Hot gas bypass control?

[Jase]

Recently, been working on an old Delta Clima full function package unit which has 2 DX circuits with 1 DX incorporating hot gas bypass. Now as I understand, the hot gas bypass function is used upon low load conditions. The hot gas valve is an old staefa valve on control of 0-20 vdc. The controls have recently been upgraded,within the last year or so.
Upon starting of the unit the hot gas valve is fully open and modulates to close position.Due to the unit cooling/de-humidifying is it best to control the valve on temperature variations only or control on both cooling and de-humidifying.i.e upon nearing temp set point the valve would begin to modulate and when de-humidifying, upon nearing RH set point also begin to modulate.

Reagards
Jase

[NoNickName]

I would suggest to control it on pressure, not on temperature, specifically the suction pressure, disregarding whether the unit is cooling or dehumidifying. In fact, you can't have two independent controls on a system with a single degree of freedom (the valve).

[Jase]

I understand what you are saying,infact that is how I would usually control the valve, but on this particular application the valve is purely controlled within the control parameters. It is not equipped with a port to monitor suction pressure:(

Jase

[piston broke]

Jase

I think it's to stop the evaporator freezing up. Valve should operate to keep the suction pressure/temperature above 0 degrees centigrade. Check your guages I think its about 6o psi, I'm assuming it's R22 am I right?

These are usually installed where there is fresh air introduced into the system. There should be a thermostat beside the fresh air intake to the coil and it should be set at about 12 degrees C. This may open the valve when the temp drops or cut the cooling off to get free cooling from the fresh air, it is cold this time of year.

Check the usual, filters clean, belts tight, expansion valve.

Keep that pressure up buddy.

PB

[NoNickName]

It is not equipped with a port to monitor suction pressure:(



Install it. I still think pressure is the measure, not temperature

[piston broke]

Sorry I didn't take the time to read the original post properly. Forget everything I just said.

I think the valve modulates (especially if the heat load is low) to reduce the cooling capacity to stop the compressor cycling and thus obtain a closer control.

If so it is operating correctly. What exactly is the complaint Jase, is the room temperature rising?

PB

[Jase]

Room temp is low with RH slightly high, so unit always dehumidifying. LPHW valve open 100%. Have now set up delay on timer on stage 2 and will monitor operation.
Should the hot gas valve modulate on dehumidifying to prohibit overshoot in temperature?

Jase

[piston broke]

What site is this on if you don't mind me asking, promise I won't steal it.

I might be able to help if I've been there. What contols the room temperature? What controls the valve?

PB

[NoNickName]

Room temp is low with RH slightly high, so unit always dehumidifying. LPHW valve open 100%. Have now set up delay on timer on stage 2 and will monitor operation.
Should the hot gas valve modulate on dehumidifying to prohibit overshoot in temperature?

Jase


Bad design. I've seen this a number of time, and it means that the heat load is much less than design. What you probably miss is not a bypass, but a reheat or warm water reheater. The valve must be open for dehumidifycation, and if you don't want overshoot, reduce fan speed while dehumidifying.

[puddleboy3]

Can i ask what temperature is the room being controlled @, as this is always going to have an effect on how well you can maintain RH%. Lower temp higher RH% Higher temp lower RH%.