Maximizing condensing abilities

Brad White_153

The delta-T between return temperature and flue temperature is certainly part of it, but while technically essential that is almost incidental. The cooler the HWR you can achieve, the cooler the flue gasses, it follows.

It all starts with how cool a return water you can achieve while meeting the heating requirements. This in turn means sending hot water out to the system (radiators, emitters) just as warm as will do the work.

130F is borderline- so stated because it may or may not condense depending on the CO2 levels in the flue gas. 128F or lower has been noted.

The "130F" edict is too close for comfort in a non-condensing gas or oil system so it is stated to stay a bit above that.

When you <i>want</i> condensing, the low 120's seem more realistic. 128 to 130F may <i>start</i> the condensing process but it may not be so thorough.

To ensure a lower return water temperature surplus of radiation helps a great deal. The best of all worlds is sufficient raditor surface which will heat the house using water all below the flue gas dewpoint. Radiant floors for example.

Next best thing is, if you are supplying water above the dewpoint temperature, returning it well below the dewpoint temperature. Recirculation such as using a 3-way mixing valve to give another pass or two to extract heat may help. A buffer tank may also be of benefit.

Keep in mind that if the system delta-T is too wide, your average water temperature suffers. The AWT is key to output. Half will be higher, half lower.

If your radiation is lean, tight to the heat loss but requires 180 degree water on the coldest days (and adding to it is impractical), it will take deep reset to get you into condensing on the average winter day. You just will not get condensing when it is very cold out. Missing some benefit but not all.

Many strategies, but it all gets down to sending out water just warm enough to do the job and take it back as close to room temperature as you can get it.

T Towne

Maximizing condensing abilities

Hey Guys

Just had a new Knight 80 insalled. Wondering how the condensing functions. I have read that it is based on delta T between return temp and flue temp. Is this correct? HVAC guy said return temps must be below 130 but it seems to me it would condense as long as the return was lower than the flue temp. Could use some clarification. Thanks.

T Towne

Brad

Very informative, thanks much! I have one zone which is fairly short (one 6 ft baseboard then going to radiant under bathroom floor then as well as in the wall). The Kitec under floor and in wall is probably 150 ft long after the basebord. This is a tough one because the water starts out with a decent delta t then quickly equals out the return to the supply. This runs for 5 minutes then boiler shuts down. The other zones maintain a decent delta t. I'll probably add more baseboard so I can keep temps lower. Thanks again.

Boilerpro_5

one thing to add

Is if you can increase the delta tee across the system, you can still run high supply temps and have a low return temp. The typical U.S. design of 20F delta tee in our systems, and only 10F in most pex radiant systems, is not a good match with a condensing boiler. In reality most systems run well under these delta tees due to recently discovered widespread errors in piping reistance charts. Opening up the delta tee on typical systems to 30F or greater and on radiant to 20F, like that typically used for EPDM tubing radiant, makes sense to maximize condensing. To do this on typical baseboard system that is tightly sized may only require adding some baseboard to the last few rooms on a loop and then running the pump slower. Also, I have found that it is quite rare that baseboard systems need 180F supply at design outdoor temp. I just got off the phone with a customer that I installed a Prestige for this week and had set the stats for 80F before I left to keep the system constantly on. The reset curve was set with a design supply at 160. The house still hit 75F within a few hours after I left. There have been no significant thermal improvements to the home since it was built in 1938 and the system installed.
Start by setting the curve lower than you expect that it can keep warm, and turn all the stats up and see what happens. If the house overheats, turn down the reset curve. Not enough heat, turn up the curve a little. If checking on a rather calm day, give the system a little extra water temp to deal with the additional heat loss of windy days.

Boilerpro

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