Control of AHU Pre-Heat Coil

[cr0ft]

Hi all,

I've got 2x AHUs where I work with pre-heat coils after the filter on the supply ducting. These are heated using the boiler CT circuit. On both of these AHUs the method of control of heat to the coil is a 3 port mixing/diverting valve with the central port blanked off. There doesn't appear to be any bypass pipework on these coils, apparently it used to be a 2 port control valve controlling the pre-heat coils on these AHUs.

I've already worked out both valves are faulty, they are passing significant quantities of water when the BMS is calling for them to close fully. Winding them forward and back hasn't helped.

The question is, is a blanked off 3 port valve sufficient to control these? Temperature just after the pre-heat coil is fluctuating by +/-10C on one AHU and +/-5C on the other AHU so something is pretty badly wrong! Especially as they shouldn't even be on at this time of year

Question is, am I right in thinking AHU heating coils should be controlled by a 3 port mixing/diverting valve rather than a 3 port valve with the middle valve blanked off? It would be relatively simple to fit bypass to the heating coil return if there's an advantage in doing so.

Also, can anyone explain why the valve appears stuck open but the temperature is fluctuating up and down so much to both AHUs?

 

[SimonG]

From memory, and it's a long long long time ago the three ports were mixing to give a higher level of control of the air temperature.

 

[cr0ft]

Yup that's my thought as well. As we need to replace both valves which means draining down that leg it's a no brainer to me to fit a 3 port mixing valve on each one.

 

[Welder]

If they letting by, are the valves appropriate for the water pressure?
What do your drawings specify? All of ours were commercial not domestic as min of 1" pipe.

 

[Last Plumber]

It’s always better for the water to have another route in my opinion rather than an open or closed circuit.
A lot depends on the rest of the system.

As far as fluctuations in air temperature is concerned, it could be water temperature changing or a change in air flow rates.

Is the AHU installed to re-circulate return air?

 

[Welder]

It’s always better for the water to have another route in my opinion rather than an open or closed circuit.
A lot depends on the rest of the system.

As far as fluctuations in air temperature is concerned, it could be water temperature changing or a change in air flow rates.

Is the AHU installed to re-circulate return air?

As you say, most flow constantly in order to ensure that temp control is tight. To have to slug cold water first to get to the warm for reheat would deliver rubbish control even for comfort cooling.

From my days, all valves had flow control on them too to ensure the most efficient operation. Likewise, as our were pneumatically controlled, most had variable pitch fans to ensure flow rates were constant. Today I understand its more electronic speed control of motors.

Unless specifically for 100% fresh air, don't most would have an element of re-circ in order to keep energy costs down?

 

[cr0ft]

This AHU uses a thermal wheel across supply and extract ducts. Problem is, as the pre-heat control valve is passing so much, the thermal wheel isn’t coming on but the chiller system is then chilling the hot air after the pre-heat. Such a waste of energy.

The current setup works to a degree but we’re going to re-pipe them properly with bypasses and replace the knackered valves.

Am now into the world of BMS, seems like a good niche area to be involved in tbh.

 

[Welder]

Whose BMS?

 

[ShaunCorbs]

Whose BMS?

Building management system

Or

Pitfa

 

[cr0ft]

Haha!!