Dwyer® Liquid Filled Wall Mount Manometer, Mark II 25, 0-3" w.c., 3% Accuracy

I am using this Manometer to allow for a visual identification of when my whole house furnace filter needs to be changed. As the filters are between $30 and $40 I do not want to change too often as I will be wasting money and if I wait too long potentially damaging my HVAC system which is less than ideal..... My current scheme is to pull the filter out and look at the color of the filter which is inaccurate to say the least, so after a recent cold snap and my furnace not operating due to a filter that needed to be changed I opted to do something about it. Competitive products seemed a little cheap to me so I went this route. As a mechanical engineer by training I was very pleased with the lab instrument quality of this Manometer, when you open yours up you will be surprised by the size of the this unit, fully 6"x6" or more. Everything that I needed to accurately measure a pressure differential was included as well as a surprise built in level for that really assisted with the installation of the product.What is extremely important about this Manometer is that the first inch of the water column is essentially more "horizontal" allowing for more accuracy in the first inch of the water column. The reason for this is that your furnace is most likely designed for about 0.5" of static external pressure. This is typically found on the HVAC systems label.So in my case when I installed the Manometer and zeroed it out with no filter in place, then added a new clean filter it measured .1" rise in the water column. That is about 20% of the total rise allowed by my HVAC units spec. (By the way that was a dead on match to the Filtrete specs for this filter, which is always nice) So I will need to change that filter when the water column reads .5" of water. Having that first inch essentially expanded out is really nice so I can see what is going on visually.Long story short, if this keeps my wife happy because her house is warm as her furnace filter was changed when it should have been then all things considered this was a great purchase. :-)Have a great day and some fun as a home geek. Jeff

This is really useful. I was trying to find out how restrictive the Honeywell MERV 12 (1550 rating) filters are in my variable speed air handler. I'm using a 20 by 20 by 4 filter. The installed filter is a couple weeks old so the readings you see on the gauge are probably close to what it was when "new". The varying values you see on the gauge are due to the varying fan speed. The lowest value is probably around 500 CFM; the fairly high value of .45 in. wc is around 1,000 CFM. What this tells me is these are probably about the highest MERV rating filters I'm going to be able to get away with. On a unit like this you can definitely put in MERV 13 on up but it's going to create pressure differentials that exceed 0.5 in wc (along with the usual problems) and honestly if you want to do MERV 13 or better filtration you are going to be better off simply ducting a true HEPA system running at a couple hundred CFM right off the return. Note the Honeywell 18 watt UV-C lamp housing. Those things are unbelievable...low cost and, according to actual published studies that I have reviewed, the UV-C at 253.7 nm. will kill the coronavirus that causes COVID-19 and just about anything else. Watch your eyes though and watch your FILTERS. UV-C will totally destroy most (non-glass) based filters given enough time (you have to keep the light away from the filter!). Anyway, this is an excellent gauge and gives a really interesting window into what is going on with your filter and you can see all the pressure changes as fan speed varies (and as dirt accumulates, over time, in the filter media.

As someone else mentioned here, the quality of this manometer is on par with a piece of lab equipment.I’m using this with a waste oil burner/heater. The manufacturer of this Eliminator 120 calls for 0.03” to 0.05” before the barometric damper in the stove pipe. Another manufacturer, Energy Logic, calls for 0.04” of draft in their unit, so 0.03” to 0.05” is a good call for an oil unit, but always refer to the manufacturer specs.You only need to use the low side port. This allows the values to be on the right side of the gauge, which is more usable. If you were in an application where you see more positive pressure, you would use the high port. That was a confusing aspect to me when viewing this online. Depending on if you are seeing more negative, or positive pressure values, you can flip to either side of the zero by simply using one port or the other.I get the impression this has been made for a long time. I will never hesitate to buy one again in the future. Has allowed me to see exactly where I need to see the barometric damper for different outside weather conditions and pressures.

New furnace went in with a new (more expensive) 4" filter, so I thought this might be a nice way to have numerical evidence of whether or not the filter needs to be changed. Sounds good, but is complicated by the new furnace having a variable speed blower. There's effectively 3 different speeds used depending on whether it's in high heat, low heat, cooling, or just fan on, so I marked out all three with masking tape on the gauge. In the several weeks since the furnace went in I have seen a very small increase in the static pressure indicated on the gauge, as one might expect. It will be interesting to see how it varies over the next several months. But then again, I'm a guy who likes data. You almost certainly don't need one of these, but it is kind of cool to be able to walk by the furnace and glance at this gauge to know whether it's in high or low heat and (hopefully/eventually) that the filter needs changing.