0 PSI Burnham V-15A-T with tankless water heater

fkevin

Hello. I am trying to understand where the water pressure is going in a 1992 Burnham oil boiler with a tankless water heater. The boiler is in the basement of a '57 ranch with basement & ground floor zones. Each is @ 1550sf. We moved in the November 2012. The boiler was serviced each year and again August 27th. A new metal chimney liner was installed a week ago September 10th. The boiler was turned off for a few hours during the installation. This past Sunday evening, September 15th, I found the water pressure gauge at 0 psi. Over the last seven years the pressure has mostly been steady at the set 12.5 psi. I have added water with the reducing valve when needed, mostly in the winter, but it has never dropped so precipitously. Up to Sunday, we used hot water without problem. We've been using cold water since, but the boiler is cycling as usual. When the temperature was on the low side, I added water until the pressure gauge reach 12.5 psi. For the first time, I saw water dripping from the heater coil plate. The plated has been corroding the last few years, but I had not seen water dripping and have not experienced any problems. Soon the dripping stopped and the minor amount of water dried up on its own the next day.
On Monday morning the pressure was down to @ 9-10 psi and in the evening to 7.5 psi. I have not found a water leak along any of the baseboard radiators or the pipes. The fuel company which services the boiler each year suggested it would be a large leak, and to also try looking thru the observation door for any leaks. There was none that I could see.
Here are some other detail that might be of use:
Burnham Boiler boiler number V-15A-T
Serial Number: 22163873
Model Number: 2205522
Product Name: V-1A Series
Date of Manufacture: 1992-11-11
Honeywell Triple Aquastat-Relay low 155° high 180°
Bell & Gusset reducing valve FB-38TU set 12lbs
Watts M3 174A pressure relief valve 3/4” 30psi
Pressure gauge make unknown
water supply pressure thought to be 50-60psi
hard water on a scale of 1-4 with 4 the hardest it is 2 and drinkable
the heat for the house has not been on in months

We're waiting for reply on warranty coverage, but in the meantime any comment on why this is occurring, what the problems maybe, and advice would be appreciated. The hot water coil will need to be replaced or an indirect system installed, but I am not sure it is related to the loss of pressure. Also, should I just keep topping it off or not because of say the possibility of a faulty pressure gauge? Also, could water be going into the green air cushion tank hanging from the ceiling above? Thank you, Kevin

Wednesday, September 17th 9am update: psi holding at 7.5