Mixing Valve Cv and mix temperatures.

rhl

I installed a new pipe system, i've written to @hot_rod before. We were discussing flow rates and pressure drops.

I had installed tamas zmix 4-way mixing valves. They are cheap and plastic, and one of them just broke! (Now i know why they were delisted from SH).

Anyways, I am doing Tekmar controls and I went ahead and bought Tekmar 3-way replacements.

My question is about Cv. I understand that the larger the cv the larger the valve opening, and the more fluid that can pass through it. The tekmar valve has a Cv of 14.

The existing 4-way valve had a Cv of ~4.

What should I expect from this? It seems like this will be less of a flow restriction, but will it mix better? how does Cv relate to mix temperature?

GroundUp

Cv is merely the relation of pressure drop to flow rate. A lower Cv will have more resistance at a given flow rate than a higher Cv at the same flow rate. Basically, assuming your flow rate stays the same as it was before, the higher Cv of the new valve offers less resistance through the valve than the old one did. Think bigger hole. Mixing will suffer no change if both valves were/are in proper operation

hot_rod

in a in a perfect hydronic world you would select a control valve with a Cv close to the application requirement 
To small and you take a pressure drop hit, possibly excessive flow velocity, noise and possible erosion 

To large and the valve loses authority, forced to operate at a small opening, for example. So look at the flow requirement, use a Cv calc to see what condition the valve will be actually operating under.

rhl

is there some kind of idronics or other good resource for reading more about this? I just bought the 1" ones.

It seems like higher Cv is good, maybe the old mixers were the flow restriction..

hot_rod

So here is the way to confirm your suspicion.
I used a 3 Cv valve, with a 10 gpm flow, notice in the 3 rd column the pressure drop across the valve.

Now in a domestic water application with 45- 75 psi pressure that pressure drop may be acceptable.
In hydronics the circulator needs to be able to over come the pressure drop of the valve and circuit, so the 10 gpm is probably putting you in to a high head circulator. High head circs and zone valved systems, for example, may not get along so well.
# gpm would be the ideal flow rate in a 3 Cv valve. Obviously you will not find a valve at every Cv number you want, so this excel sheet allows you to look over the "what ifs"

In your case the higher Cv may be just about right, do you know your flow rate?

Here is the Excel file if you want to run your own numbers. change any yellow cell.

Use column 3 if you know flow rate and valve Cv, or the other two to calculate the unknown.

rhl

Thanks for the calculator.

According to my calculations at a 10 deg delta-t I would want 4GPM at design load on the floor heat circuit. At the rated Cv on the tekmar valve that implies a \Delta{P} of only .19 ft of head.

The present system is able to achieve 2GPM @ a Cv of 4, which is a \Delta{P} of ~.5 ft of head. The 4GPM figure the calculator outputs a \Delta{P} of 2ft of head.

So either way raising the Cv up is lowering the pressure drop. I dont have any way of measuring the pressure differential.

hot_rod

Here is some good reading on valve authority if you are interested

Basically when a valve is oversized you force it to work at its low end, and a tiny movement can cause a large change in temperature control, so the valve may spend a lot of time hunting.
If it is a motorized valve you may notice the actuator constantly moving, trying to find the “sweet spot”

http://www.healthyheating.com/Control-Valve-Theory/Control-Valve-Theory-pg1.htm#.X2tefiQ1jYU

rhl

I just skimmed through that note. I still have no idea whether things are under or oversized. 

You said earlier:

in a in a perfect hydronic world you would select a control valve with a Cv close to the application requirement.

i don’t really get how to model this.

i wish there was someone way we could speak on the phone.. 

hot_rod

rhl said:

I just skimmed through that note. I still have no idea whether things are under or oversized. 

You said earlier:

in a in a perfect hydronic world you would select a control valve with a Cv close to the application requirement.

i don’t really get how to model this.

i wish there was someone way we could speak on the phone.. 

Yeah valve authority and such gets a bit technical. Robert Bean does one of the better jobs explaining the importance of correct sizing a valve. Read that attachment a few times to understand the terminology.
If your load requires 4 gpm, sounds like the previous 4 cv valve was a good match? Did it work okay other than the issues with plastic construction?

Installers usually find a way to pump themselves out of an undersized valve installation Oversizing, especially grossly oversizing a valve, control valve leaves you few options.

I think the valve you are installing is a rotary style, it may behave ok. Ball style valves you really don't want to run at a 70-80% cranked down position, bad things happen quickly.

Two way to know what is going on, or will be, run the numbers with accurate input data on the actual circuit under design flow.
Or gauge up the system and observe actual running conditions.

hot_rod

looks like the 3/4 valve with a 7 Cv would be closer to what you need?

rhl

Thanks @hot_rod. 

Like I said, the 4Cv valve @ present we can only get half the flow rate of ~2GPM. 

when I ran the boiler @ 140 degree water the fully open position mixed to 120 deg. I was hoping to see 130. 

So basically if I raise the Cv up I’m expecting that the circuit flow rate should increase (because the Delta{p} is dropping), but, if I go “too high” then I may not be able to get the mix range I want, and you think the 1” may be too high cv and the 3/4” may be closer to correct ? 

Best,
-rhl 

hot_rod

Keep in mind it may not be lack of flow rate that is preventing you from seeing the mixed temperature you are asking for. If it is a thermostatic mix valve, they require a certain amount of delta to mix properly and accurately.
Most thermostatic type mixers want a 25- 27° difference between the hot entering the valve and the mixed outlet requirement. The small wax cartridge "heat motor" inside needs both adequate flow and temperature difference to regulate. Often times increasing the SWT will correct a thermostatic valve that is misbehaving.
A valve that is not regulating accurately, or the temperature keeps moving up and down, the valve is hunting
Some thermostatic valves are tested and listed (ASSE) to maintain within a 2° range, it's a 30 day flow test, so that is a reliable valve.

If it was a motorized type mixer, the control and sensors also need to be calibrated and working properly. Thermistors do have a bit of lag time. Motorized valves, most, have the ability to completely close off one port or the other, so temperature regulation should be very accurate across a wide SWT.

Regardless, moving forward, select the valve that is closest to your requirement, flow rate and temperature regulation range.

That motorized Tekmar, both 3 and 4 port has been around 30 years or more, it has been an accurate, reliable valve. It is more dependent on the actuator and control to be accurate, the valve (brass) itself is a dumb valve
When properly applied and tuned in it should regulate temperature spot on.

One last point, don't count on a set SWT to maintain the systems operating delta T. The delta through the emitters can and will constantly change based on the load it sees. A cold start will show a large delta, often larger than design. As the load is dropping, so will the delta, regardless of the emitter type. This is the indication of a healthy, properly operating hydronic distribution. You may see a 3-5 ∆ just before the load satisifies. So basically the delta you see is an indicator of the heat energy being moved, the math is simple.
500 X F x ∆
500 X 4gpm X 20∆= 40,000 BTU/hr
500 X 4gpm X 15∆= 30,000 BTU/hr
etc.

rhl

Ok, based on this I am going to use a 3/4” Tekmar mixer not the 1” because the Cv (while higher than before), is lower, and smaller fitting means perhaps a more accurate mixing ability. 

mattmia2

Taco also makes a few mixing valves with an actuator that could be worth looking at. Just read the instructions to make sure you can set it to do what you want, some are set with dip switches and only have a couple setpoint options, but they are available in several CVs and I think quite a bit less expensive than the pieces you need to put together a Tekmar system.