Boiler Overheating


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Old 01-09-10, 06:38 AM
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Boiler Overheating

I live in a ranch house with a finished basement. One floor of heat with the basement having heat as well; the boiler is in the basement. I have natural gas hot water baseboard heating and I recently replaced my boiler pump (B&G Series #100 to a Taco 007). My boiler is a HydroTherm (model HC-125C, Input 125,000, DOE heating cap 98,000 BTU/hr). When I tried to bleed the air out, I notice that there wasn't any water coming in, so I replaced the water feed valve (advice from a plumber friend). I filled and bled the air out several times and got the boiler up and running. My backflow valves (Taco 219 Swetchek thumb screw) are both screwed down, all the valves are open, and water seems to be circulating just fine. The cold pressure is 14psi and the hot pressure is about 24psi. The boiler temp hits about 190/200F and kicks off, before the thermastat (programmable) reaches room temp.
Why would the boiler hit a high temp and then kick off?
What range should it be in?
Is the boiler over-sized for the house?
Thanks for your help.
 
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Old 01-09-10, 06:46 AM
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There is no way to specifically say the boiler is over sized but, a ranch home with a 100k output would be a very large ranch. Since my gut feeling is you have glass in all the windows and doors hung in exterior doorways.
Two zones is not uncommon as it is one per floor. The reason for boiler short cycling is the boiler produces more heat than the radiation can take away. With that said the boiler is over sized today. The boiler size changes daily or even hourly. The key question is on the coldest day does it go off before the thermostat satisfies?
My guess is yes it is over sized.
 
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Old 01-09-10, 07:18 AM
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Your circulator should still be running. When the water temperature loses enough heat, the boiler will fire again. This will happen until the thermostat is satisfied.
 
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Old 01-09-10, 08:35 AM
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The cold pressure is 14psi and the hot pressure is about 24psi. The boiler temp hits about 190/200F
Two things I would look at here...

First, the pressure swing from 14 to 24 is a bit on the high side. Not saying it's BAD, but if your expansion tank is correctly sized for the system, and depending on the type of tank (bladder vs. compression) is either properly charged with air, or not waterlogged, I would not expect that much pressure swing.

So, what type of expansion tank do you have?

Next, your high limit of 190/200 is also on the high side. Does it take that high a temp to heat your home? In other words, was the temperature setting on the aquastat raised from the 'standard' of 180 for a reason?
 
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Old 01-15-10, 03:16 PM
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Yes, the boiler does go off before the thermostat is satisfied. I live in northern VT and even on the coldest days/ nights it still does that. I guess maybe it was oversized when the development was created to accommodate a second story? Some of the neighbors have put second stories on their ranch houses.
 
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Old 01-15-10, 03:29 PM
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My expansion tank is an Extrol with a diaphram (that is what is says on it). It is installed horizontally and it feels (by shaking it) as though it is 1/4 to 1/3 full of water. I can feel the water swooshing back and forth. I bought the house in 2007 and have never touched the Aquastat, so I am unsure if someone has changed it.
 
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Old 01-15-10, 04:45 PM
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Let's start with the temperature... on your boiler, your aquastat is probably a Honeywell... gray box, about 4"x6"... loosen the screw and slide the cover off... take a look at the temperature dial(s) inside and tell us what they are set at.

Next, your expansion tank... should NOT be mounted horizontally (or vertically with the water fitting pointing down). The proper mounting is vertically, with the water fitting UP, i.e. HANGING from the piping. Reason for this is that either of the unacceptable mounting methods will allow air to collect inside the tank on the water side, and this will eventually cause the tank to rust from the inside.

Another reason for not mounting horizontally is the torque that it applies to the fittings just ain't a good thing... especially when the tank fails and fills with water.

If you do indeed feel water sloshing around in your tank, it's a very good indication that the tank is shot, and needs replaced. This and the fact that your pressure is climbing up to 24 PSI...

If you post some pics of your system, and the tank mounting, we may have some easy to implement suggestions to fix ya up. You can set up a free account at Image hosting, free photo sharing & video sharing at Photobucket and upload the pics there, and come back here and drop a link to your album.
 
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Old 01-15-10, 06:50 PM
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I saw one silver "wheel" dial inside the aquastat and it is set on ~ 198. Here is the link to 4 photos:

Ram's photos

I hope it works.
Thanks for all your help.
 

Last edited by NJT; 01-15-10 at 07:17 PM. Reason: works now!
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Old 01-15-10, 07:37 PM
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I do believe I would try turning that dial down to 180. It's unlikely that you need 200░ water to heat your home. You may, but it's highly unlikely that 200 setting will do anything but increase your fuel bills. If you do find that you can't maintain the thermostat setting when it's 20 below, then all you have to do is turn it back up again...

I also would do something with that expansion tank... you could pretty easily run a pipe nipple out to the right, elbow down, and hang the tank the way it's recommended by the manufacturer. If you do this, you would need to support the pipe somehow, because there would still be some torque on the fittings... you could run a support up to the ceiling, or down to the floor, but it would need something. I have little doubt that the tank is either already shot, or on the verge of being so...

This pic shows the piping going up... you wouldn't have to do all this, unless you wanted to... but straight out to the right, and then down... those extra valves make servicing the tank much easier. This shows the support I'm talking about...



Note that this setup is not 'ideal' either... air can still collect in the piping above, so there should probably be an air vent installed on the upper horizontal section of pipe... or change one of the elbows to a tee fitting and install an air vent in that.

There's a bucket on top of the boiler... why? Is the pressure relief valve 'spewing' from time to time? If so, that's further evidence of an expansion tank problem.
 
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Old 01-16-10, 07:19 AM
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I will turn the aquastat down to 180. The bucket is there because the pressure relief valve was dripping, but the dripping has stopped. I have an air vent just to the left of the expansion tank, but it doesn't show in the photos. It has a screw cap with a bicycle tire type valve in it and I heard air coming out of it, so it does work. I will replace the expansion tank and set it up so it is vertical down (connection on the top), but I really don't want to mess with the boiler until spring when I can shut it down. Would it be ok/safe to wait until then? What would you recommend for a new expansion tank?
 
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Old 01-16-10, 08:08 AM
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As long as the pressure stays under control, you may be OK to wait... might not be a bad idea though to collect the stuff you will need now, just in case you do have to do something before spring comes.

Using the same type/brand of tank would be fine, but there are others... all basically the same thing.

When you do the work, I would also recommend that you replace the relief valve. I like the idea of replacing them every 5 years or so as a matter of maintenance. Yours looks to be well over five!

OH, one more thing... by code, the relief valve discharge should be piped down to within 6" of the floor... so plan on doing that as well. In fact, you might want to do that sooner, because if you DO have a problem with the tank, and overpressure, you won't want that spewing all over the top of the boiler as it appears to have done in the past.
 
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Old 01-16-10, 11:06 AM
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OK, will do. Thanks for all of your advice and help! I really appreciate it.
 
 

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