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Pressure relief for ho
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john_58
Member Posts: 37
I have a large old house with hot water gas heating. Was an old gravity system. I have 40 ft from bottom of boiler to top of highest rad. Heating professional just changed exp tank to new large bladder style, also supply valve and relief valve. The supply will automatically add water at 18 psi and the relief is set at 30 psi. During the heat cycle the releif is letting go, replaced again, still letting go. The gauge indeed says its at 30 psi when hot (165F) No one wants to change the PRV to a higher rating for a number of reasons (insurance, etc). Can this be changed to say 35PSI? Any help or opinions is welcome. Thank you.
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Comments
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Expansion tank
Was the tank also pressurized to 18 psi before you installed it? If not, it was probably set at 12psi and won't have enough air in it now. In that case, it will be low on expansion capacity. If it was 18 psi, then perhaps it isn't big enough for the volume of water there. Gravity systems have a lot of volume. I would get the tank to handle the expansion and not the PRV.0 -
engineers answer
you cant just change the tank. The pressure relief valve is fine, the automatic feeder is fine. The bladder tank is probobly too small depending on the size of the system. you can quickly get the volume of water by adding up roughly the length of pipe and multiplying by that pipes volume. then you need to calculate the temp changes and operating pressure. go to the www.amtrol site, they have a spreadsheet for doing that. VColume of pipe 2" is 2.14 gallons per foot steel pipe, 1" pipe is .559gallons per foot, 3" pipe 4.78 gallons. use these, numbers could be rough within reason, but most likely you need to go to a larger bladder tank0 -
Those volume numbers...
seem very high for those pipe diameters. I don't even think that there are that many pounds per foot, no less gallons?0 -
Found better numbers...
Gal/ft typically: .045 for 1", .174 for 2", .384 for 3"0 -
Pressure
You need a bigger extrol tank.0 -
read wrong units, those were in liters
so yes you are right0 -
Something still not right...
liters to gallons is about 4:1, our numbers are different by over 10:1. I don't want to give this guy the wrong info. - I don't get involved with these calculations normally - and only have one source for the info. handy at this time. Can someone else confirm the right numbers?0 -
You are correct
Having worked out the math (Roughly - I hate math...) Area of a circle to get square inches times length of pipe then dividing that into the 231 cubic inches of water in a gallon. Your #'s are quite accurate...0 -
Thank you everyone
The new tank is a 13.2 gal CONBRACO charged to 15psi. The contractor seems to think its plenty big, in fact far oversized. The house is 7000sq ft with 4 inch mains and 1 inch risers supplying 9 bedrooms, etc. Huge rads. Any other infor is always appreciated. p.s. Contractor thinks that the pump might not be pushing enough water.0 -
I will try some rough calculations regarding volume. Isn't it as simple as 1psi = 2.31 feet of water?0 -
That conversion is...
correct, but it does not apply to volume of the system. With a 40 foot rise, your system pressure of 18 psi cold should be just enough to get water to the highest point. Is your heat output up there is sufficient? I don't see how pump output would be causing this particular problem - try to do the volume calculation, if you are able.0 -
location of expansion tank and relief valve
another two things to look at is where the expansion tank is located, it should be located on the suction side of the pumps, that tank sounds big, but you are more worried about the "acceptance volume" this is the amount of water that will expand in the system. From what you are saying, this is a large house. even a 18 gallon tank can be small because its acceptance can only be 4 gallons (for arguements sake). a house os that scale is huge. The other thing to look at is what boiler is there, whats the ratings in btu/s input and output, you also have to make sure that the relief valve is properly sized for trhat boiler, if the boiler recovers very quick, quicker then the pumps can push the hot water out you might blow the relief valve, but chances are thats not the case. Make sure the ratings on the relief valve tag match the boiler settings, and finally, put on a pressure gauge on the boiler drain valve, double check the pressure, remember that the gauge on the boiler is mechanical and we all know at one point or another these stop working. so thats all i can think off, i wouldnt be surprised if you get into a small light commercial expansion tanl. 7000 s.f. man i must be doing something wrong, i got 12oo and i cant find my keys, damn0 -
15psi?
That's part of your problem right there. It should be pre-charged to 18 psi. As soon as the system gets filled at 18psi you have lost volume for expansion in the diaphram.0 -
Thank you, heat is great up there.0 -
Thanks! The tank is not on the suction side. The PRV is sized proper to the boiler, which is a Lochivar 270btu. The boiler takes a while to recover. When its 0*F, system runs constant. Have two pressure gauges, so I assume they are accurate. We actually can't find our kids some days, let alone keys.
p.s. I have a small apt bldg (same size/style as my house) which has 45PSI PRV, I don't get the difference here?0 -
John
It depends on what body certified the boiler. You should close off that expansion tank and get it charged to 18psi and then try it out. That's a quick win that just might solve everything.0 -
Can you detail...
exactly where each of the components (expansion tank, feed valve, pump) are in relation to each other and whether on the supply or return piping (or directly connected to the heat exchanger)? Do you know for sure what the expansion tank air side pressure is/was charged to (when there was no pressure on the water side)?
The different settings on the pressure relief valves may be based on the maximum pressure rating of the heat exchanger of the different boilers. The rating plate on each boiler should give the maximum allowable pressure and minimum relief capacity for that model.0 -
The Max water temp 240F, Max Pressure 160, Min R.V. Cap 269,000lb/hr, input rating 270,000btu. Tank was charged to 15psi
Follwing the boiler out - first 'T' is trid. gauge then fresh water supply then TACO air scoop with auto vent on top, exp tank on bottom, then pump, flow switch and into system. Boiler input has only temp gauge. second pressure gauge on fresh water supply. Make sense? Thanks for help.0 -
The location of those components...
sounds proper. I read in one of Dan's books that if the fill pressure is set to 18 psi, and the expansion tank air side charge is left at 12 psi, the tank loses half of it's capacity rating - so maybe at 15 psi charge you are losing 25 % of rating? Some of the other posters here alluded to this, also. Can you increase the air side charge pressure (you have to do this with no pressure at the water side).
If you still want to the system volume calculation, the gal/ft for 4" diameter pipe is .661. If you need the content for the radiators, you will have to post information on them (brand, dimensions, number of sections) and someone will look up the capacity in the refence book.
Lastly, that boiler has a very high maximum pressure rating, so it would seem that a relief valve with a higher setting could be used - but perhaps there are reasons not to replace the 30 psi relief with a higher setting in a residence?
I hope that I am helping here - I feel that this is really a simple problem, and a more experienced person could have solved this more quickly. Good luck!0 -
Thank you very much Jim, and everyone else. Everything is being done by professionals and I am just trying to keep on top. My next step is getting the bladder tank set right to begin with. This has been a great experience coming on this board. It has been hard finding professionals in my area that know these converted gravity systems.0 -
Do i have to remove the new CONBRACO bladder tank to re-set the pressure with a bike pump or can I do it with the water supply for just turned off? Anyone know? Thanks.0 -
Charging the bladder
must be done with ZERO pressure on the water side. Can be tough to do if not properly set up...
Are you really runnning the system at temperatures of 240 degrees F? If you are, you'll need more pressure than the 18 psi you;re currently running. Also, the boiler COULD take a higher relief valve setting, but it may just "hide" the problem.
Your system should need more than 180 degree F water at design condition. Also, if it is running all the time, and not hitting the high limits when its cold outside, it could be undersized.
ME0 -
Filling a tank
The tank should be separated from the system to pump it accurately. Ideally, you have a valve right before it and a small drain port. Let the system cool before you do it, and be ready because you easily might end with with a few gallons of water in that tank wanting to go just about everywhere.0 -
I just looked at the quick sizing chart...
for the Flexcon diaphram expansxion tanks we use, and it shows a 15 gallon tank for your size boiler, and that's if your system is at 12 psi. This tells me your tank is too small, on top of the fact that the air pressure is not set right. Unfortunatly, they're chart does not go to 18 psi, so you would have to call them or whoever made the tank you have.
Here's the links for the quick sizing chart and the tank spec chart I looked at.
http://www.flexconind.com/pdf/htx-sxht_sizing.pdf
http://www.flexconind.com/pdf/htx_specs.pdf0 -
I think John is...
just providing the rating plate specifications when he states the 240* F water temperature, not his high limit setting (I hope)?0 -
Thanks Glenn. Good info to have.0 -
Really, that sizing chart...
shows a SXHT 40 tank, with a 20 gallon capacity - for that size boiler, with cast iron radiators. And that probably doesn't even take his 4" main into consideration. He might need a much bigger tank, by this information?0 -
You see, I requested from my heating contractor to WAY oversize the tank, as there is little harm in so doing. He thought this WAS oversized with his calculations. He said this what they would put in 5 storey apt buildings. Is it possible to just add another tank or should they swap out the last one with a larger (ie. 20 gal) unit. thanks all again. ps. yes Jim, those numbers i quoted were from the plate of the boiler. My max water temp is 165F at constant burn.0 -
the 18 psi is just enough to get the water to the top rad 40 feet divided by 2.31 ft./lb. = 17.3 psi. you need to have additional pressure to keep the air entrained in the system, figure another 4 psi. now you are up to 21 psi still within the range of a 30 psi relief valve. check the cold fill pressure and resize your expansion tank.0 -
My error, Jim is correct
it does list a SXHT40 for cast iron rads.
John, please keep in mind the sizing chart is for a 12 psi system. I honestly can't say how much more capacity you actually need for your 18 psi system.
It does occur to me that I had a job a couple of years ago that had a 180,000 btu boiler, set at 18psi (hydronic air handler in 2 story house attic), and a call to Flexcon showed an SXHT 60 was needed, or about 24 gallons of capacity. Based on that, I figure you need close to 30 gallons of capacity, give our take. I also looked on the Conbraco/Apollo website, and see the largest tank they make is the one you have, so to stick with the Conbraco you would need to use at least two tanks, with Conbraco's blessing. In other words, you or your contractor need to call Conbraco with the specs of your system, have them size the tanks needed, and get there ok and a piping diagram as to how they want multiple tanks installed. Otherwise, switch to a different brand (Flexcon, Amtrol, etc) and have them size the appropriate tank for you system.
http://www.conbraco.com/products/ph/techtemplate1.asp?section=D&pid=16XT#overview0 -
Okay. So I need a tank more sized to 30 gal. I will do the math and find out how much volume I roughly have. This is going to be tricky though. There are nine bedrooms, 5 bathrooms and numerous closets all with rads in them, not to mention living rooms, dining, family, hallways, etc, etc. I will do it though. But what would be the harm in getting a 40 gal tank to be sure. I am under the impression that you cannot do much harm in oversizing a tank. Any input? I also calculated about 37 feet of rise and rounded it up to 40 just in case there is plumbing behind a wall in the attic somewhere. I estimated at 18 psi for the makeup water and min setting. Should this really be more like 21psi for a buffer.0 -
Strictlyug
> Okay. So I need a tank more sized to 30 gal. I
> will do the math and find out how much volume I
> roughly have. This is going to be tricky though.
> There are nine bedrooms, 5 bathrooms and numerous
> closets all with rads in them, not to mention
> living rooms, dining, family, hallways, etc, etc.
> I will do it though. But what would be the harm
> in getting a 40 gal tank to be sure. I am under
> the impression that you cannot do much harm in
> oversizing a tank. Any input? I also calculated
> about 37 feet of rise and rounded it up to 40
> just in case there is plumbing behind a wall in
> the attic somewhere. I estimated at 18 psi for
> the makeup water and min setting. Should this
> really be more like 21psi for a buffer.
0 -
No harm
No harm in oversizing the tank other than to your wallet.0
This discussion has been closed.
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