[Equest-users] Condensing boiler curve issues

Porter, Frederick NOR fporter at noresco.com
Mon Sep 28 15:06:56 PDT 2015

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 Porter, BEMP, LEED© AP
Principal Engineer
Sustainability Services
2540 Frontier Ave, Suite 100, Boulder, CO  80301
fporter at noresco.com<mailto:fporter at noresco.com>

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From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Adam Barker
Sent: Friday, September 25, 2015 2:39 PM
To: Brian Fountain
Cc: 'equest-users'
Subject: [External] Re: [Equest-users] Condensing boiler curve issues

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 01D0FA04.0AEEA6D0]

At 100 F return water temp:
[cid:image004.png at 01D0FA04.0AEEA6D0]

At 120F return water temp (which is probably closest to reality, but I don’t trust the results):
[cid:image005.png at 01D0FA04.0AEEA6D0]

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



From: Brian Fountain [mailto:bfountain at greensim.com]
Sent: Thursday, September 24, 2015 9:54 AM
To: Adam Barker
Subject: Re: [Equest-users] Condensing boiler curve issues

Hi Adam,

That is not what I have seen with the curve.  Note that there is a rated return water temperature field in the boiler at which the boiler achieves the rated HIR (thermal efficiency).  I have seen the curve be strongly dependent on loading -- i.e. much more efficient when the boiler has a low part load.

I did see an overlay of the default eQUEST condensing boiler curve and a Lochinvar Benchmark curve and they corresponded reasonably well.

I'd be interested in taking a peek at your file if you are able to share (perhaps a generic version of it).

On 22/09/2015 4:02 PM, Adam Barker wrote:

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<mailto:abarker at pemi.com><mailto:abarker at pemi.com><mailto:abarker at pemi.com>


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