Turn Down and Condensing

newinnj · March 4

Hi,

I have a boiler with a 15:1 turndown. With "the Wall" help I am fine tuning the boiler parameters.

I was on the web and found two comments that the high turn down boilers manages things through the air gas mixture and they add more air to the mix to get the turn down. They also state that increases the dew point and might prevent the condensing boiler from condensing and loses its high efficiency. I don't know the rules of adding links in but a google search for The dirty little secret of high turndown ratios. An engineer associated with a boiler manufacturer had this on YouTube. Also found a commercial boiler service company in Nebraska had on their website of info also stating the same. Just wanted the pros opinions on this. Thanks in advance.

EdTheHeaterMan · March 4

I'm not sure what your question is exactly?

You want to know something about a boiler you already own in relation to the 15:1 turn down ratio? What exactly do you need to know? And what will this information get you to change, if anything?

mattmia2 · March 4

If that were the case the O2 would go up on the lowest fire. I don't really see why there would be a need to do that with the right burner geometry.

hot_rod · March 4

https://forum.heatinghelp.com/discussion/200828/turn-down-and-condensing

Keep in mind Patterson Kelly boilers start at 300,000 BTU/hr. They play mostly in the 750,000 and up range.

A few points I would mention. He talks about the need for turbulent conditions in both the gas side and the water side. In residential sized systems, piping below 1-1/4" that would be a very low flow condition. Somewhere below .5 gpm in copper tube to go laminar.

He mentioned that modulating boilers are mainly used to correct a piping or design deficiency. I think in residential work they are more to address zoned and micro-zoned systems. Again PK deals with larger commercial systems, so that may be more of their market, solving problem jobs.

For years now we have been suggesting all hydronic systems should be designed to supply a design load at 120 SWT! So that keeps you in condensing range, 90+ fairly easily. In his example you may see flue gas a few degrees above return, now approaching 99% efficiencies!

Although when I suggested 120 max. SWT, Robert Bean asked me why not 110 or 120. Always a step ahead, that boy:) His crystal ball was already presenting A2WHP potentials.

I think the damper you see in mod cons is how they get above 10-1, the gas valve doesn't really modulate down so low, they start to chock off the air flow as he mention. That is why cleaning and keeping those dampers moving is an important part of mod con service.

As far as 20-1 and higher, that is often two 10-1 boilers inside the jacket, both running 10-1, not necessarily one HX modulating that range. Or a HX like the Sermeta DUO 260K and up.

I though he did a great job of making the presentation understandable.

newinnj · March 4

Well it's a Navien NHB-150 boiler with a turndown of 15:1. By changing the space heating and DHW min heat capacity (%) down to 7% that makes the boiler (150K) be like 10,500 for minimum operation. I just was reading that with high turn down there might be an issue with boilers not condensing. Is there something that I can keep an eye out for if this is an issue or not. Some stat that the boiler has for diagnostic info or the how much the condensation pump is moving? I just did not know enough if these website postings were valid or not.

newinnj · March 4

My prior post was written before everyone responded. I thought the presentation was good so that is why I asked the question. So it looks like a residential application is behaving differently than a larger commercial installation. The company in Nebraska was also a very large commercial boiler servicing company. Thanks I will keep an eye out maybe on the condensation output with various turn down percentages. Thanks I think I will follow everyone's advice and keep it as low as it goes.

hot_rod · March 4

As the presenter noted, the return temperature is the key to condensing, or the quality of the condensing.

mattmia2 · March 4

The damper probably covers the a high jet or high air passage so a low jet can handle the small flows sort of how you use 2 mixing valves in parallel for a wide range of flow rates.

GGross · 1:09PM

The issue I have seen with high turn down boilers not condensing is that the boiler is way oversized, and the installer sets the supply temperature, or the reset curve, far too high so the boiler never condenses. If the boiler is more "right size" it might still condense a bit when the supply temp is set too high, but only because it will not reach that specified supply temp during a call for heat. With an oversized high turndown boiler it will go to high fire and hit the "too high" supply temp almost immediately. This is just what I have seen going to many jobs to look over what installers actually do in the field.

As far as the boilers not condensing on low fire? I mean I am no scientist but I have been to hundreds of installs and physically watched the burner at 10% (where possible) and can SEE the condensate so I am calling BS on that.

mattmia2 · 4:30PM

https://forum.heatinghelp.com/discussion/comment/1846819#Comment_1846819

Most boilers have a way to limit the max firing rate and usually some other settings to make it do this less.

Turn Down and Condensing

Comments

Categories