How efficient should I go?

Truewill · October 2010

The 32 year old Burnham boiler in my house is giving signs of needing replacement. I had a great talk with a technician at Burnham who said because I have hot water baseboard radiators fed by copper piping that it made no sense to install a 90+% efficient furnace. He reasoned that heating the water to 180 degrees made 90+% efficient heating impossible; said "it won't condense." (I didn't exactly understand that last part.) But he did say the combination of getting a properly sized boiler (smaller now than in '79 because of add'l insulation, new windows, etc.) and raising the efficiency rating from 65-70% to 85-86% would likely result in nice savings.

However, I had a service tech out here--the one who diagnosed the failing valves--who said they install 92% efficient Lennox furnaces all the time. Now, it's not unheard-of that a contractor would sell a more expensive and larger unit than required, so I am hoping for another opinion. Right now, the tech who 1) wouldn't see a dime from the job, and 2) represents a company that makes both types has the greater credibility.

I am eager to hear what you have to say.

CMadatMe · October 2010

Swamp Land

That tech is feeding you a piece of land that can't be built on without doing a heat loss of your home. Any qualilty contractor would never make such an assumption without doing one and all quality pros know that 90 percent of older heating systms are oversized not just in the boiler but in the homes radiation. This is why an updated heat loss on your home is a must.

A condensing boiler is one that extracts the excess heat that normally escapes throught the venting system. This allows the extraction of normally lost btu's and what gives these types of boilers the added efficiency.

To be able to extract these wasted btu's requires the installer to use much lower supply water temperatures than what you are currently running. Your system today is likely running 180 degree water everyday throughout the heating season. You may only need this water temperature or even less on your regions coldest day. Not when it's 40, 30, 25, 20 or 15 degrees.

A condensing boiler regulates the supply water temperature based on the temperature outside and the temperature needed on your regions coldest day. The reason a heat loss is needed is so we can ascertain how many btu's you would need on that coldest day. We then calculate the supply water temp needed based on the amount of radiation you have existing in the home.

Once we calculate these we are able to find a heating curve for your home. Though my 20 years experience and I'm sure the rest of the pros here will agree you most likely will be able to obtain 90 percent efficiencies most of the entire heating season if the system is designed correctly. Attached is a nice little read that may help you to understand this more.

NRT_Rob · October 2010

fixed fire conventional boilers

are dead.

modulating, sealed combustion units whether they are condensing or not will run circles around most of them for efficiency.

with outdoor temperature reset built in, as Chris said even on a "180" system they will USUALLY be far below that and condensing.

If you had a huge water content system maybe that would be different. But with baseboard, you want a mod/con.

Brad White · October 2010

Chris has it right here

The Brookhaven study by Dr. Butcher confirms what we always knew, that aggressive attention to outdoor reset can leave you with many hours where condensing efficiencies will occur, even if you still need 180 F. on the coldest day.

A second view of this is, even IF the house needed 180 F. originally, subsequent insulation, residing with Tyvek or other air barriers, storm or replacement windows, air sealing, etc., your coldest day will require cooler water still.

It often works out (not as an absolute, every house is different, but in general), that 140 to 150F becomes the "new high temperature".

If you are running the water at a true 20 degree drop, you will be in or close to the condensing range on the coldest day and lower still when it is warmer than that. The number of hours you need the warmest temperature are barely 2% of a winter and half of one percent of a full year. That leaves a lot of hours for efficiency improvements.

Yet another view is, if you have a relatively new CI boiler, you can at least make your radiation-side temperature have deep reset, even if you have to keep the boiler hotter to protect it. Point being, you have options and all of them are good.

I am glad Chris had a copy of that study. The abstract says it all, but it makes for good reading for a deeper understanding of the principles and variables they dealt with.

CMadatMe · October 2010

Brad If you like that one

This study rienforces how many oversized systems there are out there.

How efficient should I go?

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