Adding corrosion inhibitor to central heating circuit

[Col_T]

We have an MV Hydro 5 diesel water heater on board, which provides hot water to the calorifier, a heated towel rail and three radiators. It has 5 year antifreeze / corrosion inhibitor, and there is an expansion tank in the system which is pretty much at ceiling height.

 

My understanding is that the antifreeze element of the mix will last pretty much indefinitely, but the corrosion inhibitor should be topped up periodically. Some questions arise:

 

How frequently is ‘periodically’?

Is it ‘merely’ a case of drain ‘x’ amount of fluid from the central heating circuit and adding the same quantity of new corrosion inhibitor into the expansion tank?

If the new inhibitor is added into the expansion tank, will it ‘self-mix’, or will it not circulate as antifreeze seems to?

Is there a better place than the expansion tank to add the new inhibitor - I’d prefer to use the expansion tank as it seems the easiest place to get to, no breaking connections, etc.etc.

 

I suspect that what I refer to as the expansion tank should probably be called the header tank, so sorry for that!!

[nicknorman]

If you just add more of the antifreeze/corrosion inhibitor you will be adding a lot of antifreeze. You don't want to go over 50% antifreeze as the ability of the mixture to transport heat is reduced. Sorry but the correct way to do it is to drain the system and refill with fresh pre-mixed antifree/corrosion inhibitor and water. Every 5 years.

[Tony Brooks]

44 minutes ago, Col_T said:

We have an MV Hydro 5 diesel water heater on board, which provides hot water to the calorifier, a heated towel rail and three radiators. It has 5 year antifreeze / corrosion inhibitor, and there is an expansion tank in the system which is pretty much at ceiling height.

 

My understanding is that the antifreeze element of the mix will last pretty much indefinitely, but the corrosion inhibitor should be topped up periodically. Some questions arise:

 

How frequently is ‘periodically’?

Is it ‘merely’ a case of drain ‘x’ amount of fluid from the central heating circuit and adding the same quantity of new corrosion inhibitor into the expansion tank?

If the new inhibitor is added into the expansion tank, will it ‘self-mix’, or will it not circulate as antifreeze seems to?

Is there a better place than the expansion tank to add the new inhibitor - I’d prefer to use the expansion tank as it seems the easiest place to get to, no breaking connections, etc.etc.

 

I suspect that what I refer to as the expansion tank should probably be called the header tank, so sorry for that!!

 

I think on domestic combi-boiler systems, the plumbers might use a cartridge gun and squirt it into the system through a radiator bleed screw, but that will overfill the header tank. Also, if the inhibitor does not mix it may stop or hinder circulation.

[Cas446]

On one of our radiators I installed a drain valve on the radiator, so I can isolate the one radiator both sides and drain it down - then refill with 25% mix. I do this every year, avoids a full drain down and refill, but keeps the inhibitor up. I have a pressurised system hence header tank not an option.

[BEngo]

Screwfix, and probably Toolstation and others, sell corrosion inhibitor without antifreeze.  If you have a pump circulated heating system that circulates via the header tank drain out 500 ml, or a pint of fluid and add the same amount of inhibitor.  Run the heating to mix it all in.

 

If the pump does not circulate through the header tank, you will need to drain more out (and save it).  Drain at least enough to get the inhibitor into the main circuit.  Add 500ml of inhibitor and then get the rest from the draining bucket when you refill.  Run the heating to mix it all in.

 

Or buy an injector cartridge and squirt it in through a radiator bleed valve, having drained off enough beforehand.

 

N

 

[Col_T]

Thanks for the info folks. I hadn’t heard about cartridge injectors so plenty to think about.

[ditchcrawler]

Complete with injection nozzle 

[David Mack]

https://www.screwfix.com/p/flomasta-universal-filling-kit-5-pcs/8145K

[Col_T]

Thanks for the link, David, much appreciated!