thermal traps

Brad White

Once filled, the system should hold at least it's cold-fill pressure and may increase by 5-8 PSI when fully hot, depending on the size of the expansion tank.

Any drop below your cold fill pressure is usually a leak. Check your fittings, valve stems for signs of weeping and corrosion. Leaks may not be obvious. A drop or two an hour add up. Hard to see that small a leak on a given day.

DO get a Backflow Preventer. Watts 9D is a common brand/model. About $40 last time I checked. Just do it. You could be drinking that stuff.

15-20 PSIG in a 2-story house does not concern me. Just have enough cushion below your 30 PSI relief valve setting, that's all.

Sparky Bill

thermal trap

While helping my brother-in-law with his heating system: i.e. installing automatic air vents, adding make up water via manual feed, purging the system, etc. I noticed the heat was migrating up the boiler's make up water piping.

He and I plan to replace the manual feed with an automatic feed (pressure reducing valve). I would like to also repipe the make up water with a thermal trap to contain the migrating heat.

Is there a minimum vertical dimension for an effective thermal trap? Any other important criteria?

Brad White

Normally

there should be a backflow preventer and I hope that there is. This will eliminate a tell-tale backflow condition you may have, lower pressure in the domestic water line which entices thermal flow not to mention direct flow. The fact that the flow is happening at all is cause for concern. The heating system and domestic water system must be disconnected from one-another.


For typical thermal migration conditions, a drop or dip of 28 inches would be enough. This represents 1 PSI of pressure to be overcome. If the thermal gradient is quite high and the building is tall, doubling this is recommended.

ALH_4

Leaks

Is the system is leaking enough that continuous automatic feed is needed? I would fix the leak(s) before going any further.

Sparky Bill

The original problem was no flow on the second floor. the system pressure was 10 psi. I opened up the manual feed and raised the pressure to 15 psi.

I thought this was the first time I had seen a manual feed, however I just looked in my B&G catalog it looked like a B&G FB-38 with manual fast fill. Is there a difference?

There is not a back flow preventer. I was considering adding that too when we changed out the RPV.

Could the lack of a BFP cause the system presure to decrease from 15 to 10 psi? I originally thought it to be a slow leak somewhere in the system.

Sparky Bill

No leaks are apparent. Although they just bought the house about 18 months ago.

ALH_4

pressure

How is the expansion tank? How is the relief valve? if the relief valve has not been changed in quite a while, I would replace it. If the system has a bladder type expansion tank and it looks quite old, I would replace that also. A water-logged expansion tank will make the system pressure fluctuate quite a bit.

Caleffi makes a nice automatic feed with backflow preventer attached.