# How critical is return water temperature?



## James Reimer (Oct 1, 2012)

I've got a new gasifier and I've heard that you need to keep your return water at a certain temperature. Why is this critical? Does low return water temperatures cause premature boiler corrosion? Creosote buildup? Warping and cracking? I can see where low water temperatures causes creosote buildup in the lower tubes, but does it cause corrosion or warping?

What is the minimum safe return temperature? My owners manual doesn't say anything about maintaining a minimum return temperature (P&M Optimizer 250) but I have seen other manufacturers manuals stress that as a requirement.

What are your thoughts?


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## Floydian (Oct 1, 2012)

James,

From my research I would say it is important enough to contact the manufacturer about this. Return water protection is a MUST for all the down draft gassers I looked at.
Is the Optimizer a down drafter?

Noah


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## James Reimer (Oct 1, 2012)

Yes, the Optimizer is a down draft gasser.  Why is it a must for this style of OWB?


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## BoilerMan (Oct 1, 2012)

What is the water capacity of the P&M? 

TS


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## James Reimer (Oct 1, 2012)

The brochure says it has a 240 U.S. gallon capacity.

James.


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## RobC (Oct 1, 2012)

The idea of minimum return water temp is to keep the inside of the boiler hot enough to prevent condensation and excessive creosote buildup which will over time damage boiler.


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## BoilerMan (Oct 1, 2012)

They may not say it needs return protection due to the large water volume, like a Garn.  Return protection is always a good idea, however it is just another mechanical part to fail, I'll let heaterman answer this for you, I'm shore he's around somewhere.

TS


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## Trex83 (Oct 2, 2012)

James Reimer said:


> I've got a new gasifier and I've heard that you need to keep your return water at a certain temperature. Why is this critical? Does low return water temperatures cause premature boiler corrosion? Creosote buildup? Warping and cracking? I can see where low water temperatures causes creosote buildup in the lower tubes, but does it cause corrosion or warping?
> 
> What are your thoughts?


I would answer yest to most of your concerns, you need a mixing valve for boiler protection. Without it, you get in the pitfalls of some US systems:
-early corrosion due to flue gases
-water shock which stresses the metal (high delta T or no control on water temp return)
-operational issues like taking a long time to heat up if your kids or significant other "forgot" to load the boiler before leaving the house...
- not having enough storage to operate efficiently
- waiting a little while longer to get hot water (read the Laddomat manual if you have external storage or the Danfoss VTC manual for info)

The only issue I can see is that you can wait a lot longer for 240 US gal. to get to minimum return temperature (ie, you start your boiler and its 100F.... you will have to wait for the hot water to get to the minimum return temp (some are 130F, other a little bit higher if you charge storage tanks) to get some of it - in other words, the 3- way mixing valve will completely recirculate the hot water inside the boiler until it gets to the minimum return temp of the mixing valve.
Not sure about 4-way mixing valve - see Central Boiler for the operation principle of their valve.


Cheers,
Trex83


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## Trex83 (Oct 2, 2012)

Taylor Sutherland said:


> They may not say it needs return protection due to the large water volume, like a Garn. Return protection is always a good idea, however it is just another mechanical part to fail, I'll let heaterman answer this for you, I'm shore he's around somewhere.
> 
> TS


My friend has a medium size Central Boiler with internal storage. He still has a 4 way mixing valve at the house, but never had the time to sit down to see how it works.
Trex83


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## BoilerMan (Oct 2, 2012)

I spoke to a guy on Sunday who has a CB gasser.  He lit his first fire of the season last week and his 4 way (I think it could have been a 3 way) return valve stuck causing a boil over.  He said he took the valve apart and it had an automotive looking thermostat in it. Dropped it into boiling water and no-go.  He bypassed the whole thing.....I wouldn't have done that nor do I recomend it, but his whole setup is <5 years old, I'd keep a spare thermostat on hand.  Just shareing his expierence.

TS


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## Trex83 (Oct 2, 2012)

Taylor Sutherland said:


> I spoke to a guy on Sunday who has a CB gasser. He lit his first fire of the season last week and his 4 way (I think it could have been a 3 way) return valve stuck causing a boil over. He said he took the valve apart and it had an automotive looking thermostat in it. Dropped it into boiling water and no-go. He bypassed the whole thing.....I wouldn't have done that nor do I recomend it, but his whole setup is <5 years old, I'd keep a spare thermostat on hand. Just shareing his expierence.
> 
> TS


Okay its good to know ahead to plan for that, in our case, we don't have a small hole through the thermostat assembly (the flat washer part beneath the spring). Next time my dad opens it up, i'll tell him to drill a small by-pass hole in case we run in the same problem.
Now that I think of it, my friend that own a CB had issues when he first ran it his unit. He wasn't getting any hot water on the house\demand side. He had to open up the 4-way valve to remove some debris that got stuck during the install. But the next day he called me to say he went to the dealer to get a new thermostat because it still wasn't working when I left his place.
Thanks,
Trex


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## heaterman (Oct 2, 2012)

The issue with your boiler or any boiler for that matter, is flue gas condensation. It simply has to be prevented in any boiler that is not designed to withstand liquid instead of vapor in the heat exchanger and flues. Doesn't matter if you're firing with wood, coal, oil or gas, Flue gas condensation will without fail eventually wreck your boiler.

So ...the simple solution to preventing said condensation from happening is to keep the flue gas temperature above the dew point or otherwise said, the point at which vapor in the flue gas turns back into liquid. In most scenarios (there are a few variables) you will find that in order to maintain sufficient flue gas temp the water temperature must be kept above 140* for solid fuel appliances. (Moisture content of the fuel is a significant factor in this also)

This is done in a couple of different ways. One being the use of a termovar diverting valve that routes hot water back to the boiler before it hits the system. The other is the installation of an aquastat the turns off the boiler circ should water temp drop below the setpoint and allow the boiler to "catch up".

I had my combustion analyzer in a couple of flues this past week looking at the dew point among other things. The first piece of equipment was a standard downdraft gasser the owner was having issues with from creosote plugging things up. While the design of the boiler is less than stellar and contributed to some of his problem, his main issue was wet wood and a lot of idling. Moisture content of the wood (oak seasoned last summer and this) was still over 28% in many splits. The dew point of the exhaust gas was just under 150* which is very high. This means he has to keep his water temp somewhat above that or the flue gas will be too cool and it will rain in the chimney. This is a very bad thing.In his situation the boiler was connected to a huge load (several thousand sq ft of concrete slab) that dropped the water temp below
the critical point. Lot's of black goo everywhere. He will be installing a pump aquastat to kill the main circ when water temp drops below 160*.

The other boiler was pellet fired and due to the design of the boiler and the extremely dry nature of pellets the dew point of the flue gas was running only 116* to 118* degrees. This boiler has an integral pump aquastat that can be adjusted to different set points. We set it to tuen off the circ at 140* to give him a margin for error/variability.


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