baseboard radiators and condensing boilers

Ron Schroeder

Condensing isn't just an all or nothing thing. Even with BB sized for 180F at the design temp, with outdoor reset, you will get SOME condensation during at least 75% of the heating season. With the typical BB on all of the outer walls, you will get condensation over a greater percentage of the season. I would enable Outdoor Reset but set the boiler minimum temperature at the lowest that is still comfortable for the air handler. You can oversize the coil in the air handler and get by with a lower water temperature too. Just don't set the blower CFM too high.

Ron

Steve Goldstein

baseboard radiators and condensing boilers

I had an HVAC contractor out to my house yesterday, and he said something interesting that I want to verify. I currently have a 25-yr-old Utica Boiler supporting 6 zones (one of which is a Mega-Stor stainless tank for DHW). The radiators in the rooms are the standard copper fin short baseboard radiators.

Right now my house is being heated by both the boiler and also a forced hot air furnace. My goal is to not only improve the house's thermal envelope (by sealing air leaks), but to also replace the FHA furnace with an air handler that will hook into the boiler and the FHA ductwork.

I was asking him about replacing the Utica Boiler with a high efficiency condensing boiler. He asked if I was planning on replacing the baseboard radiators too, and I said that I wasn't. He then said that a high-efficiency condensing boiler wouldn't be a good idea, because my baseboard radiators were designed for a boiler temperature of 160 to 180 degrees, but that condensing boilers are in their condensing mode at a lower temperature than that. So if I run a condensing boiler at the higher temps that my radiators require, the boiler will be running too hot to condense, and as a result it won't be running in its high-efficiency mode. So he was suggesting that I don't bother with a high efficiency condensing boiler, because of my radiators, and go with a mid-efficiency model instead (he's fond of the Viessmann brand).

Does this sound accurate? Do condensing boilers need to run at lower temps in order to run at their highest efficiencies?

Tony_23

Call someone else

Bbd doesn't ALWAYS have to run at those temps, and a mod/con is STILL more efficient at it's lowest than a mid-efficiency CI at it's highest.

Ted_9

condensing

Well, he's right but there is another part to this.

The copper fin tube may work better in your house at higher temps. But, You dont need 180 degree water to heat your home every day. Using a condensing boiler with outdoor reset will still heat your home and have high efficiencies. Copper/fin still puts out btus at lower water temps.

A cast iron boiler with outdoor rest is still a great way to go.

What you really need is an accurate heatloss and design. A good contractor will size each room for the heatloss and figure out how much the existing radiation will put out. Then, the contractor, can figure the system supply water temp for the copper/fin and the hydroair unit. You may need two different water temps for your system.

After this is done, the contractor can tell you which boiler makes the most sense.

Happy New Year

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clammy

In agreement

I,m in agreement with the other posts .I have a few jobs using munchkins with outdoor resets and i sized all my baseboard for 160 at design day of 15 degrees outside 70 indoor .I also used slantfin 80 to boot .One systems rarely runs at 160 and it operates at about constant circ because of proper heat lose and sizing .Even at high temp condensing ,modulating boilers save fuel, stop short cycling espically with mirco zoning which is like a illness .The switching the forced hot airfurnace to a air handler with a hot water coil is a good deal .Go to the find a pro listing and contact some one from there they all are intellent and know there business .Also get a heat loss for your home you may be suprised and if you are doing all this go with a indirect domestics hot water tank you'll save some coin on domestic water production to boot good luck peace and have a happy safe new year peace clammy

Paul Pollets

He's right

The contractor is right...the modulating boiler will not be in condensing mode most of the time, unless the baseboard is oversized. The air handler HW coil will require a min. of 150-170 degree water...whenever it demands heat. Modulating the coil temps means you'll get cold air out of the ductwork...not very comfortable.

I usually specify a Vitogas 100 cast iron boiler for this application with the Vitotronic 200 control. The additional cost of the condensing boiler will be hard to justify. Yes, it can be done, but I suspect it will be in condensing mode less than 10% of the time.

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Tony_23

10 % ?

I don't buy that for a minute. Design temps account for less than 5% of a season, statistically. That's a whole lot of heating season at lower water temps than 180.

I agree about the AH supply temps needing to be consistent. I shoot for 140 avg. if I can. The thing is the water temp wouldn't modulate w/o ODR, but the burner input would. When my Ultra 1st comes on it ramps right up until the target temp is reached, then it modulates down to maintain that temp. The same is true of my customer with an AH, and all the rest that aren't on ODR.

BTW, every Ultra I've installed is still condensing at 160 deg., just not as much as at 120

Paul Pollets

How

can a boiler condense at 160?

Presuming an air handler HW coil will make a demand to heat as often as the baseboard, when is the boiler going to be running at low temp?

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Tony_23

160

Is outlet temp. 140 return. Don't believe me, try it yourself and see. Others here have seen the same results.

Since the boiler isn't storing any sizable amount of hot water, the front end of a call for heat would be at lower temps. And on a warmer day the load is less, satisfied quicker, and target temp may not be reached on bbd. On fancoils I like to wire an Aquastat to cycle the fan after the coil is up to 140 supply. If it's all bbd I leave the Ultra's boost feature enabled w/o ODR. This gives the system a chance to satisfy the load before water temps get too high.

[Deleted User]

Agree with Tony

We've been in our house 2yrs. The house is 1700sqft two story with fin tube baseboards and it is not zoned. I done a heat loss calculation, 48,056 btu's. The boiler is 18 yrs old and grossly oversized, 140k btu's. I installed an Ultra 105 with an indirect plus 40. Zoned the house upstairs/downstairs with two spare spots for additional zoning down the road. I throttled the Ultra 105 back 40% so it's output is around 63K btu's and running max baseboard temp of 170 degrees. At these water temps my condensate pump is running a good amount of time. My gas usage last year per month average was 100ccf, with the Ultra 105 I'm using an average 67ccf a month. And Unlimited amount of hot water to boot!!

S Ebels

True and False

True in the sense that they have to run a lower (<140*) to attain their highest efficiency. False in the sense that they aren't worth it if you have a "high temp" system.

Short answer, I wouldn't put in anything BUT a modulating, condensing boiler.

At their worst, they are more efficient than the best "standard" boilers. With the use of outdoor reset, 100% of the shoulder seasons will serviced by lower temps allowing the flue gas to fully collapse, or "condense". During the coldest parts of the winter, when you are at the rare scenario of "design temperature" you will still be operating in the upper 87-89% ramge.

One caveat, You need to have your contractor do a good heat loss and then carefully measure up your actual feet of "active" baseboard. This will ascertain whether this type of setup can be used successfully in your application. If your BB is on the skimpy side you will find that more of the heating season will need to be above condensing temps and maybe beyond the reach of the boiler itself. A Viessman condenser would be my first choice. If you do this, DON'T OVERSIZE IT, PLEASE!!!

Mark Hunt

You say it best!


Mr. Ebels lays down the truth.

Listen and learn.

Mark H

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Mark Hunt

Very good!


Good post!

Mark H

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