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Hi, I only have questions. Did they say where the tanks are leaking? Is it at welds, fittings, or? Are there water quality issues? Salt softened water? No doubt engineers at HTP know what's going on. 🤔

Yours, Larry

The hrv or the fan in a can make the most sense. The other appliances that vent outside like the dryer and the exhaust fans won't work well if they are under negative pressure either. The main maintenance to perform on a cast iron boiler is to make sure it is burning properly and that the safeties work. There is little reason to replace an 11 year old boiler unless it is leaking.

They're upselling you. And they might not even fix the problem.

As has been suggested: Fix the negative pressure issue, and inspect the flues. And then fix the boiler. At 11yrs old you could get another 30 out of it easily. That water heater might live another 5-10 years.

On the gauge — the red hand indicates maximum pressure reached. The black hand indicates the actual pressure in the boiler.

When you fill the boiler, you want the black hand to come alive (it may be) and let it come up to about 2 marks above the 10 mark (about 12 psi). That should be ample.

And on the thermostat —those aren't the most sophisticated, not do they hold that close a temperature — but they will last forever. Keep it.

That setup on the tank is to make draining the tank a little easier. You do not need to do my first step from my previous post to drain the expansion tank. Someone else has made it easier. To start with an empty tank all you need to do is place the end of that hose in the floor drain or laundry tub.

1. Make sure valve #2 is closed
2. Open valve #4 and let the water out. Once the water has stopped flowing the pressure gauge on the boiler will be at or close to zero PSI
3. Open valve #1 and let air into the tank. You will hear air gurgling into the tank.
4. After some time the air starts to gurgle into the system, that means the tank is empty and you close all the valves.
5. Now open valve #2 to to add water to fill the system back to the proper pressure.

The two red valves are a relief valve and a pressure reducing valve the pressure reducing valve is also known as the automatic fill valve. It takes the house pressure of 30 to 70 PSI and reduces it to about 12 PSI. if it is working properly some folks leave the manual feed valve open so there is always 12 PSI pressure on the system. That way, if you are not getting any heat from any given radiator, you can just open the radiator vent and let air out. VIOLA »»» HEAT!

I’m not surprised there isn’t an air bleed on tank. You may find a plug but my tank didn’t have have one. To drain it was simple process of closing valve between boiler and tank; attach hose and then drain. Be prepared for it to take a while since it will be sucking air to make up for vacuum created as water drains out.
Once you get it drain, you can open the valve leading to MPT connection to let air in. Just open it a bit so hopefully only air goes in and no water comes out.