# Condensing boiler curve issues

6 posts / 0 new

Has anyone come across issues with the default condensing boiler curve in eQuest? This curve is a function of part load and return water temperature, and based on how condensing boilers work you would anticipate the part load efficiency to get worse as return water temperature increased (assuming part load is the same).

I am seeing the opposite in eQuest, where the default 80F RWT is giving me a poor seasonal efficiency, and a temperature like 160 RWT (in the non-condensing spectrum) is giving me a very high seasonal efficiency.

Any insight into why this might be happening?

Adam Barker, MBSc, C.E.T., LEED AP BD+C
Energy Project Manager
Provident Energy Management Inc.
T: 416-736-0630 x 1874 | abarker at pemi.com

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Joined: 2011-12-22
Reputation: 0

One thing to keep in mind is that the value for efficiency in eQUEST is
HIR, which is the reciprocal of efficiency. In other words, higher HIR is
worse that lower HIR. Is that what you are seeing?

Another thing you might be seeing is the that value provided by these
curves includes PLR. So at 50% load, the curve value is 0.5 for a boiler

I've examined the default curves in the past and found they performed
correctly, i.e. in the right direction.

-Erik

*Erik Kolderup, PE, LEED AP*
erik at kolderupconsulting.com | 415.531.5198 | www.kolderupconsulting.com

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Joined: 2011-09-30
Reputation: 0

eQUEST's default curve performs just as you say it should.
Be careful that when you calculate your corrected efficiency, you must take the PLR into account.
The correct formula is :

Corrected Efficiency = Nominal Efficiency * PLR / f(PLR)

You calculate the f(PLR) with the eQUEST curve.
See below the quick graph I charted out :

[cid:image002.png at 01D0F555.125D3CF0]

CondBlr-HIR-fPLR&HWR

a

b

c

d

e

f

-0.09439243

0.90319633

0.01547839

0.00159793

-0.00000645

0.00111453

Nominal Efficiency

90%

PLR

Water T

f(PLR)

Corrected Efficiency

1

80

1.00000

90%

1

90

1.01616

89%

1

100

1.03103

87%

1

110

1.04461

86%

1

120

1.05690

85%

0.9

80

0.89782

90%

0.9

90

0.91287

89%

0.9

100

0.92662

87%

0.9

110

0.93909

86%

0.9

120

0.95026

85%

0.8

80

0.79596

90%

0.8

90

0.80989

89%

0.8

100

0.82253

88%

0.8

110

0.83388

86%

0.8

120

0.84394

85%

0.7

80

0.69440

91%

0.7

90

0.70721

89%

0.7

100

0.71874

88%

0.7

110

0.72898

86%

0.7

120

0.73792

85%

[cid:image003.jpg at 01D0F554.9B7098D0]

Patrick Lapierre_ing.
plapierre at bpa.ca

De : Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] De la part de Adam Barker
Envoy? : 22 septembre 2015 16:03
? : 'equest-users'
Objet : [Equest-users] Condensing boiler curve issues

Has anyone come across issues with the default condensing boiler curve in eQuest? This curve is a function of part load and return water temperature, and based on how condensing boilers work you would anticipate the part load efficiency to get worse as return water temperature increased (assuming part load is the same).

I am seeing the opposite in eQuest, where the default 80F RWT is giving me a poor seasonal efficiency, and a temperature like 160 RWT (in the non-condensing spectrum) is giving me a very high seasonal efficiency.

Any insight into why this might be happening?

Adam Barker, MBSc, C.E.T., LEED AP BD+C
Energy Project Manager
Provident Energy Management Inc.
T: 416-736-0630 x 1874 | abarker at pemi.com

Offline
Joined: 2015-01-28
Reputation: 0

Sure Brian, thanks, and thanks to the others that responded.

I believe what's happening has to do with the part load, which as you say seems to have a big influence, as well as how the boilers are staged (or aren't staged). My boilers are at very low part loads most of the time. As mentioned if I increase that return water temperature, not touching anything else, my fuel use goes down. Looking at PS-C, it is going up at higher part loads, but getting better, i.e. going down, at very low part loads - where my boilers are spending most of their time operating, hence the net decrease in energy consumption.

PS-C results for my lead boiler (does most of the work)
At 80F return water temp:
[cid:image002.png at 01D0F7B0.907CD7F0]

At 100 F return water temp:
[cid:image001.png at 01D0F7AF.531113F0]

At 120F return water temp (which is probably closest to reality, but I don't trust the results):
[cid:image003.png at 01D0F7B0.907CD7F0]

Am I getting to the fringes of this curve's limits so its starting to go out of whack?

Regards,

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Joined: 2011-12-22
Reputation: 0

I think you are conflating the rated hot water return temperature input for the eQUEST boiler model, and simulated hourly hot water loop return temperatures.

When that rated return temperature input for the eQ boiler model is changed, that is a change in the boiler rating point, not a change in the simulated hourly temperatures of the modeled hot water loop. Increasing the rated boiler HWT, implies that the boiler is more efficient at warmer return temperatures. The parametric doesn?t show any changes to HW loop-design T, control strategies, reset schedules if applicable, design dT, minimum flows or other loop inputs that would actually change the hourly HW return Ts. So I think the program is doing what it should do given the varied inputs; that is the simulated hourly HWRT is really staying the same, but the modeled boiler efficiency is increasing at those temperatures as you increase the boiler rated HWRT.

If you leave the boiler rated HWRT input constant at an appropriate value for the selected boiler you are trying to simulate, and vary the HW loop design and control characteristics to simulate raising or lowering the loop HWRT, you should see results more in line w/your expectations; the annual boiler efficiency will go up as the simulated hourly loop return temperatures go down. There is no direct input for HW loop return temperature; it is a function of the inputs mentioned above, and probably a few others. Achieving these desired low HWRTs is not a trivial matter in actual operation of commercial HW heating loops, so be careful not to be too optimistic.

Fred
Fred Porter, BEMP, LEED? AP
Principal Engineer
Sustainability Services
NORESCO
2540 Frontier Ave, Suite 100, Boulder, CO 80301
fporter at noresco.com
www.noresco.com

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Joined: 2015-02-03
Reputation: 0

I will like to say something about curve and the part load factor values
and also on hot water return temperature.
The curves in the condensing part load based curve is dependent on two
things the part load factor and hot water return temperature and the
resulting is HIR.
You can have a look at the data points in the default condensing curve.

[image: Inline image 1]
As mostly the design supply water temperature is nearly 130 ?F for
condensing boiler and the hot water default delta T is 40 ? F.
So the supply design temperature is 120?F as per eQUEST if there is 80 ?F
return water temperature.
Also i found as after few simulations that the HIR below 1.00 i.e. Z is not
the possible condition for simulation for HIR input in eQUEST at full load.
The best condition for the default curves to operate as per condensing
boiler curve to have operate is the return water temperature is 80 ?F.At
lower temperature the efficiency are the best for part load and full load
which can be seen from the data points.

As from your snapshot of PS-C command the boilers are operating at full
load for more number of hours for the return water temperature 80 ?F.and
at 160 ?F the boilers are operating more at part loads.
Going to 160 ?F return water temperature the boiler have lower efficiency
at all loads which can be seen from curve data points.

But as per your snapshot the number of hours being operated for part load
is much higher at 120?F and there is less energy consumption and at part
load there is less fuel input required.
It is the default curve nature that is resulting in better performance at
higher return temperature as at that time the boiler are more being
operated at part loads and at part load the energy consumption is less.Hope
this will make your words coherent with mine.

Thanks,