In my last post, I said we needed to get out brain-set back to heating. one of the items that needs to be looked at is gas pressure — both inlet and manifold pressure. It is important to understand that these settings can make a big difference in how that heating section is going to work. This applies to both residential furnaces and packed roof top units, single stage and multi-stage equipment.
Improper gas pressure, on the side of the gas train can cause the unit to over heat if it is too high. To little gas pressure can cause nuisance lockouts on the flame sensor. Likewise, too much on the inlet side of the valve will cause gas valve failures or no heat because the valve cannot open against “high pressures”. To little inlet pressure can cause sooting in the heat exchanger and contribute to nuisance lockouts.
So, what is the guidelines for inlet and manifold pressures? What should the inlet operating range gas line pressures for the furnace to operate safely. The gas line pressure MUST BE a minimum of:
in order to obtain the BTU input specified on the rating plate and/or the nominal manifold pressure specified on the rating plate and to work safely.
Manifold pressure will now be able to help set the correct firing rate. On single stage units, most manifold pressures are set to
You should always check with the manufacturer as some do vary, but as a “rule of thumb” this is where most manufacturers set the high fire pressure. This is based on the actual burner orifices requiring this pressure to deliver the rated output. On multi-stage units, there is also a setting for low fire and that also needs to be checked.
So, hopefully now you see why inlet and manifold gas pressure is important to proper operation of a gas heating system. So how do we check this?
As you can see from the picture above, you need a something that your can read inches of water column (IWC) to check for proper pressure. This could be a simple “U-tube” manometer (as shown) or it could be a digital manometer, could be a magnahelic , could be a simple pressure gauge — anything that can read inches of water. You can then set the unit up with the proper manifold pressure or can verify where the inlet pressure is coming into the valve depending on what side you hook up to.
One thing to remember, on 2-stage gas valves, there usually are 2 pressure regulators – one for HIGH FIRE and one for LOW FIRE (see example below). Be sure you know the manufacturer’s specs for setting these. Most high fires will be 3.5 IWC but the low could be different depending on manufacturer. Some use 50% low fire, some 60% low fire and some as high as 80% low fire so it is important to verify, either on the data tag on the unit or with the manufacturer where these should be set.
One good check is to always leave the manometer hooked up to the inlet side and then watch it as the unit operates. This is especially important on “high pressure gas” supply systems and propane applications. A lot of times, you can watch the inlet pressure drop, possibly due to a faulty regulator, when the main valve opens. A lot of techs will hook up the meter, see they have good pressure and then remove the meter. Then they see there is a problem on the manifold side and condemn the gas valve when there is nothing wrong with it.
Gas pressure, both inlet and outlet, are important to proper operation of any heating system. In the next post, we’ll discuss temperature rise, how to check, and why that is an important diagnostic.