Optimum boiler water temperature.

[Dave Baker]

I have an oldy worldy Camray oil fired boiler, early 1990s vintage, to heat the house. It's been dead reliable so far for three years and probably not much inside it to ever go wrong compared to these complicated combi things people use nowadays. Being a bungalow with a large floor area the hot water has to pass through an awful lot of pipework under the floor to get to each radiator. Logic tells me that the lowest possible boiler water temp will cause the least heat rejection to the underfloor. The boiler was set to 60 degrees C when I moved in. I move that up to 80 when I want the rads to heat the house quickly in the morning and put it back to 60 at night to leave it ticking over.

On such a boiler is there any efficiency advantage to any particular water temperature?

[harryagain]

Any boiler works more efficiently with a lower water temperature. However. If the temperature is too low, you won't get enough heat out of it to heat the house. Also, you may get condensation within the boiler leading to corrosion as its not designed for this. (Condensing boilers are made out of stainless steel or aluminium.)

The critical factor is the "exhaust gas" temperature which must not fall below 100degC at any time. If it does, condensation will occur and your boiler will rust. The condensation from oil fired boilers is particularly nasty and corrosive.

[Andrew Gabriel]

The return water temperature will govern condensing on the heat exchanger surface. For a non-condensing gas boiler it must be above the flue gas dewpoint of 55C*, so 60C is normally the lowest return temperature allowed. The flue gas temperature is a function of the flow temperature and heat exchanger efficiency, which is not so relevant to condensing.

Oil may be slightly different - in particular the dewpoint for oil fumes is probably different as burning oil generates more CO2 and less water.

*Older non-condensing boilers often have more excess air flow than condensing boilers which will reduce this, although that doesn't normally seem to be taken into account.

[John Rumm]

Not as such - yes you will lose more heat from the pipes at higher temps, but since that is still into the fabric of the house that is perhaps less of a problem than you might expect.

If we assume this is a non condensing boiler, one thing to watch is that you don't allow the return temperature to fall low enough that the boiler starts to condense anyway. Since its not designed to do it, that will just mean water in the boiler and lots of corrosion. Hence I would aim to keep the return temperature up close to 60 as a minimum.

[Dave Baker]

The pipes run under the house in the void space so it's basically lost heat hence my pondering the matter. They are foam lagged but I've noticed it takes a fair while for some of the rads to get hot after switch on. The one in the shower room gets hot really fast so I assume it's right after the boiler on the pipe run (it's also the only one that ever needs bleeding as per a previous thread which ties in with that) but the one in the kitchen takes bloody ages. I think the water that reaches that one has gone all round the underside of the house, past all the rads in rooms I don't use which are switched off and then back again.

Okie doke. Cheers.

[meow2222]

Quicker pump speed would help

NT

[John Rumm]

It could equally be that the system balancing is poor and hence there is inadequate flow to some of the rads.