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[Willydean]

I was reading another thread and there was a brief mention of the dreaded 140 degree return temp. What is the lowest the return temperature should be and could someone please explain why?

[kybaseball]

Don't know a lot but I have been told that when the low temps are in the return it causes the tank to sweat. I have a temp gauge on my return and check at least once a week. I run my stove at 175 with a 17 degree difference 2 exchangers and heating hot water. Mine has never been below 153 on the return. I run my pump on high speed which is what is recommended. Hope this helps you out.

[slimjim]

Each stove manufacturer should suggest the operating temp of the boiler that they build, we like ours to be run hot @ 185+ I like them hotter @ 190-195 with 10 degree differential or less on the aquastat and return water temps not more than 20 degrees under full load, this is to ensure stack temps high enough to keep condensation from forming in the fire tube heat exchangers, our exhaust temps are at 270 degrees with a clean boiler, other manufacturers may be ok with cooler temps for thier own reasons, I think that sometimes it is because of the old idea that pumps and piping could not handle the higher temps, and the extreme temps inside the fire box of many of todays gassers could damage the stainless fireboxes, todays quality pumps and piping can easily handle the high temps, I don't think that I would suggest running stainless units to this high a temp, you should consult the manufacturer on that one.

[yoderheating]

I've never heard Heat Master put out a high or low recommendation for return temps. I wouldn't worry about hurting the system by running too hot, 190+ isn't going to have any effect on stainless except to help keep oxygen out of the system which is a good thing. I run my furnace at 155 with a 10 degree differential so I'm sure my return temps are below 140 when everything is on.   

[LittleJohn]

I run a Central Boiler and they require a Thermostatic bypass between the OWB and heat emitter (inslab, sidearm, fan coil, etc).  The Thermostatic bypass come preset from factory, and basically have 3 operating zones;

(PLEASE NOTE - this is my interpretation on how the valve works - installed mine several years ago and have since LOST paperwork)
1st - Supply water from OWB below 140, water is returened to OWB
2nd - Suply water from OWB above 140 and below ~170f, some water flows thru to heat emitters and some returns to OWB
3rd - Supply water from OWB above ~170, water flows thru to heat emitters

[Sprinter]

Depending on environmental conditions, water vapor will condense in the 126-138 degree range. Condensation anywhere inside the boiler is not good regardless of material. It contributes to burn efficiency losses, metal erosion, and creosote.  It's sustained low temps that do the damage, a few minutes isn't gonna hurt, or form anything. Don't forget there is a lot of discrepancy in gauges and their locations. So many variations in water jacket design also can help or hinder this.  All piping should be designed to have a max velocity of 4ft/sec. And boiler protection devices should always be incorporated regardless of brand, to prevent this temp drop or boiler shock. Something as simple as a "boiler bypass " pipe. You can adjust mix flow with a simple valve. This is commonly used in cast iron or copper fin boilers, right in the manuals.

[slimjim]

Yoder, I would agree that 190 degrees should not harm the stainless, the point I was making is that todays gassers run some incredible temps at the coal and ceramic bed, that would be my concern with stainless. Knowing about the oxygen in cooler water temps why would you choose to run at 165 high temp.

[Willydean]

I'm convinced with everyone's help that the 185 deg temp I'm running is not going to damage my OWB and the return temp is quite a bit higher than any of the numbers mentioned in y'all's post! I'm learning!

[yoderheating]

 Most of the year I run at 150 for a high temp but I'm up to 160 during colder weather. It burns less wood when a conventional furnace is run lower and I haven't seen any evidence of trouble so long as good water treatment is maintained.