It Seems So Odd...

[Rory]

I filled up the Tarm before heading into town with my wife yesterday afternoon around 4. It was a mild February evening, but not unseasonably so. We got home around 8, and, of course, I went right downstairs to check out the heating situation. The boiler was shut down at 194, I didn't bother to check if there was more wood in the firebox or not. My tank was at 182 on top and 154 on the bottom, and I knew I was fine for the night. I'm just now getting it going again, and I'm not sure how I'm going to hit the cycle, but I suspect by early afternoon I'll be done with the fire until tomorrow morning.

I'm looking forward to the weather that will allow me to skip the morning fires, and It looks like it shouldn't be too far off.

 

[cguida]

A more-or-less random thought --

For people with stratified tanks --

Wouldn't it pay you to have some means (say, circulator-timer-switch arraingement) to "homoginize" or mix up the water in your tank? Suppose with a little mixing you could get 175 degrees all through the tank -- top to bottom. Wouldn't that increase your tank-heat-holding capacity?

I realize you wouldn't want this under all conditions. But when your boiler was putting out surplus heat, wouldn't it be helpful to have some place to store the excess, rather than idling the boiler?

 

[Rory]

A more-or-less random thought --

For people with stratified tanks --

Wouldn't it pay you to have some means (say, circulator-timer-switch arraingement) to "homoginize" or mix up the water in your tank? Suppose with a little mixing you could get 175 degrees all through the tank -- top to bottom. Wouldn't that increase your tank-heat-holding capacity?

I realize you wouldn't want this under all conditions. But when your boiler was putting out surplus heat, wouldn't it be helpful to have some place to store the excess, rather than idling the boiler?

I was thinking the same thing, or thinking that as the tank heated up it might work better to be putting the hot water in at the bottom rather than the top. But after considering it, I decided that the principle of heat moving from higher to lower concentration is probably going to work regardless of which layer it sees first. In other words, until the entire tank temperature exceeds the boiler limit, it's probably not a limiting factor.

 

[steam man]

This forum has discussed stratification at length. There is an advantage to it. I have a 120g tank for my solar DHW. I have measured a big difference in thermal layers from top to bottom. As the heating coil is in the cooler bottom part, the cooler water makes the heat exchanger much more efficient by having a bigger delta-T. The main reason for stratification in heating systems seems to be to get a higher useable temperature which is going to be at the top of the tank. The average mean btu holding capacity of the tank whether mixed or stratified should be about the same but the higher temp at the top of the tank will allow most heat emitters to work better as well as the heat exchange to the tank. One example of this is in an unpressurized tank where the coils will be typically in a counter flow arrangement where the the heat in and out are reversed for efficiency's sake.

Did I get that right?

Mike

 

[cguida]

On sover reflection, in the clear light of day... I think you're right. If the hot water in the coil enters at the bottom coolest part of the tank, that will always be best. Mixing wouldn't help. In fact it would be counter-productive since it wouild lessen the temperature difference between the two waters.