HWBB with 140° max water temp

Unknown

Brad, thanks for the clarification. Sometimes I'm not clear on whether people are referring to output, return or average temps in their postings. As you say, it really depends on the situation, and whether condensation is the desired result or what you are trying to avoid.

In any case I guess the point I was trying to make is that the return temp is what is important, given the differences in design temp and delta-T of various installations.

The quality of the actual burner appears to also be an important factor. A combustion system which can burn clean with less excess air is at an advantage in allowing condensing at higher temperatures. This is a point made by Viessmann in their literature, and is probably used to advantage in the Vitodens to increase efficiency.

The graph is also interesting in explaining why you can get better efficiencies without condensing with oil than gas.

Mark Hunt

Proof it works

Some of you may recall a job I posted about back in the fall. A GB142/24 was installed with HWBB and a small radiant zone to replace a FHA system.

I sized the baseboard for 140° max water temp at design day of -10°.

I got an e-mail from the folks this morning letting me know that they system works like a charm AND we have not actually hit design day conditions yet which means that the boiler has been operating below 140° since it was turned on in October.

Mod/con with HWBB works. It can be done.

Mark H

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Brad White_9

But of course it does!

As the folks at Brookhaven demonstrated in their study (Dr. Butcher), it can be cost effective even when the BB is sized for 180 F. EWT. given the number of hours when ODR brings the water below condensing.

That you expanded this to cover about all hours is to your credit. Happy clients!

Thanks for sharing- what a nice, neat job that is your standard.

Mike T., Swampeast MO

Did you use the RC-10 control?

Mark Hunt

No


I used the AM-10 control. It uses od temp.

I never liked the RC-10 when there were multiple zones. Trying to pick the "coldest" zone to be the master was a PITA. Musical thermostats so to speak.

I have another HWBB home starting this week. It will also have two small radiant zones. Max water temp @ DD is.........130°.

How low can you go????

Mark H

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Mark Hunt

Thanks Brad


We have quite a few of those systems out there as well. HWBB sized for 180° still heats a room with 110° when the OD temps are "mild".

Ooooo.........I think I just felt my carbon footprint shrink.

Mark H

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Uni R_3

You should test it if you get a chance!

Mark,
If you sized the baseboard for 140° water at -10°F OAT, I would not be at all surprised if it only needed 125°F or less.

**********************************

With Siggy's book and Slant-Fin's software, my temps were calculated to be 154° @ 0° OAT with 68°IAT. Given the fact that the average for January and February here is 25°, I didn't figure I'd fall below condensing range very often. This included still having 4K of heat from the kitchen which was disconnected during the install in preparation for radiant.

Even missing the equiv of 4K out of 56K, and having to run the house at 70/72 (70 in the kitchen or the floor feels too cold) instead of the 68° we've used forever, I still am only using 138° water @ 2°. This was derived by putting the house directly on ODR. I'm sure that with the other emitters not having to take the portion of the load for the kitchen floor and getting the house back down to an even 68, it will be even lower again.

There's simply way too much fudge factor in heatloss numbers!

My house is supposedly 56K (without counting the joist areas, using tight inflitration numbers, etc.) and at 2° OAT I'm consuming about 36K **gross** (less than 2/3rds) with the inside temps jacked up.

Maybe I should crack all the windows open 1" and see if that's what's needed to get up to the heatloss numbers?

**********************************

If it's not propane, you should get them to clock the meter when it's really cold. Compare that gross number to your heatloss which is a net number.

Unknown

I've got a couple at 120

You can go lower still, as long as you've got the wall space. The output capacity of the baseboard itself makes a big difference too.. panel trim seems a bit stronger than those hayden units..

Don't stop mark, limbo that temp down to the floor!!!

Mark Hunt

I thought about that Uni


and the customer is close enough(2 mins. away)so that I could play around with the curve.

These folks are on LP, so no meter reading but they will be able to see what they used in gallons.

The customer said in the e-mail that she did have one "complaint". She said that the old FHA system "lulled" her to sleep. The new system is silent. LOL!

Mark H

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McKern

Ummm

I sure hope she didn't have any CO assist.

Yanno... I actually miss the sound of my Riello. Mind you my modcon roared for its first 3 weeks. Bad flowcheck placement - why the supplier was allowed input I have no idea. Maybe little LED on/off lights near the computer wired with the pumps and fan... ;-)

John Ketterman

I don't understand...who said HWBB doesn't work at 140?

Of course it has twice the output at 180 compared to 140. That's not just HWBB; CI rads and panel rads are also the same. But you can put in lots of CI rads or panel rads, whereas with HWBB you are limited by the perimeter of the room. That's the only issue.

Apparently you were able to fit in the necessary HWBB to heat the room with 140. If the space is well-insulated new construction, no problem. In an older house, problem; you may have to run HWBB up and down the middle of the room.

Mark Hunt

Bryan_16

ratings

Were you able to get output ratings of your baseboard at low temp? I tried to find this info awile back and all anyone knew was "it's rated at 180".

[Deleted User]

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Troy_3

HWBB low temp

Mark: I moved into a house built in 1975 last winter. Existing baseboard with ci boiler. This fall I changed to a Mod/con with the max temp at 149deg at 0 deg od temp. It has been in the single digits all week. the house has been between 63-68deg. I know many customers wouldn't accept this but so far I've convinced the family it is all they get. I'll say look the boiler is still running. In fact I don't think it has been off since last week. I will have to add some emitters when I get a chance. I'm very surprised how well it has kept up without elevating the water temp. I'm still condensing a ton at these temps.

Unknown

Real life condensing temperatures

I wish the modcon boiler manufacturers would explain to installers the temperature at which their boilers will really condense under normal operating conditions. 140F seems to be the temp most often seen, but I have a feeling that it is actually much lower.

Looking at the burner setup specs of several of the modcons, they all seem to be recommending settings which result in exhaust O2 levels of 6% to 7%. This translates to about 50% excess air relative to stochiometric combustion. I assume this is being done to reduce CO levels in the exhaust to acceptable levels.

Condensation temperature depends on how much excess air the exhaust gas is diluted with. A few years ago, Glenn Stanton posted a graph of condensation temperatures vs. excess air which I have attached. According to this graph, at 50% excess air, condensation doesnt even begin to occur until 127F. It appears condensation doesn't occur at 140F unless the gas is burned with ZERO excess air, which just doesnt actually happen in real life.

The point of all this is if you want to design a system for maximum possible efficiency, it looks like you really want a return temp of less than 127F to insure fully condensing operation.

Brad White_9

The 140 degrees, Mike

so often referenced (I know I have for it is my ModCon default basis in design) refers to the supply water outlet temperature.

The emitters are selected typically for 130 degree average water temperatures, sometimes lower (20 to even a 40 degree Delta-T).

This way your return water temperatures are indeed below the 127-128 degree threshhold you cite.

As you note, the variables dictate. Higher outlet temperatures or even higher return temperatures of 130, 135 or even 140 degrees are given to assure a margin of safety when one does not want condensation, such as in a conventional CI boiler.

When I use return water temperature control, I use 135F as a setpoint.

Does that make sense?

Brad