Flow of circulation thru heat exchangers

[new england tommy]

I did some searching and while I thought I read it on here last year, I cannot find this information.

I am installing a couple of crude heat exchangers (old cast iron radiators) parked next to my Hearthstone one to assist in heating the cold return on a FHW two zone home. My idea is that they pre-heat some water and dump into a 40 gallon rock lined tank so when the zone calls, I have a little stored hot water ready, hopefully to delay or negate the oil burner kicking on. I may do a copper coil around the stack pipe to boost the temp, but I don't want to push pressure up too high to worry about anything. Don't worry, I will have a temp and pressure relief and a spot to plump in expansion AND if I am lucky I will plumb in a heat dumping loop on natural thermosiphon.

My question is do I send cold in the top or bottom of the radiator. I know heat rises but I thought I remember reading a few posts suggesting that forcing heated water down (by flowcheck valve at the beginning of the loop) will create a directional flow as desired.

My other question is: should the "heating pickup flow", (flow next to the stove in the crude heat exchangers, ex. above) work in the same direction in the 40 gallon rock line storage tank? Should the recirculating loop off of the tank come of the bottom or the top?


Thank you

 

[new england tommy]

wow, is this idea that stupid or nobody has an opinion on fighting physics to create better heat exchanging??

 

[maple1]

Afraid you lost me on the rock lined tank.

 

[jebatty]

wow, is this idea that stupid or nobody has an opinion on fighting physics to create better heat exchanging??

... might this be understood to be a little rough on the volunteers who participate in this forum?

 

[ewdudley]

I guess first of all, what are the emitters of your FHW zones, and how 'cold' is the return water from them?

If parking radiators (absorbiators) next to the Hearthstone would work` -- which it won't -- then you'd want to park them where they can receive infrared radiation by line of sight. Flow from radiators (absorbiators) to tank would be from bottom to top of radiators (absorbiators) and from top to bottom of rock lined tank.

Water is a better heat storage material than rock, so lose the rock and replace with water.

 

[new england tommy]

I guess first of all, what are the emitters of your FHW zones, and how 'cold' is the return water from them?

If parking radiators (absorbiators) next to the Hearthstone would work` -- which it won't -- then you'd want to park them where they can receive infrared radiation by line of sight. Flow from radiators (absorbiators) to tank would be from bottom to top of radiators (absorbiators) and from top to bottom of rock lined tank.

Water is a better heat storage material than rock, so lose the rock and replace with water.

Just looking for ideas or opinions.....sorry I am from New England we are know to be not as nice as the rest of the country.

The cold return is nothing but the main return from both zones, so it would be room temp. The absorbiators (I like that) have been parked next to the One for the end of last season and the start of this one. The surface temps range from 130 to 230, just parked there and not insulated.

The rock lined steel tank is something I found from a local guy. He said it came as kit with his old woodstove and was used for domestic hot water. In his estimation he had 180 degree water hours after the fire went out. It is nothing but a 12" by 60" steel cylinder that weighs 300 lbs and has plenty of bung holes.

 

[maple1]

It's your time - but I'd forget about it.

That rock will also serve to drag heat out of water being heated after the tank temp drops. The heat in the rock has to come from somewhere. Net gain - zero.

You get water flowing through the absorbiators (TM), and the heat transfer to it will be negligible. And any heat that does get transferred will be that much less heat that gets into your living space directly from the stove. Net gain - zero.

I also would drop the idea of wrapping cold pipe around a stack - that will create a creosote generator.

 

[ewdudley]

The rock lined steel tank is something I found from a local guy.

Oh, I thought you meant some tank with a bunch of granite in it or who knows what. I think you mean it has a non-rusting porcelain liner or something like that?

 

[new england tommy]

Oh, I thought you meant some tank with a bunch of granite in it or who knows what. I think you mean it has a non-rusting porcelain liner or something like that?

Honestly, I am not sure. It is from "HOT ROC" manufactured in NY and it looks old. From what I gathered from the original owner and a plumber I have talked to, it is good to hold water hot, better than external fiberglass insulation. It is probably porcelain and it is really heavy, 250 to 300 lbs. easy.

Regarding the absorbiators, if a piece of cast iron is sitting at 180 degrees and the zones aren't calling, wont the water come up to temp? Say it arrives around 65 degrees and leave 100, then circulates back from the rock tank at 100 and back thru the absorbiators to 140?

Thank you for the wrapped stack tip, I already have low stack temps. Would you expect similar results if the suspended coil had a 12" diameter over a 8" pipe?

 

[new england tommy]

Also, thank you for the advice and opinions.

I have been told a couple times that this is a useless and stupid idea.

 

[ewdudley]

Also, thank you for the advice and opinions.

I have been told a couple times that this is a useless and stupid idea.

It's a perfectly good idea, but it's just that it's unlikely that you could pull enough heat to make it worthwhile for heating some other part of the house.

If, for instance, all you wanted to do was bring fifty gallons of well water from 50 degF up to 120 deg each day to put a dent in your DHW bill then it would seem more realistic. It just seems that trying to pull fifteen or twenty thousand btu per hour all day long would probably take a whole lot more radiators than you might have in mind.

 

[new england tommy]

hello all,

just wanted to update the thread. We have had below average for almost a week. The absorbiators are plumbed, insulated and covered on the sides opposite stove. My boiler has seldom fired to heat when zones call, aquastat is now around 135. Without getting into all details, it works for my needs and heating situation about as well as expected. Mainly my wife is happy that the thermostat controls temp and the baby's room is not cold anymore. I think the baseboard heat runs more like in floor radiant, a much longer run time but at a cooler temp.

the return is coming in between 70 and 100 and leaving the absorbiators at 125 to 140. Our heat doesn't call that often due to the Hearthstone One running in the basement.

thanks all, tommy

 

[ewdudley]

Outstanding! Never would have thought you could pull enough heat to make it worthwhile.

“In theory, there is no difference between theory and practice. But in practice, there is.”

-- Yogi Berra​