Do I need more expansion?

[Boardroom]

I have been running my system for about 6 months now and have had the same issue since day one. If I start from cold I have about 12 psi in the system. If I charge my 1000 gallons of storage up to full I can be as high as 28psi. I have never blown the prv but have avoided fully charging my tanks because of this.
I think I have about 1100 gallons total in my system and have an Amtrol sx160 in the garage and a smaller 30 in the house.
I stopped by a plumbing wholesaler this week and inquired about pricing for a new expansion tank. The guy at the counter who claims to have worked in hvac for 25 years says the problem is probably not my expansion volume but rather that my piping is too small. First time I have heard of this. I have 1 1/4 copper running from my boiler to my storage and then 1 inch running to the manifold in the house. 3/4 inch pex to all my rads. My Alpha pump is only running at 1-2 gallons per minute to supply my rads and the house is heated fine.
If I only get higher pressure as my storage is heating up and then the pressure drops when it cools, how could it have anything to do with pipe sizing? He also said I should never fill my system with well water, only city water as my well water will clog the pipes so maybe that is my problem. I hardly think so since I have had the same issue since the first day.
Without getting into all the calculations, does it sound like I just need more expansion or does this guy have a point?

 

[maple1]

That fellow doesn't know what he's talking about.

What is your temperature range at those pressures?

 

[Boardroom]

I usually let the top of my tanks get down to about 140 before burning. The middle and bottom will be about 110 at that point.
If I charge fully and get to the 28 psi I mentioned, the top is 185, middle about 170 and bottom about 160. My tanks stratify pretty well.

 

[maple1]

Play with this:

http://westank.com/calculator/

I did, using 1000 gal., 140-185, and 12-20 psi., and came up with a 180v tank. On the face of it, with a 160 + 30, you should have enough expansion. But the symptoms kind of point otherwise. Are they both exposed to the system & storage tanks all the time? (No check valves that might be blocking?). And when you say 12psi 'cold', is that when you first filled the tanks & before you started heating them (what I would call cold), or when they are at 140? How high is the highest part of your system, above the system circulator, and how much elevation between the circ & the pressure guage?

(Do you have more than one pressure guage? I've got a bunch, don't exactly trust them from past exeprience with one.)

 

[jebatty]

I'm going to stretch a little and say the guy might have a point, and at the same time IMO you may not have adequate expansion capacity. As to expansion capacity, my system too has about 1100 gallons of water. I do not operate the system during the summer and the system cools down to about 68F. Maximum temperature is 193F. My system is all on one level, height of system is 14'. This requires static pressure of about 10psi at minimum system temperature to lift water to the top and maintain about 5psi additional to insure adequate pressure on the circulator to prevent cavitation.

I thought an SX-160V would be adequate, but not so within my temperature range. The 160 may be adequate if minimum system temperature is higher than 68F, how much I do not know. I added an SX-40V and these two together are adequate to maintain static pressure as needed and maximum pressure then is about 22psi at the boiler. I have a pressure gauge on my 1000 gallon storage tank and also on my Tarm boiler. My circulator pumps directly from bottom of storage and into the boiler return. The pressure tank is located between the boiler supply and the storage. The boiler pressure gauge will read somewhat higher than the tank pressure gauge, therefore reflecting some pressure differential.

As to the point the guy has. Depending on where your pressure gauge is in the system relative to the circulator, the gauge pressure reading will/can be impacted by your circulator and by the pipe size. A gauge close to the circulator will reflect the increase in pressure caused by the circulator on the outlet side, with a corresponding decrease in pressure on the inlet size. This is the pressure differential, and if measured directly on the inlet and outlet will give you information that, combined with the pump curve of your circulator, can tell you the gpm flow rate. Pipe size can also impact the pressure differential: smaller pipe size, more flow resistance, greater pressure differential; larger pipe size, less flow resistance, lesser pressure differential.

To your question: with minimum system temperature of 110F and maximum 185F, and with 1100 gallons, a 160V + 30V probably is OK but maybe not. Where is your pressure gauge located? Since your boiler is probably rated at 30 psi and that's were the relief valve would be located, I think that would be the pressure point you want to measure. What is the total height of your system? A typical two story height (basement to second level emitters) requires about 12 psi static pressure at minimum system temperature. Then expansion capacity needs to be added to handle target maximum system pressure. As to pipe size, what is the circulator and flow rate between the boiler and storage? I think 1-1/4" pipe is undersized to handle your full boiler output (200,000 btuh) at a delta-T of 20F, but I don't know whether or not that may contribute to your high pressure.

You might find this helpful for pressure tank sizing: http://westank.com/support/tank-sizing/

This isn't the clearest post I have ever made, but I hope it is helpful in analyzing your issue.

 

[ewdudley]

28 psig is just about exactly what you should expect when pushing 180 degF with system volume of 1100 gallons and a 86 gal expansion tank.

Expansion volume of 86 gallons is only 7.8% of 1100 gallons system volume. More like 10% would be needed to push on past 180 degF.

There are many of us here on hearth run the 86 gallon Amtrol tank with 1000 gal storage and accept that 180 degF is about the upper limit for storage temperature.

(If the pressure relief is not losing water there should be no harm or risk in running at or near the rated pressure of the boiler.)

 

[Boardroom]

When I use the calculator you guys referred to, I come up with required expansion volume of 112 gallons which is about 10% as ew is suggesting. Not being an hvac guy I figured that anything pushing 30 was taking a chance. Is there any harm in running at 28psi as opposed to 20? I think I would like to be able to charge my tanks to about 175-180 top to bottom but cannot do it with my current expansion.
To answer a few of the other questions - my 160 and 30 are both accessed by the system, there is nothing in between that would block them from each other. I have one circulator pumping from the bottom of storage to my boiler. I have another circulator in the basement of my house that pumps from/to storage. The water has to be pumped through the whole house system before it arrives at the pressure gauge, which is installed in an electric boiler that is piped in series. My storage tanks are just over 9 feet high and my garage floor, where the tanks sit, is probably about 8 feet above the pressure gauge in my house basement. It is the gauge in the basement that hits 28. The one on the wood boiler in the garage would be 4-5 less at the same time.
I am not sure what the flow is from my boiler to the storage. I set my boiler limit to 195 when burning. I use a Caleffi 281 series loading unit. If I run it on medium speed the boiler will idle when I reach top temp. If I turn it up to high I get very little idling, if at all.

 

[jebatty]

I would assume your boiler is the point where the 30 psi pressure matters, and if it was me, that's where I would measure pressure as being relevant.

 

[Boardroom]

Yes I agree but if I run storage to where I like, the boiler pressure will be fine but the prv in my basement on my electric boiler will open.

 

[maple1]

And when you say 12psi 'cold', is that when you first filled the tanks & before you started heating them (what I would call cold), or when they are at 140? How high is the highest part of your system, above the system circulator?

 

[Boardroom]

And when you say 12psi 'cold', is that when you first filled the tanks & before you started heating them (what I would call cold), or when they are at 140? How high is the highest part of your system, above the system circulator?

Not just after filling. At 12psi, my tanks have cooled to about 100.
I have two circulators, one from boiler to storage and one from storage to house. I assume you are referring to the latter so I would say the top of my tanks are about 13-14 feet above the circulator.

 

[maple1]

You could likely lower your precharge by a couple of pounds to get a bit more mileage out of your expansion. I would be tempted to try 8psi at 100°, or 10 at first & see what happens.

Is there a temperature at which the load circs won't run if it drops that low? Mine stops running when temps drop below 120 or so.

My system has about 16' between the load circ, and the top of the system (upstairs rads). The guage I have at the bottom of the system is usually showing around 8psi when storage is fully depleted. Might be a bit on the low side of pressure recommendations, but seems to work OK.

 

[Boardroom]

Yes Maple. That is probably a good first step. Like you my concern is how low can I go before the circulators stop working? Trial and Error I guess.

 

[maple1]

Something else you could try if still worried about higher pressures, is get a little bit of air into the very top of your storage tanks to act as extra expansion space. But that won't work if there are air vents up there (unless you valved them off), or if the trapped air bubble would be close to a pipe inlet. I think I have something like that happening in my system - my horizontal tanks are higher on one end than the other & I think an air bubble has settled in there at one end of one tank.

 

[BadgerBoilerMN]

There is no substitute for experience, except perhaps knowledge. Time served in not necessarily time well spent.

The volume of expansion is a function of the volume of the system, fill temperature and operating temperature.

From room temperature to 180F about 3%. The expansion tank should be charged to match the static pressure of the system in turn determined by the highest fixture in the system, plus 5 psi.

You may have more than one expansion tank but all must connect to the system at one point and then upstream from the system pump.

The size of interconnecting pipe has little bearing on system volume or the XT size.

 

[Donl]

I though it was always best to place expansion before the circulator. Pump away from expansion, yes?

 

[BadgerBoilerMN]

Yes, pump away from the point where the expansion tank connects to the system.

 

[salecker]

My expansion wasn't big enough,i added the small expansion tank i got with my boiler and that helped.I also dropped my precharge in the expansion tanks from 12 PSI to around 5 PSI.When my system was cold i dropped the pressure till the gauge just moved.I was able to go from cold to maximum of around 190 without loosing water this year.I could use a little more expansion then i wouldn't have to mess with precharge pressures and watch the cold pressure as close.
But expansion is expensive so i will live with it for now.
So you could try what i did and maybe squeak a little more out of your existing tank.
Thomas

 

[jebatty]

There are a few downsides to reducing tank static pressure as a solution to gaining more expansion space. One is that the static pressure will be insufficient to raise the water level to the highest point in the system. The result will be the admission of air (oxygen) into the system when the system cools down due to the created vacuum, and potential consequent corrosion. The second, on a start from cool down until pressure rises sufficiently, will be insufficient pressure (NPSH) to keep the circulator from cavitating due to boiling water with consequent risk of early circulator failure. These downsides are why the recommendation is to maintain sufficient static pressure to raise the water to the highest point in the system plus 5 psi, although that may be overkill in particular situations.

 

[maple1]

There are a few downsides to reducing tank static pressure as a solution to gaining more expansion space. One is that the static pressure will be insufficient to raise the water level to the highest point in the system. The result will be the admission of air (oxygen) into the system when the system cools down due to the created vacuum, and potential consequent corrosion. The second, on a start from cool down until pressure rises sufficiently, will be insufficient pressure (NPSH) to keep the circulator from cavitating due to boiling water with consequent risk of early circulator failure. These downsides are why the recommendation is to maintain sufficient static pressure to raise the water to the highest point in the system plus 5 psi, although that may be overkill in particular situations.

Yes - so it all depends on the total height of the system, and where your pump, guages & expansion are at with respect to each other.

If the expansion tank is up high, and the precharge on it is set at 12 cold - I think the precharge could be reduced. If it is down low, where the circulator is, it will need to be set at around 12 or so (looking back & deciphering again, I think I see 17' system height - so (17x0.4) + 5 = 11.8). If it were at the top of the system, it could theoretically be reduced to 5. (I think?).

 

[salecker]

My expansion tank is at the highest point of my wood boiler/ storage loop and my pumps are a little lower than the bottom of the expansion so i guess i have the best scenario for reducing the static pressure.
Thomas

 

[DaveBP]

Just to toss my own example into the pot:

My expansion tank is up against the ceiling of an upstairs closet.
Boiler and storage tank are in the basement.
The current system temperature is about 68F (all 1100 gallons, closed system).
The pressure at the expansion tank is about 2.5 PSI.
The pressure at the circulator at the bottom of the boiler is 10PSI.
During the heating season, the lowest temp is rarely below 100F and the pressure at the circulator stays above 12PSI.
That 7 or 8 PSI difference stays constant throughout the system cycle from minimum to maximum pressure during firing.
Using a lower starting pressure at the expansion tank allows more water to enter the tank before it reaches the maximum pressure down at the boiler circulator. It's a way of adding useful expansion volume to the expansion tank.
I don't know how much.