I am quite unfamiliar with central heating systems, let alone those that use hot water as the heat source, so please forgive my clumsy attempt to ask this question.

I have a First Co Aquatherm "Combo Heater" that is connected to my tank-type gas water heater. As I understand it, hot water is piped in from the water heater to the Aquatherm where it passes through some sort of heat exchanger, producing hot air and cool water.

My water heater recently failed and I had it replaced. The guys doing the install did not check the heating system after they installed the tank; the tech only told me that I might experience some air in our faucets the first time we used the heating system.

The last time I used the heating system (prior to the water heater replacement) everything was working fine - granted this was a few month back in spring. With the new water heater, we get hot water from the tap, but our heating system blows only ambient air. I suspect an issue with the water heater install, but my vendor has washed his hands of it and told me that it's probably a problem with something in my heating system.

I've traced the lines from the water heater up through the ceiling to the Aquatherm in the attic, and from what I can tell they have the connections right. I don't have a way to check that the hot water conduit to the Aquatherm is actually full. My understanding is that the cold water supply provides the pressure for the hot water outlet; this was sufficient before the tank install, so I would assume it's the same now.

Are there any other mechanisms by which a new water heater can cause a heating system like mine to fail? Or should I just shell out for maintenance on my heating system?

2 Answers 2


I know I am late, but aquatherm is simple. You have an inlet and an outlet, there's also a shut-off valve for each line for when the unit might need to be serviced. You might have additional shut-offs at the water heater, same purpose. Typically you will have a pump in the aquatherm to move the water. Your lines were emptied and the pump is supposed to be primed. That's your issue and it's your Water heater installers lack of basic fluid mechanics knowledge. You should be able to open your valves and prime the pump. Then it will work as advertised.


You should have four lines hooked to the heater one coming in from the water supply one that goes out to the faucets there should be two other hookups one that sends the hot water to the heating system and one that returns it if you take the heater hose that returns to the heater and disconnect it first shut off the water to the heater and sometimes they'll put stops on the return line and the main line to heater line that you can open and close. If they have these shut offs on them shut the water off to both lines also turn off the cold water supply that fills the heater then disconnect the hose that Returns the water to the heater is a good idea to open the t&p which is the pressure release valve before you disconnect the hose then open the line that goes to your heater and open the water that fills the heater itself put a bucket under the line that I told you to disconnect open the valve to the water heater slowly until water comes out without any air in it this will usually release any air that's in the system once the air is out turn the line back off hook the hose back to the heater and then try and see if your heater works make sure you reopen the line that returns from the heater you do not need to open that line to drain the heater and get the air out just leave it closed if there are no valves to shut them off then just turn out the main line to the water heater open the t&p valve usually has a metal lift on it that lifts up to release the pressure then you can disconnect the hose that comes back from the heater and then open the valve from the heater and that will do the same thing to release the air

  • Paragraphs are your friend. This is an almost unreadable wall of text. Commented Aug 14, 2018 at 12:31
  • Hello, and welcome to Stack Exchange. @TheEvilGreebo is right: this may have good information, but it's impossible to find. Would you edit the answer to be a bit clearer, perhaps with paragraph breaks and other formatting? Thanks. Commented Aug 14, 2018 at 19:21

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