How does boiler mixing valve affect efficiency?

Jim100Flower

I have new a Laars Minitherm JV225NDISU2 natural gas forced hot water boiler with six zones of baseboard. There is a valve (picture) above the boiler which connects the system return water, just above the boiler pump, with the output of the boiler. I’ll call it a mixing valve. What is the proper position for this valve and how does position affect efficiency? How big is the effect on efficiency between half open and full open? Right now it is fully open.

Initially the valve was half closed and the boiler guy said that position was best for efficiency. But I had a problem where if one zone called for heat, all zones would get heat. The other zones would get heat even though their thermostats were set below room temperature. Opening the mixing valve now allows the zones to be independently controlled. Does this situation point to a different problem with the system?

I don’t get the theory behind connecting return to output and how that helps efficiency.

Jamie Hall

The problem with the other zones heating when only one is calling is not related to the mixing or bypass valve as such. How are the zones controlled? Zone valves? Independent pumps? Makes a difference...

And the bypass valve will affect the overall boiler efficiency, since the idea is to keep the boiler operating at its ideal inlet temperatures and delta T. To figure out how much boiler efficiency you will lose with a wide open valve, we'd have to know more about how the boiler is controlled.

Tinman

The piping you see is very likely primary secondary piping using closely spaced tees. Not sure what the ball valve brings to the table? Laars MiniTherms require P/S piping.

Jim100Flower

@Jamie Hall: There are six circulation pumps and one boiler pump.

Jim100Flower

@Stephen Minnich What does P/S piping look like? What you see in the photo is all that is there. Is it installed incorrectly?

Gordy

What you have. That’s not a mixing valve.

That valve should be wide open.

If you have a boiler pump with closely spaced tees for the primary, and zone pumps on the secondary that’s primary secondary. It allows the boiler pump to run with out interfering with the flow rate of your zone pumps. The closely spaced tees hydraulically decouple the primary loop from the secondary loop.

Closing that valve doesn’t do anything for efficiency, and hinders the reason for primary secondary piping practices. I suppose you could control the flow rate of the boiler pump with that valve, but it is certainly not a recommended practice for P/S piping.

It certainly does nothing for efficiency.

Gordy

I suppose in a pinch if the zone pump, or boiler pump goes down you could close the valve, and make it a direct piped system temporarily.

SuperTech

First thing I thought when I saw the picture was "Huh? Looks like primary secondary closely spaced tees using a ball valve?" I'd like to see the rest of the near boiler piping. Maybe he was trying manage flow in the primary loop in an attempt to gain a better Delta T on the boiler.

hot_rod

We need to see a wider pic.

There are a few ways you will see primary secondary systems piped, see these examples.

I'm of the opinion the valve should not be between the tees as it negates part of the reason to have P/S. Wide open it should work well enough.

Gordy

Another thing in using a ball valve to throttle flow, not so predictable. 1/2 closed by the handle position doesn't necessarily mean 1/2 the flow. Plus it isn't good for the valve. I'm assuming if this is its purpose the installer was going by temperature, or delta t...........

Gordy

Gordy

Gordy

With baseboard you have high temp emitters. Not low temp. Nothing in the diagrams with a valve between tees for high temp emitters.

SuperTech

Is the baseboard cast iron or fin tube?

unclejohn

Make sure the pumps are going in the correct direction. They have arrows on them. Back up a few feet and send some more pictures.

Jim100Flower

@SuperTech here are wider photos. Baseboard is not cast iron, I think it is copper.

Jim100Flower

@unclejohn : all seven pump arrows point down which I think is correct because they are on the return side of the system.

Jim100Flower

@Gordy My system looks like Figure 10 Primary Secondary Multi-Zone Pump System (except my pumps are on the return) and I agree there is no valve called for between the Ts. But the valve is now fully open so I think it should function as in the diagram.

Jim100Flower

I found it. It is called a balancing valve on bypass piping, Figure 7 in the Mini Therm Installation Instructions, and is used to assure that the temperature rise across the boiler does not exceed 30F. I measured the rise with my longest zone running and it was a few degrees. So, I think I am fine with it fully open.
https://laars.com/images/uploads/products/1025Y-NH.pdf

EBEBRATT-Ed

I am guessing with the # of zone pumps that the system flow is higher than the boiler flow. That may or may not be true.

If the system flow is higher than the boiler flow it would seem to me that throttling the 2" valve between the tees will increase the TD across the boiler. Apparently Lars wants 30 degrees or less.

If the valve is open and the boiler runs with a low TD. Shouldn't be an issue unless it creates short cycling.

If it was me I would set the valve to maintain 20 deg td across the boiler on a cold start with all zones calling.

hot_rod

Seems chocking down the valve puts the boiler pump in series with the zone pump or pumps? Possibly very un-predictable flow rates depending on which circs are running at any time.

A 225K boiler with one 3/4" zone calling, hmmmm

Gordy

Well that is my thought. The whole premise of P/S is being able to deliver two different flow rates between the boiler loop, and the system loops. I suppose partially closing that valve between tees can have some effect on boiler delta, but then you are also have ever changing flows in the secondary as zones open, and close especially when zoning with pumps.

I'll bet that you can't dial that boiler delta in very well like that. It would be all zones open, or 1 zone open happy medium. Certainly a wag control system.

Tinman

@Jim100Flower - No. Figure 7 is different than your piping. In Fig 7, the valve is between the bull of the tees and in the actual piping, the valve is between the run of the tees.

I'm sticking to an attempt at P/S piping. Remove the valve and it's solid P/S.

Gordy

That’s quite a bit of weight on the boiler with that rack of circulators, and black pipe. I hope there is good pipe hangers up in the floor joists helping to support all that.

unclejohn

I would remove the valve or close it 100%. Then remove the first tee in the supply leaving the boiler and put a tee there. Connect the middle of the tee to the supply and the other end pipe to the return at pump #6. That gives you a constant flow rate through the boiler. Also I would add a supervent [or something similar ] in the supply and add a shut off valve after the vent. One last thing in the return above pump #1 is a tee so you need a shut off valve in each return and a bleed on each to get the air out.

SuperJ

You may need some protection against low RWT to protect the boiler, and the valve might have been a failed attempt at that. There are other methods.