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Boiler Question

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Jonski

01-26-2003 16:49:45




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Can someone explain why a H2O pressure of about 15 psi is recommended in a hydronic boiler. The only reason I can figure is to diminish steam in the system. At a higher pressure the boiling point is higher thus less chance of steam. I have a wood boiler and the safety valve lets off on ocasion (>30psi). I eliminated the auto H2O feed since the well water is poor and corrodes through the copper pipe walls. I use a small pump to add water manually when the pressure drops in the boiler about once a month. Do I have any alernatives other than a water conditioner. Can I use a bladder water tank pressurized to 50psi with 15psi auto feed valve?

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SHeiserman

01-27-2003 17:08:07




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 Re: Boiler Question in reply to Jonski, 01-26-2003 16:49:45  
The expansion tank needs to go on the cold water feed line. Atmospheric pressure is 15# when water boils at 212 deg. Any more than that and you're talking steam heat, not hot water heat.



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MarkB

01-27-2003 03:49:13




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 Re: Boiler Question in reply to Jonski, 01-26-2003 16:49:45  
Your system should already have an expansion tank. I don't think you can use a regular water tank because of the temperature. You should probably install a larger hydronic expansion tank if your pressure relief is popping off.

I think that the main reason the pressure limit is set low in hydronic systems is for safety, since there's really no need for a higher pressure. (Increasing the pressure actually reduces the likelihood of steam in the system, but increases the possibility of water turning to steam if there's a sudden leak.)

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Jonski

01-27-2003 15:19:24




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 Re: Re: Boiler Question in reply to MarkB, 01-27-2003 03:49:13  
My question is more related to why the need for any pressure at all in a hydronic system. Why can't I operate at atmospheric pressure in the system vs the recommended 15psi.



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MarkB

01-28-2003 17:50:47




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 Re: Re: Re: Boiler Question in reply to Jonski, 01-27-2003 15:19:24  
My guess is that pressurizing a hydronic system accomplishes two things:

It allows air to be purged out of the system, reducing corrosion.

It reduces the likelihood of steam forming in the boiler by raising the boiling point of water. (Even though the water temperature is regulated at about 175 degrees, the boiler walls are going to be a lot hotter.)



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Scott Green

01-27-2003 06:32:11




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 Re: Re: Boiler Question in reply to MarkB, 01-27-2003 03:49:13  
Now that I think about it Mark , My uncle had a coal boiler("natural draft"). He had to run it at recommended temperature. If the zone wasn't calling for heat , the boiler would just get hotter and hotter , until the safty valve would blow off. My cousin , who installs heating units of all kinds , told my uncle that he needed a bigger expantion tank. The bigger tank solved the problem. Also Jonski , your boiler doesn't turn off and on like a oil/gas boiler. You do have the system set up to over ride the thermostat don't you? This will send excess heat through the zone , taking heat away from the wood/coal boiler. My uncle added an extra zone just for the excess heat. It went in the back section of farm house(unheated). The extra zone was installed from the start. He knew he would have to get rid of the excess heat. The bigger expantion tank was the answer to his safty valve blowing off. You can have the system over ride the thermostat , sending the excess heat through your house , but it may get to be too warm. My uncle ended up taking out the coal boiler , and installing an LP gas boiler. Now he can't afford to heat the house useing the boiler system. He heats with a natural draft free standing coal stove. Nice and toasty in the house. The LP gas unit is just a stand by , should he not get home for tending the coal stove.

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Jonski

01-27-2003 15:05:23




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 Re: Re: Re: Boiler Question in reply to Scott Green, 01-27-2003 06:32:11  
I currently dump the excess heat into the longest run of base board which handles the excess and this overides the thermostat. The circulator cycles on at 180F min to dump the heat into this loop. Also the house has two zone heat and I have both the check valves (Taco)for each zone in the lifted position to get some natural circulation. My problem is about ounce a month the safety valve lifts. The current expansion tank size is 18 gals non bladder type. How big of a tank did you uncle have to rsolve the problem. How do I determine if the existing expansion tank is water logged as this could be the problem based upon your comment regarding size of the tank.

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Scott Green

01-27-2003 17:06:48




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 Re: Re: Re: Re: Boiler Question in reply to Jonski, 01-27-2003 15:05:23  
I'll be seeing my uncle tomorrow. i'll check that stuff out for you.



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