pressure

Eric_10

On a fhw boiler cold the pressure is 12-15psi.With an appropiate expansion tank is it ok to see the pressure rise to 20 psi by the time the boiler reaches hi limit.
Im assuming because of the temp rise you will see the pressure flucuate even though the tank is working
curiousity kills.
thanks for any input....

john_35

expansion

Yes the pressure will rise to 18-20 lbs normally when it heats due to expansion in a closed system. As long as it stays below 25 there's nothing wrong with that. If it goes to 30 the relief valve will open up and spray water (on the floor, hopefully). That's the signal the expansion (compression actually) tank is failed or flooded.

Kal Row

ok here goes again...

There is hydraulic pressure,
Static pressure,
Dynamic pressure,

“Hydraulic pressure” is the pressure evenly distributed in a cylinder – but doesn’t take into account the weight of the fluid which will make the pressure higher at the bottom

“STATIC PRESSURE” – is pressure at a point in a stationary system and at a fixed temp

“DYNAMIC PRESSURE” – is pressure at a point in the system moving and at a fixed temp

you need about 1 psi of “hydraulic pressure” (measured at the very top of the system) to keep the air out

you need a “static pressure” of “top minus bottom in ft” divided by 2.31 to push the air all the way out the top

now – if the expansion tank precedes the pump (“pump away”), then the prev formula is ok, but if the exp tank succeeds the pump (“pump to”), then you need to add the pump pressure differential to the static pressure to get the correct "DYNAMIC pressure" (usually about 5-7 psi more)

I is not unusual for the pressure to rise 5psi from 65f to 180F – especially if the exp tank was not charged to the correct static pressure to begin with, so the diaphragm is really stretched by the time the system gets hot, just make sure that the exp tank was charged with air correctly – by correctly, I mean empty of water –
They usually come factory set to 12psi and if you are “pumping to” the exp tank. Then you will need to add at least 5psi to this setting

And keep the max, at least 5psi less then the boiler’s relief valve

Ken_8

Or...

Undersized.

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Eric_10

pressure

Any rule of thumb or formulas on correctly sizing an expanion tank? Could oversizing be an issue?I generally see #30 or #60 type in the work we do. How do you know when to step up? Thanks for your input.....

Big Ed

Tanks

We tend to oversize tanks for breathing room . A #15 = a 15 gallon compression tank which would be the norm. For a couple of bucks more we throw on a #30 . The basement rule of thumb if knowing the tank is properly charged .........If you see 10# pressure rise on temperature rise , you need more expantion...

Tip from the tank guru's ....Two tanks work better then one..Meaning two #30's are beter then one #60....Also easier to store on your truck :0 .... Important:::::: All expantion needs to be installed at the same pipe location .If at two ends of the system you would create pressure slop !

Kal Row

oversize usually not a problem

a simple rule of thumb is 1/10th volume in gallons

3/4 copper is 2.69 gallons per 100ft, add that to the boiler's and boiler room's piping volume (usually less than 10gal) and divide by 10 - to get requred exp in gal

a #30 tank, is usually enough for most homes up to three apartments - it's only when you get into those large outdoor snow melt applications that the water volume becomes an issue - but then the expansion is only based on half, since the returns are so much colder than the supply

Eric_10

presssure

Your input is helpful.........thanks

Weezbo

heres a few ideas..

a leak can cause weird fluxuations,too much air in the system (lack of air eliminators)deliberate setting of pressure for certain types of recirc pumps....now you have some fresh room for thought. a functional Xtrol 30 is a relative standard devise,...on a home owners boiler.

Mark Eatherton1

The only way to know for sure..

is to calculate the system volume, minimum fill pressure, maximum allowable fill pressure, minimum temperature, maximum temperature, fluid type and the consequential expansion of fluids. Siggy has a nice program to calculate it in his software at www.hydronicpros.com

Oversizing a tank only cost a little money. Undersizing can cost you a major component, like a disolved boiler due to continuous leaking relief valve.

ME

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Brian Macdonald_6

measure volume?

HO here. My new boiler was installed with a undersized expansion tank, exhibiting the symtoms described above. The Contractor put in a larger tank with few hassles. But it got me to thinking...Could the volume of water in a system simply be measured instead of calculated? Two possibilities. 1.) When draining the old system (for a boiler changeout) sit a guy with a two 5 gal buckets at the end of the drain hose and count the number of buckets of water that come out. Might take half an hour on a big system but cheaper than the callback that my contractor had to make. 2) Read the water meter before and after filling the new system. My water meter reads to tenths of a cubic foot (.748 gal). Probably accurate enough to make sure the tank is the right size, no? Of course the best method is the one that gets the job done right with the least time (money) invested. Would either of these ideas take less time or provide greater accuracy than calculating the volume by adding up the pipe volume per foot plus radiant units plus boiler volume? By the way, I am addicted to reading this forum. Brian Macdonald

Kal Row

reality check: you can't count on a more than 90% draining

Brian Macdonald_4

Still, how does that accuracy compare with trying to calculate the volume of a gravity conversion job with pipe sizes ranging from 3" to 1" in varying lengths with most of the pipe concealed inside the walls? How much time would be spent trying to crunch those numbers compared to just reading the water meter twice?

Big Ed

Residential

I would say about two hours work measuring the volume...

The home owner would not want to pay for the time figuring it out . Much easier for both parties to just add another tank on a small job if needed.....