Low delta T question


Old 12-28-07, 08:46 AM
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Low delta T question

First, thanks to all who have previously helped me out on my new boiler project. I'll post some pictures when I get my camera back from servicing - just too busy lately to pick it up...

We have wonderful heat now. We went from a gravity fed monster to a tiny little boiler that hangs on the wall - hard to believe. The heat regulation is wonderful - the ODR and modulating boiler seem to make all the difference in the world. Believe it or not the system is quieter than before - because there is less cycling of heat, the pipes don't clink and bang at all. The temps here swing wildly but the house remains very comfortable since the new install. Stage I in my conversion from gravity fed to circulation seems fine. When time/season allows I will break the system into zones - but for now things are so much better than before it hardly seems worth it. My only gripe is the 3rd floor rads dont get as warm as they did before but that area has supplemental forced hot air so it's not a problem really. I'm sure they were piped to take advantage of the old gravity system.

So here's my question - my delta T seems low - generally around 10 degrees. I guess you can attribute that to the primary/secondary loops and giant mass of water in the old 3" and 4" piping and all the standing cast iron rads? Is this situation okay for the boiler? Everything else seems fine. With the outdoor temps they way they are the unit seems to want to keep the water temp between around 130 to 140 degrees. Maybe that's low for the old cast iron but it sure feels more comfortable than before. It hasn't really gotten cold here yet for any extended period of time so that may kick up. It's been down in the teens and the house has been toasty warm and it feels like a more even heat distribution throughout the house. Maybe it's just me...

The boiler is a Knight 105 mod/con. Primary and secondary pumps are Grundfos 26-99. I've kept the primary at med speed (boiler whines a little at high) and the secondary at med (totally silent, some slight rushing water noise at high).

The boiler modulates somewhere between 30 and 50% a lot of the time during the day.

I'm currently renovating a bathroom and kitchen so 2 rads are disconnected - but even those areas feel pretty warm right now. It's an old brick home and I think the whole thing sort of heats up throughout - plus I've added tons of insulation and new windows.

I just don't know if this is normal and I don't want to do any long term damage to the unit.

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Old 12-28-07, 11:33 AM
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The short answer is that a low dT won't hurt anything. There's no magic to the 20F "standard" delta-T.

You are probably right that the huge water volume and the mass of the cast iron rads and piping take a while to give up heat.

Let those rads run as cool as you can get to maintain space temperature. That evenness of space heating and consequent increase in comfort is the result of the ODR aligning more closely with the heat loss at the current outside temperature. The space temp fluctuates less, so you don't feel the swings. You will probably be amazed at how low you can adjust the reset curve and still hold space temp.

Enjoy your new level of comfort (all that insulating and new windows helps too). Once you get your zoning set up, then it's time to start playing with optimizing temperatures, flow rates, etc.

Congrats on what sounds like a great installation.
Old 12-28-07, 05:54 PM
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A delta T of 10 is just fine and quite normal (I've had to jump through hoops just to get mine up from 6 to 8)... you need some velocity in a gravity system to keep things even, and that velocity means the heat doesn't get off the train the first time around.

As xiphias stated work on lowering your ODR curve. You want that water temperature as low as you can get it to maintain indoor temps.
Old 12-28-07, 06:19 PM
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Most people don’t understand what happens with delta T. It changes constantly. If you design for a 20º delta T you only achieve it at design water temp. If you layout a baseboard system, do the heat loss per room, calculate the amount of baseboard at 180º water temp and install it. Turn on the system. The delta T will vary as water temperature varies. When the average water temp in the system is 180 is when you achieve the 20º delta T. That is why your old VI system did not have a 20º delta T. Of course it also has to do with the proper flow, and the above explanation assumes proper flow.

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