Building pressure in system....Too small of a Expansion Tank????

[Huskurdu]

I have the system mentioned in my signature. Below is a picture of it....if I did it correctly. My system keeps building pressure, sometimes up to 60 lbs.! This makes my T/P valve seep on the gas boiler. I've got an Extrol 60 on my wood boiler loop and a tank about half that size on the gas boiler that's been with that system right along. My thought is that when my system cools down, the fresh water supply valve opens and puts more water in the system because the pressure is so low. Then when it heats up, it is too much water and the expansion tank can't handle it. I've let air out of my Extrol 60 so there is very little left, but the problem continues. Do I need a larger exp. tank?

 

[Jim K in PA]

I can't offer comment on the sizing, but I do think you are exacerbating the problem by letting air OUT of the expansion tank. The air behind the bladder is the space into which the expanding water pushes. Less air space will INCREASE the water pressure due to a decrease in available expansion space. 60 psi is WAY too high. Most hydronic systems run at under 20 psi, AFAIK.

Let the system cool, drain some water out, add air to the expansion tank until the pressure is about 2 psi higher than your target pressure, then open the fill valve and let the system pressure rise to your target pressure. Then CLOSE the fill valve. Heat the system up and see what happens.

 

[RJP Electric]

My plummer ordered a Extrol 60, it was way undersized and did blow my relief valve, he sized it for a non pressured coil system. 500 gallons is a lot of water to expand, I have a SX-60V along with my small one on my boiler like you have but it is just barely enough. You may want to check your auto fill valve also, it shouldn't let any more than 12 psi in when cold. I have accidentally left my lever up too, make sure it is down. Household pressure will blow the reliefs too.


Bob

 

[MrEd]

I deleted my prior thoughts...seems to me your exp tank is WAY to small. 500 gallons of pressurized storage needs much more than the 8 or so gallons of displacement of the extrol 60.

I have an extrol 60 just for my tarm, and a SX-160v expansion tank for my 500 gallon pressurized storage (I sized the exp tank for 1000 gallons when I bought it). Yours appears much to small. 60lbs is dangerously high, imo.

 

[mpilihp]

I cant tell from your diagram

What size is your storage tank?

What size is your expansion tank?

With any REAL amount of storage you will need alot of expansion room.

~ Phil

 

[DaveBP]

A comment from a "hydronics as a spectator sport" perspective:
Expansion tanks are a source of never ending confusion on this forum. And I think it stems from a combination of sticker shock and ambiguity around sizing. Tanks are referred to as 'so many gallons' but the size of the tank and the functional capacity are blurred a lot from what I see.
I think there is a concensus among the pros on this forum that from min to max temperatures in a closed boiler system around 4% of total system volume is about as low as is safe. That's the volume of storage, boiler and all the pipes. What gets confused is what "size" tank that corresponds to. To accept that much water going in and out at each heat cycle the overall size of the tank needs to be about double the amount of expansion of system water. The air in the tank shouldn't be squeezed more than about 50% of the empty tank volume or the pressure goes up too high.
So just the 500 gallon storage alone needs about 20 gallons of acceptance into the expansion tank. That's about an SX-40V tank and you might need the next size up to account for boiler volume and pipes.
I do think your expansion tank is probably undersized by about 300% at least. It just sucks because they get expensive real quick.
With any luck, one of the pros on this forum will chime in with some authority.

 

[rickh1001]

Huskurdu,

I have a similar sized system, with 500 gal of pressurized storage. When you get done going through the complicated tables and charts on the Amtrol site, I think you will find as a rule of thumb that the acceptance volume of the expansion tank should be about 5% of the system volume. This is overly generous, and you can trim it, but if you size it at about 5%, you will have extra capacity. That is the acceptance volume - the total nominal volume of the tank will be roughly twice this. So you will probably find you need about a 60 gal expansion tank. Of course, if you are planning to add storage like I am, to 1000 gal, then you might as well buy the larger tank now so it will be ready. Also, as others have said, you should not release air from the expansion tank once it is under pressure. I think the idea is to start out at 12 psi (or whatever initial system pressure you choose) in the expansion tank before introducing water. The 12 psi (cold) boiler feed will then compress the bladder somewhat. Upon heating and increasing the system pressure, the bladder is compressing the air cushion, providing the space for the water to expand into. If you release the air, you defeat the basic design of the expansion tank, and start a cycle until you end up with no air cushion left, and the bladder stretched to its full expansion. Pre-charge the tank when cold to the feed pressure before hooking it up, and leave it. Unless there is a leak in the tank, it should not need air again.

Good luck.

 

[in hot water]

having expansion tanks in two different locations are confusing to the system, you have to points of no pressure change. Go with one, properly sized. Also add the fill valve right at that expansion tank connection. The fill valve needs to be at that PONPC.

The pressure on the bldder tank needs to be adjusted to match your fill pressure. for most cases 2- 15 PSI is plenty fill pressure.

I'd size the expansion for 210F max as wood boilers can run away at times. So calculate the lowest possible twmperture and the highest possible temperature. i think you can find expansion tank sizing programs at Amtrol and others online.

hr.

 

[MrEd]

I like to be able to completely valve off and entire sub-system, and know that the remaining sub-systems (oil boiler, wood boiler, or storage), all have enough expansion. You are the first person I have heard state that multiple tanks are not a good idea - can you explain further?

Even on the tarm piping diagrams they show multiple expansion tanks, in different locations.

 

[in hot water]

The place you determine to add the expansion tank established the ONLY point in the system where the pressure cannot be effected. As such it is the point you want to pump away from. Really no need to have two locations. The exp tank need to be able to handle the expansion volumn of the entire system, especially the buffer tank. There is no harm in having oversized expansion if you valve off the buffer.

If you do use multiple expansion tanks, which is fine, connect them them to the same location in the piping.

How will you pull energy out of the buffer? It looks like it would always have to flow through gas boiler. Only if it has a tankless DHW coil would you need to do that.

hr

 

[BenW]

You said your T/P valve was seeping. What is the relief setting on your T/P valve? usually a T/P valve is 150 PSI and 210 degrees. If that's what it is make sure you change it out with just a regular relief valve set at 30 PSI.

 

[Huskurdu]

Jim K, you're probably right about letting air out. It has not gotten much better.

mpilihp, my storage tank is 500 gallons, my extrol exp. tank, on my wood boiler, is 7.6 gallons according to Amtrol's site. The expansion tank on my gas boiler/house loop is about half the size of the Amtrol 60.

BoilerMan, the 5% rough calculation (considering 500 gallons storage, plus 50 gallons piping, etc.) is putting me in the 27.5 gallons area. WOW! I suppose my kids don't need to go to college. LOL.

BenW, My current T/P valve on the gas boiler says 30# and 535,000 btus an hour. So the fact that the pressure was so high, that is what caused it to blow off originally. I will have to replace it because of the continual leaking.

To all, I have not resupplied the exp tank with any air yet. My system temps and pressure are as follows....My Eburn shows 172 deg and 32 lbs and my gas boiler shows 160 deg and 22 lbs. Both readings are from the mechanical gauges on the internal tanks. Why would it show more poundage (sp?) on the wood boiler? Should it not show a lesser pressure? The wood boiler is 1 floor above the gas boiler. It is also about 100' away in the unattached garage....hmmm..very weird. The gas boiler T/P valve is still seaping very slowly....no doubt from overuse at this point. At least it's not peeing on the floor any more. It evaps before it makes it to the floor.

I will put another Extrol 60 on this spring's budget and pipe it in series with the other...should I also include another air scoop and air vent with the second one? If not, should it be before or after the existing one? I will replace the gas boiler T/P valve.

If any of you are familiar with my system, the 1" pex lines ARE too small for my application. I will have to run 2 additional 1" lines or replace the lines with 1.25 or 1.5 inch lines in the spring. I'm going to limp through this winter as is. It really sucks and I'd better not cross paths with the dope that spec'd the 1" pex!! I've blown about $1000 dollars making this stuff work and it isn't quite cutting it. What does everyone think of putting a second Taco 007 on the return line on the gas boiler end to (pardon the expression) 'push' the return water back to the wood boiler? I currently run the other Taco 007 24/7 to keep the ground lines from freezing and another 007 running isn't going to kill my electric bill and should improve the flow through the 1" pex. ???? Thoughts?

I appreciate all of the info you have give and any more thoughts would also be appreciated.