pressure flux in primary loop

DD_5

I'm currently looking at replacing an on demand water heater (takagi) that is being used for a radiant heat only application.



Yes I know....the application itself is a mistake but what's interesting is the malfunction and I'm curious about any ideas about it because it may apply to radiant apps in general



System:  The primary loop is in 3/4" copper and its a very small loop, maybe 10 linear ft or so with a grundfos pump sending water through the on demand at 2.6 GPM. The loop passes through a flat plate heat exchanger. On the other side of the heat exchanger is the secondary loop that has its own circ pump that sends a circuit through five steel wall hung radiators. The 2.6 GPM flow activates the flow switch on the on demand which then heats the water in the primary loop to 165F.



Problem: Whoever installed this did not include an expansion tank on the primary loop. So what happens is when the on demand fires and heats up the water, the pressure rockets from 20 PSI to over 100 PSI...can't say how much since the gauge only goes to 100.



Solution? : if I install an expansion tank on the primary and set it at 30 PSI will that compensate that pressure jump? Doesn't seem to me like it will be enough to keep it around 30 constant and it also means a steady PSI is dependent on the expansion tank not failing...which of course it will do at some point....and if I work on this then I frigging own it....



General Rule: I'll probably replace the on demand with a boiler but would there always be this large of a pressure jump in any primary loop that does not include an expansion tank?



 

Bill Clinton_6

expansion tank

Yes, you need an expansion tank.  It doesn't need to be large because you have such a small water volume.  It sounds like your system does not have a feed valve/pressure reducer.  Therefore it's cold fill pressure is the same as the house pressure.  Pump up the pressure in the expansion tank to match the house pressure before installing--almost certainly more than 30 psi.  Should do the trick.



Yes, there will always be a pressure jump with no expansion tank.  When you install a new boiler, isolate it from your domestic with a pressure reducer and include an expansion tank AND a 30 psi relief valve so pressure never gets over 30 psi and you are safe from over-pressure--unless there is a hydraulic reason for higher pressure.  40 and 50 psi relief valves are available, but use only if boiler is rated for them.  Always adjust pressure in expansion tank to match cold fill pressure.  Do this with expansion tank isolated from the system.

Bill

DD_5

why the fast, big pressure rise?

great thanks.



but the mystery to me is that there is a prv and backflow seperating the system from potable and when the system is cold it rests at 20 PSI. Pressure rises, dramatically, 80 PSI at least, when the heater fires.

Jean-David Beyer

What is the setting on the PRV?

If it is a PRV from a typical hot water heater, it could be rated at 150 p.s.i. 210F.

Bill Clinton_6

pressure flux

The pressure reducer is a one way valve.  Once water gets into your system, its only path our is the relief valve.  Heating the water makes it expand. That increases your pressure. JDB is right: your pressure relief valve is probably a 150 psi model.  If so, replace it with a 30 psi model AND add that expansion tank.





Bill Clinton

icesailor

Hydraulic Pressures:

I was told that you can compress a gas until it becomes a a liquid.

But you can't compress a liquid. That liquids expand when heated. And the difference between 10# of water, at 10# of pressure, can expand to 100# by forcing a drop of water into it. A fast pick-up boiler with not a lot of water will cause the pressure to rise rapidly. Like a heating system with a broken pressure tank.