I have an old house, built in late '20 -and lately problem with a
pressure in (hot water) heating system. Everything's been normal for
last couple of years but ... things change, I guess.

Problem: very soon after furnace gets signal from main thermostat in
the house, water starts to heat up. Water pump turns on (around 130F)
and the pressure builds up. So far everything seems normal but usually
after several hours pressure gets too high and security valve opens
and attempts to lower that pressure. You can imagine my surprise when
I first time saw that system is "leaking" and that a good part of my
basement is wet because of that.

To (temp) fix it I do the following:
-turn system off
-close valve that leads to the expansion tank
-remove water from the tank
-open the valve that connects expansion tank to the rest of the system
(that drops pressure to "normal value which is shown as 10-15 feet)
-turn system on

Settings on the thermostat is: 130F (when water pump kicks in) and
160F when heating stops

What could be wrong? How to setup termostat so system'd work as it
used to?
Thanks for the input.

In article , (Nenad) wrote:
I have an old house, built in late '20 -and lately problem with a
pressure in (hot water) heating system. Everything's been normal for
last couple of years but ... things change, I guess.

Problem: very soon after furnace gets signal from main thermostat in
the house, water starts to heat up. Water pump turns on (around 130F)
and the pressure builds up. So far everything seems normal but usually
after several hours pressure gets too high and security valve opens
and attempts to lower that pressure. You can imagine my surprise when
I first time saw that system is "leaking" and that a good part of my
basement is wet because of that.

Hot-water heating systems are typically connected to the house water supply
through a pressure regulator, so that the heating system can be filled easily
and yet it is not exposed to the relatively high pressures typically found in
house water supplies. Possibly your pressure regulator has failed.

Assuming you have this type of setup, if there is a shutoff valve between the
pressure regulator and the boiler, you can easily tell if the problem is the
regulator or a waterlogged expansion tank, as another poster suggested.

Follow the same procedure you've been using:

-turn system off
-close valve that leads to the expansion tank
-remove water from the tank
-open the valve that connects expansion tank to the rest of the system
(that drops pressure to "normal value which is shown as 10-15 feet)

but before you turn the system back on, close that shutoff valve.

If the problem comes back, you may indeed need a new expansion tank.

If the problem does not return (and I'm betting it won't), replace the
pressure regulator. Expect to spend about $40-50 for the part. You won't find
one at Lowe's or Home Depot. You need to go to a plumbing supply house. Tell
them you need a pressure regulator for a boiler. Look at the existing one
first, to see what size fittings it uses, and what pressure it's rated for.

Good luck!

--
Regards,
Doug Miller (alphageek-at-milmac-dot-com)