high temp system using mod con

pat_3

How much more efficient is using a mod con boiler vs standard cast iron boiler when the system is high temp? thank you in advance for any responses.

Tony_23

my experience

40% less gas, consistently. Use ODR, start out with high limit set no more than 180 and adjust accordingly. Most have the high limit as an adjustable parameter by the HO, so if the HO is savvy they can help.

Brad White

I defer to Tony

on the 40% savings and that is encouraging. I would say "25-30% average" to hedge and not over-promise. Let the 40% be the home owner's surprise. There are so many variables you cannot control.



As Tony said, the key is aggressive use of the reset schedule, limiting the higher water temperatures to the relatively few seasonal hours where they are really needed.



Also as Tony mentioned, backing that down below the "proverbial 180" is a good possibility. This is because the system may once have actually needed 180F water but since then, insulation, window upgrades and air sealing have made that less necessary. You very well may have a starting point (high point) of 160F or could get there with basic energy improvements.



The big driver to higher boiler efficiency is modulation, (the Mod on Mod Con), a smoother tracking of firing rate to load over time.



Conventional boilers, what you are replacing, are "on-off" with a lag for mass-heating and loss of thermal momentum when the boiler short cycles in any weather warmer than the very coldest days. I hope that makes sense.

Condensing is just a bonus but there are many hours left where that will occur.



Say that you actually *do* need 180F on the coldest day, say zero degrees design. Say also that your minimum water temperature were 80F on the warmest day when you still need heating (say, oh,  60F outdoors).



You would not be below the condensing line (Water at 125F more or less) until the outside air temperatures rise to the 32-34 degree range, but that is surprisingly many hours where you get that extra 10% efficiency boost from condensing.



If your actual high temperature were 160F and not 180F (all other parameters the same), you would start condensing (reaching 125F water temperature), when it is about 26 degrees outside. Many more hours to enjoy.



One point of clarification: To be conservative for the above examples, I am using the "supply water temperature" as the definition point for condensing. This is to assure that you will condense across the entire exchanger, start to finish. Many condensing boilers are rated (appropriately) based on return water temperatures, often 10 to 20 degrees cooler than supply. Thus you can see how many more hours you will have in the condensing zone.

tim smith

Re: efficiencies on condensing boilers

I am even more conservative than Brad, I tell clients to expect 15 to 30 % and once in a while more,  savings over old boilers. I still find it amazing that some of are 50+ year old boilers still come in at pretty good combustion numbers and also the thermal loss on their jackets in not reeeeeal bad.  I have found the above to hold quite true as I check back with customers over the next few years on their fuel useage and this is what I see. I try to never over estimate as I do not like dissapointed customers. Peace, Tim

Henry

High temp mod/con

Here in the Montreal region, it is much colder and we do run a bit longer at high temperatures. From the onsite tests that the utily has done, it was found that we use 1200 hours of boiler operation. Of that amount around 200 hours are run above 150F output temperatures. So there is definetly a saving to be had with a modcon. The most important aspect is a proper install with the right pump selection to get a proper delta T of around 30F and if possible 35F. Anybody can pipe a boiler, but to do it properly is something else!

We replaced the atmospheric boilers  in 9 identical apartment blocks, all high temperature 180F . The savings were from 35% to 55%. Meanwhile an identical replacement not far away only generated 20%. The difference is due to the inhabitants of the buildings. One leaky faucet can ruin a project.



Henry