Outdoor Boiler Anti-freeze

30 Apr 2005
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United Kingdom

I am fitting a Grant Out-door boiler and having just been in PTS and gasped at Fernox Inhib & anti-freeze price, £19 for 5 ltrs and you need 25% to protect down to -11 was wondering is automotive anti-freeze be alright to use? Car engine coolant works in a harsher environment with all different metals and probably higher temps.

Your thoughts :idea: appreciated.

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I have one of these boilers myself and contaced Fernox t ask why their antifreeze is so expensive.

It was explained to me that automotive antifreeze is in no way suitable - if you have a leak in the immersion heater coil the water from the primary circuit can get into the domestic hot water.

Automotive antifreeze is highly toxic.
Thanks for the replys chaps.

Hadn't thought of the valid issue 2sheds raised, will definately not use automotive anti-freeze and will go the pipe stat route with the onboard frost thero.


My own boiler is a Grant outdoor module type. I think if you're fitting one of these it probably has a froststat already built in - mine certainly has.

Seemed to work fine last winter though I did burn quite a lot more oil than with our previous conventional indoor boiler.

I'm currently trying to find a way to reduce this consumption (you can see my earlier post).
Thanks for that 2sheds.

A thought occurred to me, don't get many of them!!

The scene, bitterest winter for years, -10 ootside and freezing wind, power is cut off, whats doing to protect the boiler?

I'm now thinking to spend £120 on sufficient antifreeze to protect down to say -15, could then dispense with frost-stat that Grant provide and anti freeze will pay for itself in reduced fuel bills!

What do you think?


The common auto antifreeze is ethylene glycol, the non-toxic type is propylene glycol, this is also used in high tech/performance/price etc engines so it is also used in the auto world. There's a long discussion about it a couple of years ago on the forum.

It depends where you are, but it's very unlikely you will get -10 and no electric for long enough to let your system freeze. Another way would be to buy a cheapie generator (you need only 500W) to run the heating system.

I'm no professional but speaking as a layman cannot see anything wrong with your idea about put in enough antifreeze and do away with the froststat. Perhaps someone who is a professional can point out the flaw.

I reckon that last years (mild winter) heating cost for our well insulated 3 bed bungalow was some £580 which was £200 more than previous year. (although a large part of this is of course due to the price increase in oil).

Do you know that 30 litres (which is what you would get for £120) is 25% of the volume of water in your heating system?

don't forget the frost stat is not only going to protect the boiler but the rest of the internal services by bringing on the heating. you could consider 2 stage protection, bring the circ pump on first say @ 5 degs, you try freezing moving water :!: then boiler @ 0 degs...... just a thought :D
Many thanks for all your input.

I have decided to add enough antifreeze to negate the need for a frost or pipestat with the cost being recouped from lower fuel bills throughout the winter months. On each annual service the sg can be checked just as in a car, to ensure suffuicient anti-freeze concentration.

Will keep you posted on how it progresses.


Is your Grant a system boiler or a combi , if its a combi then a frost stat would be recomended as with only anti-freeze you would have no latent heat to protect the mains water pipes at least with a frost stat it will come on and heat the w/w heat ex plus have you bugeted for replaceing the a-f each time a repair is made

It's a open vented system boiler. Will build in a few valves to enable boiler to be isolated should the need arise. Can't use a freezer which will be a bit of a blow but that's the only real drawback I can see in using anti-freeze.



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