Modulated temps?

Mike T., Swampeast MO

Say your house looses 30,000 btu in some hour at some conditions.

The ultimate goal of a modulating boiler is to provide 30,000 btus via one continuous cycle during that hour.

To the boiler it makes no difference if it supplies that 30,000 btus at 180° or 80°. BUT, because it can also recover "wasted" energy from condensation you'll get much higher BOILER efficiency at 80° instead of 180°.

But also (and this is a BIG but), the boiler itself MUST have some sort of feedback mechanism to tell it what temperature it needs to produce <I>for the desired conditions in the present weather</I>. Very frequently the outdoor temperature is measured and a guess is made via an <I>outdoor reset curve</I>. Another method is indoor reset where the thermostat and boiler are in two-way communication with each other with the thermostat telling the boiler how far the current temp is away from the desired.

Without this feedback the boiler would fire 100% to some set maximum temperature--only then could it ever begin to modulate to maintain that temperature. The problem with this is that in most weather conditions the required BTUs in an hour would be provided MUCH more quickly than one hour. Depending on the weather and the mass/volume/characteristics of the emission system this may occur WAY before the set maximum temp is ever reached--thus no modulation.

Even if the temp does rise to the max and the burner modulates, the supply temp will be much higher than necessary in most conditions and you don't get the full benefits of condensation.

As I hope you can see there are MANY dynamics in a modulating system. This makes your questions impossible to answer with the information you've provided.

To even begin to estimate for your purposes you need to know:

1) Heat loss of the structure as a whole and for rooms individually at at least two different outdoor temperatures.

2) Output ability and exact type of emitters in each room.

3) Flow rate through the system.

Even with this information you'll be hard pressed to get an accurate answer to the question, "How much will a condensing/modulating boiler save in my system?" In general the lower the temperature requirement of your emitters, the greater the savings. Also, the more the present conventional boiler is oversized the more the savings. But even these are not hard-and-fast rules. If this is a zoned system, you'll only get the savings available to the zone with the highest temperature requirement. This can be an especially serious problem in old iron radiator systems that have been modified/expanded.

Xc8p2dC_2

In theory

Is there any rule of thumb to what temp water supply will be when a when a natgas boiler drops into the Mod mode>

Say an 80k or a 105 in, running at 20K btuh? 40 btuh? etc
{Meaning at 20K max water temp could be?]

TIA, snowbound

Brad White_12

Depends on the flow rate

If a given boiler has a constant water flow rate and the fuel input drops, the temperature of the water will also drop in rough proportion.

Example: An 80K output load with 8 gpm = 20* delta-T
Same 8 gpm flow but 40K output = 10* delta-T

If the water flow is varied it can more or less keep the same delta-T. Some pumps with VSD's can be made to track a constant delta-T among other variables.

Plumb Bob

This is not a well-formulated question.

Boilers modulate as needed to reach the desired water temp. In other words, the outdoor temp and the reset curve determine what water temp is needed, and the modulating boiler figures out how to get to that temperature at an even keel without going on/off/on/off.

Since boilers can be hugely oversized, it is possible for your 80KBTU boiler to run at 20KBTU (as in your question) and still heat the water to 180F. It depends on the system, (the heat loss, the amount of radiation, etc).

That's why the boiler's modulation is not fixed; the boiler looks at the actual water temp and the target temp and figures out whether to floor the accelerator (so to speak) or ease up. In a different house, the same boiler will behave differently.

Xc8p2dC_2

Hence, the \"in theory\"

Brad,> Is there a fomula for finding GPM, is it piping and emmitter size or circulator pump size?

J.> My questioning was to see how much saving there would be if I convert to a Mod-Con?

What I figured is, Is my 140,000 btuh in, boiler @ 68 deg inside, runs for 8+ mins, twice an hr at 25 deg outside>> Thats 46,600 btuh an hr X 24 Hr's X 30 days = 26,876,160.btu a month>> divided by 1040 btu CF at 82% efficiency = 315 ccf a month >>> any where near correct??????

Could figure the same at a lower BTUh with a ModCon, but was hoping to run at lower water temp

Any formulated idea's?
RC

R. Kalia_8

Frankly I have no idea what you are talking about. You always need to supply the same BTU to heat the house.

The small savings due to modulation comes because the boiler doesn't go on/off/on/off during a single heat call from the stat, but rather finds the right firing rate to run steadily all the time until the stat says to stop. This is slightly more efficient, in the same way that driving a steady 55 on the highway is more efficient than stop/start driving.

So the necessary output BTU doesn't change (you still have to heat the house), but the input BTU to the boiler changes a little because of a change in efficiency.

Brad White_9

GPM

is simply a function of pounds of water moved per hour times degrees F. (a BTU being a pound of water raised or lowered by one degree F.)

A gallon of water weighs 8.33 lbs. (slight change with temperature density but close enough). A gallon per minute being 8.33 lbs. per minute x 60 minutes per hour = 499.8 lbs. per hour. (OK, call it 500 everyone else does )

Lbs. per hour x degrees F change in temperature is BTU's per hour.

This is how we get the "10,000 BTUH per GPM Rule" with a 20-degree Delta-T. (8.33 x 60 x 20 = 10,000)

Your emitter is sized to at least the heat loss of the space it serves. The pump then delivers the water expected but against a calculated head (pressure) loss of piping, fittings, emitters, etc.

Xc8p2dC_2

retained info

Thanks for the reply's >> I do know the specifics on my end but didn't provide to you>> I thought there would be a more simplistic answer as I was looking at it from a Non modulating type system>>

I currently have a new 4 X oversized for heatloss, boiler that is running in Mike T's "without feedback" mode [ so for every firing it's at 140k btu IN regardless of 1 min or 10 mins , 10 deg or 45deg out]and is running 195 max/ 180 min water temp

If I looked at monthly CCF savings from just changing to another single stage cast iron boiler with a lower BTU IN rating [and IN was all I was asking about]with the btu out that was closer to my heat loss, it would be easier to figure the CCF savings

BUT, I am also 1/3 Over emitted, so figured there would be a benifit from the ModCon and a lower water temp hence the resaon from asking " what temp water at lower btu IN" and clearly understand your answers

My home is new 1300 sqft 3B2b ranch, heat loss around 30K depending on DD temp, 88ft fintube baseboard 2 zone [temporarily 1 zone to remedy severe short cycling] with separate DHW

Maybe not as much benifit from the Mod/Con as thought?? > [and the Con was never major priority]
RC

Mike T., Swampeast MO

You have given the EXACT reason why SMALLER modulating/condensing boilers are GREATLY needed in the US. BUT fin tube baseboard is not particularly well-suited to take advantage of low operating temperatures afforded by condensing technology.

WE'RE NEITHER STUPID NOR ABSURDELY WASTEFUL GIVEN CHOICES THAT SUIT OUR LIFESTYLE.

Vitodens 6-24 heats BUILDINGS in Germany--NOT SINGLE FAMILY HOMES...

Xc8p2dC_2

Oh boy!

> You have given the EXACT reason why SMALLER

> modulating/condensing boilers are GREATLY needed

> in the US. BUT fin tube baseboard is not

> particularly well-suited to take advantage of low

> operating temperatures afforded by condensing

> technology.

>

> WE'RE NEITHER STUPID NOR ABSURDELY

> WASTEFUL GIVEN CHOICES THAT SUIT OUR

> LIFESTYLE.

>

> Vitodens 6-24 heats BUILDINGS in

> Germany--NOT SINGLE FAMILY HOMES...

Xc8p2dC_2

Oh boy!

I hope those last two paragraphs were NOT directed at me, as I don't think I insinuated anything, towards anytbody nor would I try to create indifference in a topic I was trying to learn from!

Mike T., Swampeast MO

Just a nudge to manufacturers that smaller condensing/modulating boilers are needed.

Brad White_9

Not directed at you, Xc,

Mike is both brilliant and passionate about BTU conservation as most of us are. But more, Mike is hands-on along with mind-on, clearly among the Hot Rods and ME's of the industry and of this site.

Mike- Keep doing what you are doing!

Brad