Mod/Con boiler efficiency vs. supply temps

icesailor

Gordy said:

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

Maybe it does only get 80%.

The gas pedal on a formula 1 race car or a NASCAR race car is a Delta T circulator. It adjusts to the needed power needs.

bmwpowere36m3

@Jason

Those DOE water temps apply to all boilers? Or just mod-cons?

bmwpowere36m3

Seems a lot of Tacos marketing with the delta t pump is in regards to "over-pumping"... With that aspect I see the benefit of variable pumping and coupled with ECM technology a reduction in electrical costs.

However as hot rod mentioned, is their anything magical about a certain delta t and does that translate into fuel savings (not electrical) or system performance.

Gordy

bmwpowere36m3 said:

@Jason



Those DOE water temps apply to all boilers? Or just mod-cons?

DOE

That's where I was leading you before. In the manual if you do the math with a delta t (pick one, interpolate one) using their flow rate that is the expected output which is the DOE.

Jamie Hall

There is no magic delta T. There isn't a magic output temperature. There isn't a magic flow rate.

If you want the boiler to condense, the return temperature has to be low enough to accomplish that. If the boiler is putting out a certain number of BTUs, the flow rate through the boiler will, given the temperature of the returning water, produce a certain output temperature. At those two temperatures and flow rater, the boiler will have a certain efficiency -- which is measurable. And, if one reads the test specifications, is the reported efficiency (usually... and most manufacturers are pretty honest about it!).

Similarly, out in the system, if you have an emitter of a certain size it will output a certain number of BTUs at a certain mean temperature of the emitter (OK, OK, sor the really fussy that isn't exactly true, since the radiated BTUs are not exactly linear -- but in most cases don't worry). That number of BTUs extracted from the circulating water in the emitter will drop the temperature of the water a certain amount, depending on the flow rate -- more flow, less drop and vice versa.

Now -- the designer's and installer's problem is to get everything to match, so that the flows match and the heat input and output match, and control it so it stays that way.

How you go about doing that is your choice -- and the skill with which that choice is made is what separates the experts from the hacks.

bmwpowere36m3

@Gordy

Sorry, there are no absolute temps listed in the manual, just delta ts. I feel like a broken record. I'm asking Jason if the DOE rating is specifically measured/calculated at 140/120 temps.

Seems your inferring that it would be same if it was done at 180/160 since their both a 20 delta t.

Gordy

the AFUE standard for testing is 140/120

Gordy

Using the universal hydronic formulas there is not a reference to actual water temperature only the differnence in water temperature delta t.

Whether your using 180/160 @ 4 gpm = 40000 btuh

160/140 @ 4 gpm= 40000 btuh

140/120 @ 4 gpm= 40000 btuh

120/100 @ 4 gpm= 40000 btuh

100/80 @ 4 gpm = 40000 btuh

Gordy

The ability to absorb, or distribute those btus at the given water temperature falls under the design of the HX, and the emitter. Surface area being the key component.

Example: 80' of baseboard with the 180/160 depending on design for 40k. As AWT drops length of emitter increases. Or internal design.

Around 1600sf of radiant at the 120/100 for 40 k at the other end of the spectrum. As AWT rises SF decreases, or falls emitter,SF increases. Or density of the tubing.

Gordy

As far as the mod/cons "efficiency"yes it drops as temperature increases, and rises as temperature decreases. because it's hx is limited to its designed size just like an emitter unless you change its size. You still can get the same btu output by changing deltas, and gpm going through the HX. Just like an emitter.

Gordy

Now,go to page 36 of your whn manual. See the gpm through the boiler at differing deltas? They all work out pretty close to the DOE output of the boiler.

Want a 30 delta 51000/500*30=3.4gpm