Hot Water Flow Curiosity...

Mike T., Swampeast MO

Behind the HX coil and directly in front of the burner there is a flat, water-filled "jacket". The return enters into the jacket. Jacket isn't all the way around--just directly in front of the burner at the end of the coil.

Par64 1k

Flow Direction & Effects

Thank you in advance for keeping up this very informative site.

I am not a heating professional. I am an electrician and have been exposed to troubleshooting heating control systems for both residential and commercial applications. The following thought has crossed my mind several times and your site seems like a good forum to post it to. Here goes...


It seems obvious that the pioneers of heating technology would rely on the effects of gravity to induce flow into their systems. Cool water in at the bottom passing through the heated cavity of the boiler itself only to emerge as hot water out at the top etc...

It also seems obvious, with the introduction of the circulator pump, the continued adherence to this understanding of flow was kept. Existing gravity systems could be converted but were probably held to the original flow direction of their piping. New systems designed with the pump adhered again, in at the bottom, out at the top. Which brings me to my question.

In a pump-circulated hot water heating system could the heated water leaving a heater be hotter if it exited from the bottom of the unit instead of the top, depending on the specific design of the unit.

The idea being, at first glance, that the hottest part of the unit would have to be, 1st the fire itself, at the bottom, then the closest surfaces to that fire and finally the furthest surfaces from that fire would have to be the coolest overall.

While a heater is lit and circulating it seems that water exiting from the hottest area (closest to the fire) could be hotter than if exiting from the cooler area (furthest from the fire). Thermal shock might also be eased by cooler water entering the top of the unit which is the farthest from the fire and thus cooler overall. And this might result in a lower required fire temp, if even only by a small amount, to produce a similar amount of heated water.

With the same heater not lit or circulating it does appear that gravity would ultimately take over and begin to reverse a small flow of water, in at the bottom, out at the top until it either cooled completely or was called upon again to produce heat for the household, at which time the hotter water collected in the pipes near the top of the boiler would flow down into the unit easing the initial thermal shock of the next heating cycle and so on.

Are there units like this in the world?

What if any are the benefits and/or drawbacks of such a design?

Thanks again for all of your thoughts and expertise on a simple curiosity such as this.

Boilergeek

down fired counterflow design
look at www.hydrothermkn.com
see also www.mzboiler.com

Brad White_9

What you are describing is called counterflow

Counterflow is essential to the condensing boiler process not to mention legions of other heat transfer functions.

In a conventional boiler indeed the cold water enters near the bottom and exits near the top. Theoretically this is where the hottest water resides (especially as you well pointed out, in a gravity hot water system).

But this exit point also coincides with the coolest flue gas temperatures, just before leaving the unit, after which it will do no more work as far as the house is concerned.

In a condensing boiler, the flue path is reversed, flame at the top and exiting or at least passing by the bottom. This is where the coldest water entering meets the coldest flue gasses leaving. The "cloud" of combustion products and vapors then can collapse -condense- and wring out the most practical amount of heat. Conversely the leaving water sees the hottest flame for that little bit o' English on the ball as she leaves. A fair send-off to heat the home.

Of course the condensing boiler is made to take the potential for corrosion and shock.


If you were to reverse the process in a conventional boiler, I suppose you might enter the boiler with colder water versus colder flue gasses and condense on the surfaces. Cannot be a good thing..

In some systems, if you do not use counterflow you can at the end of a circuit, reverse the flow of heat. In airside fluid to fluid recovery coils with narrow delta-T's for example, absent counterflow you can positively cool the leaving air as opposed to heating it.

Mike T., Swampeast MO

Am fairly certain however that the return is still brought in nearest the burner. The "real" counterflow seems to be the flue gasses, not the water itself.

Since heat transfer is driven by difference in temperature you want want to bring in the coldest water where the flame and/or gasses are hottest.

Brad White_9

Only in a laboratory

I agree that seems how it should be Mike, at first anyway I used to think. Yes the counterflow is relative to both water and flue gasses.

But consider that you are talking about the extremes of water at say 100F degrees returning and flame-hot gasses at 2200F. Almost academic when the supply water temperature is only 20 to 40 degrees warmer than the return. What is a score or two degrees compared hundreds of degrees differential between friends?

But when I have flue gasses approaching dewpoint, hovering on the brink of collapse to liquid state, I would be hard pressed to make the final state-change with water that might be 140 degrees or warmer. So the colder return water makes the difference.

And the leaving water? Off she goes entering the "final afterburner", 140 or so water seeing the 2200F hottest flue gasses (via a heat exchanger let's be real here ) on the way to your radiators.

Does that make sense?

Brad

Mike T., Swampeast MO

Yes, that makes perfect sense Brad. Thanks. Only boilers where I've personally seen the guts are either old and traditional or the Vitodens.

Do any condensing/modulating boilers other than the Vitodens have a return water jacket or is that because of the unique radiant burner?

Brad White_34

When you say

return water jacket you mean all combustion is surrounded by water? My MZ I think has that, the top part being aluminum and the bottom, well, no, you are right, I think the stainless portion is water-tube, combustion side out. Mmmmm. Don't know. But I like the idea...