see each zone with a variable speed circulator and the ODR was still set as the manufacturer sent it.

Your second paragraph sums it up nicely. There seems to be some disconnect here with BTUs jumping off fast moving trains

I do agree converted gravity systems with radiator designed for gravity flow behave differently and slower flow works best. There had been plenty of evidence presented here to confirm that.
For any other emitter i am familiar with, increased flow = increased mean temperature = increased heat transfer.


Until that rule of thermodynamics is understood and agreed the discussion seems to go off to other tangents.