Residential system pressure issues

peterslug

I have a Carrier Weathermaker series high efficiency boiler with a boiler circ pump on the return side. The boiler loop runs to the expansion tank then through gate check valves to three zones. In the attached photo, the boiler loop runs around the back of the boiler and connects to the return side of the three zones. The three return side circ pumps are independently thermostat controlled. Check valves were added to the return side of the zone pumps. The main return loop then runs around the basement through a set of closely spaced standard tees and then through a cast iron radiator before returning to the boiler. This 1” return line stays warm the longest after the system runs. The system’s water supply goes through a pressure regulator (12psi) and is piped directly into the basement return loop prior to feeding into the boiler pump and the boiler itself.

Zone 1 is a primary high temp loop with a secondary (lower temp) loop connected with closely spaced tees. The radiant served manifold has 3 zones of  1/2” oxygen barrier pex. There is a manual Taco mixing valve and temp sensor. The mixed line has an Alpha 1 variable speed circ pump and integral check valve. This pump is wired independently from the primary loop pump.

Since the system was opened to add check valves and incorporate the secondary radiant flooring zone, there is air in the line causing audible water flow through the baseboard radiant fin tubes (zones 2 and zone 3). Each zone and the main return loop have been sequential drained in attempt to remove the air. 

Rough schematic:

The problem: The system will intermittently vibrate loud enough that I get concerned about cavitation and the circ pumps and I shut down zone 1. The issue decreases when the primary pump on zone 1 is not running. My assumption is there is a pressure issue in the system together with the entrained air.

The pressure gauge on the zone 1 primary return reads as 10psi.

Potential fixes:  Add an air separator at the pressure tank side prior to the expansion tank?

The expansion tank - tapping sounds like it still has air at the base but I can check it  while I have the system off the boiler drained.

Does an auto flow balancing valve need added?

Thanks in advance for the input.

hot_rod

The loop that comes out of the boiler, loops around and back into the boiler is the primary loop. We consider the loop that has the expansion tank to always be the primary.

That boiler pump needs to run when it is firing to assure adequate flow
Zones 1,2 & 3 are parallel loops to the primary, with two additional loops off closely spaced tees.

When Z1- 3 are running those pumps are in series with the boiler pump, so you will get additional flow velocity.

For the radiant to work properly Z1 will need to run, the mix valve pump, and possibly the boiler pump, depending on how it is piped inside the boiler?
Of course when the boiler pump runs that radiator will heat.

The small sub loop will need a pump also.

Do all the zone pumps have check valves installed?

It is a bit of an unusual piping, how is it controlled? Seems the radiator pictured below the boiler will always heat when any zone calls for heat?

Z1-3 and the low temperature loops would all be taken off the primary loop via closely spaced tees to be a true P/S piping.

peterslug

Bob, thanks for looking at this and clarifying the P/S loops.

I’ve tried to answer your questions below.

Getting the supply and returns for the parallel plumbed Z1-3 to come off the boiler primary loop with closely spaced tees would require re-plumbing the system which is not exactly feasible at this point.

The boiler pump is a cartridge pump and I think it runs when any zone pump runs.

Yes, the cast iron radiator heats whenever a zone calls for heat. It also just sits and radiates heat until the boiler has to fire again making the basement the warmest room in the house.

There are gate check valves at the supply side of each zone and check valves at each zone pump and at the mixing valve pump (return side).

Each Z1-Z3 pump has a relay panel in a junction box below (see the original post picture of the three blue pumps). The relays are connected to the boiler panel with thermostat wire.

It sounds like the potential edits to the system include some or part of the following:

1. Control the mixing valve pump together with the Z1 pump.
What if I remove and relocate the Z1 circ pump and check valve to the mixing pump’s location? Then there is only one pump pulling from the primary loop and it would maintain operating at the same time as the boiler pump.

2. Re-plumb the floor radiant portion to come off the primary instead of the Z1 loop. Side note, the floor radiant does heat the room and seems to work aside from the issues mentioned in the first post. 
3. Add a pump to the small sub loop off the primary. Does this pump need to turn on together with the boiler pump?

EdTheHeaterMan

@hot_rod said: When Z1- 3 are running those pumps are in series with the boiler pump, so you will get additional flow velocity.

and based on the location of the expansion tank (the Point Of No Pressure Change) the auto fill may add more water to the system increasing the pressure to the expansion tank above the desired 12 PSI.
The zone circulators are then pumping towards the lower pressure of the boiler pump inlet, further lowering the inlet side of the zone pumps. There may be a situation where the high temperature loops may have a lower than atmospheric pressure on the upper floors of the system. That can sometimes be noisy.

I might reconsider the location of the PRV feed valve and the boiler pump in relation to the expansion tank and perhaps get the high temperature zones on a true secondary loop of their own with closely spaced tees or some other design to hydraulically separate those pumps from the boiler pump.

EdTheHeaterMan

This might be a better design. There are others, however this will be the least amount or repiping.

Zone 1 (the low temperature zone) does not need a zone circulator and a mix circulator if you pipe the closely spaced tees directly from the primary loop. Zone 2 and 3 can use the existing tee fittings and flow valves, Just place the circulator on the supply side of the secondary high temperature loops just before the flow valve. then return the zone with the closely spaced tee right next to the supply. A little further down you can make zone 2 in the same configuration. After all the high temperature loops are taken care of, you can then take the low temperature loop off the primary loop.

If the basement radiator is making the basement too hot, you can make a secondary loop for that radiator and use the zone 1 pump that you no longer need for zone 1.

As I said, there are better designs but this is the least amount of piping changes. If you want to start from scratch and make it the best design, I will need to know what is on the small sub loop and if you plan on a Indirect DHW tank in the future.

peterslug

Ed, Thanks for including the proposed schematic. 

As a general note, the system “worked” as in made heat and didn’t vibrate etc. until the mixing valve radiant loop and pump were added off of z1. Connecting the radiant loop to the primary with closely spaces tees after the high temp zones seems like the logical first step to fixing the issues. With this change, the original z1 circ pump and parallel piping can be removed.

Shifting the supply input, circ pumps, and z2 and z3 to closely spaced tees will likely need to wait till after this heating season. 

At that point, how do I size the flow valves for Z2&3? 

There is no plan to add DHW to the system. The small sub loop is about 30’ linear of subfloor radiant. Like Bob said this loop will need a circ pump.