Question about Mod/cons on old gravity systems

steve_25

Hello everyone. I am a plumber in northern New Jersey. Sandy has flooded my basement and I need to replace the boiler ASAP. It's a cast iron boiler on an old gravity system with a B&G 100 on the return and it's all one zone. I was planning on updating the whole system in the spring with a modcon, separate zones and primary secondary piping. So now that I have to change the boiler immediately here's my question. If I were to just replace the old cast iron boiler with a modcon using the same supply and return pipes and a new B&G 100 would the system work for now? I am willing to sacrifice efficiency for safety until I can update the rest of the system. My current boiler sits in the middle of my basement and feeds 2" horizontal runouts that feed 1 1/4" risers at the front and back end of my house. Each set of risers supplies one large cast iron radiator per floor. It's a rowhouse with three floors for a total of 6 radiators. Any advice you could give would be greatly appreciated. Thanks.

Al

Rich_L

Mod-con efficiency

Sorry to hear about your flooding Steve. Modulating condensing boilers get their highest efficiency with the coolest return temps. This makes them perfect for snow melt and radiant floor applications. Part of their high efficiency comes from condensing flue gasses and capturing the heat that would otherwise be going up the chimney. The flue gasses condense when return water temps are at or below 140 degrees. I've gotten away from putting mod cons in homes with radiators due to the higher required water temps to make these systems work properly. You can gain some efficiency in the shoulder seasons if you use outdoor reset. If you're converting from a gravity system to a pumped system there are some special considerations for the piping. Dan addresses them here: http://www.heatinghelp.com/article/332/Gravity-Hot-Water-Heating/72/Gravity-Hot-Water-Heating-FAQ

TonyS

Many modcons

such as the Triangle Tube Prestige do not require primary secondary. Head pressure of a gravity system is figured as zero but in truth its actually negative due to the fact that it circulates itself. Therefore it doesnt require an extra pump for the system.

As far as not using the a mod con, nothing could be further from the truth.

Gravity systems were way oversized for a variety of reasons. Dans books explain this quite well. everything from sizing with the windows open to any innovations to the house since the system was installed have changed the original heat load.

Boilers such as the TT come with outdoor reset and yes there is a quick menu available for lesser skilled mechanics that dont know how to set up a curve.

What you have to do is... do a heat load on the home.... then do an EDR count on the radiators and then figure what temperature water is needed with that amount of radiation to heat your home on the coldest day.

I have found that my gravity jobs never go above 150 supply and return at 130. So they are condensing even on the coldest days of the year.

with an outdoor reset this return only gets cooler and cooler as it gets warmer outside.

I dont know why this myth is perpetuated! Even if your system was designed to operate at 180 and return 160, that is still only the coldest day of the year. 80% of the time it would still be condensing.

Install a TT Prestige...supply in return out.... turn the pump to low speed and set the pump for continuous with wws and radiators will work better than they ever did.

SWEI

condensing and CI

Are a great match.  Read http://www.heatinghelp.com/files/posts/12471/AshraeCondensingtechnology.pdf before you condemn it.

Paul48

Sizing

it correctly is critical. You have to do a Heat Loss. Think small, and don't over-size,"just to be on the safe side".

BigErl

Thank you

Thanks guys for taking the time to respond to my question. I'm not too familiar with modcons so I really appreciate the info you provided.



P.S. My name is actually AL. I have no idea why Steve appeared in the nickname box. Maybe it's a default. Anyway, thanks again.