Cast Iron Heat Capacity

[bpirger]

If someone would check my simple arithmetic, I'd be most appreciative.

My oil boiler is a Buderus G115 which I have currently plumbed in series with my primary loop, and my Garn water comes in via a secondary loop (well, after the HX isolating the garn water).

The G115 3 section is quoted to weigh 330 pounds. I believe the specific heat capacity of cast iron is 0.11 BTU/LB*F So, to raise my G115 1 degree should take about 33BTUs. To raise it 50 degrees should take 33*50, or let's call it 1500BTU/s. Now that is really an insignificant amount of energy, given say 160 degree water flowing through the buderus, it should be up to temp pretty quickly. 10,000 BTU/HR at 1gm with a 20degree drop. So if I'm pushing 5gpm through the primary, and the 007 as the primary circ should be more than this, I'd say the cast temperature should rise pretty quickly...matter of minutes. Do you agree? Am I missing anything?

I have the Riello BF3 direct vent burner (or is it OC3...no matter) on the Buderus, so the stack loss should be really minimal...

I see my water temps inside the house follows the Garn water quite closely (within a few degrees) up to about 135 or so. As the Garn water rises, up to 180 say, the warmest I see in the house at the moment is 162F. That's just nuts....

I think the problem is my old buried 1" pex lines I installed long ago, inside a 4" conduit, with the little sytrofoam pipe insulation on it. I have microflex burried to replace these lines, and 1.25" to boot, but I haven't hooked that part of the system up yet. Only so many hours in a day...

I just want to rule out that the Buderus cast iron mass could be taking a huge amount of heat to come up to temp, thus robbing my hot water flow. It only burns at .6 gallons/hour, or say 70,000 BTU/HR, and it can heat up fast....minutes....so I don't think this is it.

I wonder how much water I have in that 4" conduit, surrounding my 1" pex, sitting there at about 125 degrees? LOL I suspect that is the issue...and will know a whole lot more when my temp sensors arrive from overseas! Hopefully over the holiday I will measure the temps of the pipes in various places and know where this big drop is. Maybe that will inspire me to change to the microflex that much sooner.

Happy Thanksgiving to All!

Thanks,
Bruce

 

[benjamin]

Bruce

Your math looks good to me. The water inside of the oil boiler likely holds much more heat than the cast iron but that is irrelevant as you seem to have concluded. Even if it were a ton of cast iron and water it shoudn't soak up that much heat and wouldn't continue to lose that much heat unless it was going up the chimney or into unheated space somehow, which doesn't seem to be the case.

You don't need fancy temp sensors to nail down this big of a heat loss. You can confirm your suspicions with a pair of meat thermometers or a noncontact thermometer. The only bright side of this situation might be that the heat that is lost from the garn to the house might be going mostly to the water returning from the house to the garn?

 

[Nofossil]

Your basic approach seems sound. You can lose a fair amount up the chimney through an idle boiler in some cases, but not likely enough to cause the problem you're seeing. I think you're on the right track with the buried lines.

 

[bpirger]

Thanks guys! I agree no sensors needed for this....I can likely use my bare hand! But I look forward to having the sensors to try and really understanding everything that is going on.

I've noticed a couple of funnies....I'd like to fully understand.

8 gallons in the Buderus....so not a whole lot of water either...

I should stick a snake down into that 4" conduit and see if it comes back wet....

Happy Thanksgiving to all!

 

[Willman]

I should stick a snake down into that 4” conduit and see if it comes back wet….

Hook up a shop vac to conduit, youll see how much water is in there real quick. And the heat loss from the water will be gone and you can see if it improves your situation.

Will

 

[slowzuki]

Probably the biggest problem with water in the conduit is your in and out lines are a big counterflow heat exchange. In air, and with water around them, there is a big change in the intentional heat exchanger performance.