MTHW for HVAC heating coil

[WilliamTz]

Hello

Has anyone any suggestions regarding trying to control an air handler off heating coil temperature when using MTHW at 135degC
Control is very difficult and the temperature tend to over shoot.... its the huge difference between the water temp and the desired air temp is the problem I think.
Air handler is 10m3/s
Air off temperature approx 15 degC.
Im using 2 port control valves.
I've seen installation using pumps to maintain constant volume on the coil...would this help?

 

[EnergyProfessional]

Valve authority and coil size (based on the expected fluid temperatures) need to be correct to have good control. In addition you need PID or at least PI controller.

 

[IRstuff]

Control loops, however, are highly dependent on system lag; if there's too much lag, there might always be some instantaneous variabilities. If you have too much lag, you might need some sort of feedforward adjunct to the main control loop.

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[nuuvox000]

What is the desired leaving air temperature?

 

[GT-EGR]

You could create a lower temperature sub loop to and from the coil with a dedicated pump and a mixing valve. Having the loop recirculate on itself and just bleed in MTHW water with the coil return water as needed to bring the loop temp up to a more reasonable HW temp. Then pick the HW control temp to whatever it was designed for.

 

[WilliamTz]

Hello GT-EGR.
Thanks for your tip!
Does the attached sketch look ok.
Some diagrams show what you explained with no control over the MTHW supply and just a bypass before the mixing valve and pump arrangement.
Can I delete the by pass and put in a 2 port valve since the overall system is 2 port? The 2 port valve will maintain the desired LPHW temperature
Your thoughts?
William

 

[GT-EGR]

I don’t think you want the 2-way and 3-way in series, it should probably be one or the other.

If you go with 2-way option you need a check valve in that line your crossed out so the pump doesn’t get starved and can recirculate on itself.

In both scenarios, the 2-way or 3-way controls loop temp.

Then you can either have the pump speed modulate to control your coil air temp. And or you probably need a bypass valve in front of the coil so your pump minimum flow doesn’t over heat the coil (or you could reset this sub loop temp at that point). Or if you have constant volume pump then just have the bypass valve do all the coil air temp control.

 

[WilliamTz]

GT-EGR
Interesting arrangement.
What do you think of the attached slightly modified;
Use 2-port and retain the "decoupling line" as you previously mentioned.
Use a 3 port diverting valve further down in front of the coil and a constant volume pump.
What do you think?
Regards,
William

 

[GT-EGR]

I think that can work but as I look at it if this is really just one coil you could go simpler.

Use that 2-way valve to directly control the coil leaving air temp (your goal), and just pump through it without any valve. Basically on a call for heating the pump comes on, and if setpoint isn’t being met have 2-way slowly modulate open to warm up the loop, which if sized correctly it will put out warmer and warmer water supply temp to meet your load.

Size that 2-way to blend enough high temp water in to meet the load/create the coil entering eater temp it’s designed for after being mixed with the remaining GPM quantity of return water it will be blended a lot. Keep that temp sensor in to monitor and make sure you don’t somehow hit excessive temps. But since you have no actual need for a specific temp for that loop, tightly controlling that isn’t critical. I’m assuming you have a varying load on the airside of the coil and this will help with that.

But if you wanted to expand on this loop for more than one coil for some reason then you’d need to go back to the 2-way valve keeping a loop temp and then individual valves at each coil.

Also if this air stream is subject to freezing air temps make sure to turn on the loop and maintain a minimum temp in that scenario when air temps are low.

 

[GT-EGR]

Another side note about doing it this way is that since it isn’t isolated from the high temp side, when/if it’s stagnant for long enough period it will creep up slowly in temperature depending on how big the loop is and how long it sits there. So initial startup of pump you might be stuck with a high temp loop until the airstream takes the heat out of it after a few rounds of circulation. Probably fine unless the outlet air temp of the coil has some critical nature where it can’t be overly warm for short periods.

 

[WilliamTz]

Thanks GT-EGR....some great posts and information!