underfloor heating - not so efficient?

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I'm considering UFH for an extension. It's supposed to be so efficient because of the low water temperature required. But a blending valve is needed to lower the flow temperature from the boiler. So the water has already been heated to a high temperature (for the rads) and is now being cooled. Where is the efficiency in that?
 
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My temp from the boiler is only set at 45° (for the radiators) I was lead to believe condensing boilers are more efficient at lower temps, so long and slow vs my mother-in-law, where you could smelt metal on the radiators they are that hot.:eek:

Think my underfloor heating recommended this temperature, I've set my blender to 75% as I'm trying it out.

I suppose it depends if your house radiators can take a lower temperature and keep you warm, doesn't seem to be and issue for me, but depends on radiator size/efficiency etc
 
If the temperature of the water returning to the boiler is greater than 55°C then the boiler does not operate in condensing mode.
Cheers for this, presume you ideally want it to operate in condensing mode?
 
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Cheers for this, presume you ideally want it to operate in condensing mode?

Definitely.
Please don't take the numbers in this condescending description as gospel; as an explanation, draw some water from the cold water tap, now heat it up to boiling point - you've put in a "dollop" of heat. Keep on heating it until it's all evaporated - that's taken 6 more "dollops" of heat. When your boiler is running in condensing mode then you're getting those 6 "dollops" of heat rather than letting them go up the flue.
 
The water from the boiler is blended with cold water to achieve the required temperature for underfloor heating. Therefore less hot water is used to provide that heat. This is not a source of inefficiency.
 
The water from the boiler is blended with cold water to achieve the required temperature for underfloor heating. Therefore less hot water is used to provide that heat. This is not a source of inefficiency.
Good point. I didn't realise this.
 
Another question. Is biocide required for UFH because of the low temp, as opposed to inhibitor for the rads? How do you stop the biocide mixing with the water in the rads? Is the ufh heating somewhat isolated from the rad heating? If so, how is heat exchanged?
 
Cheers for this, presume you ideally want it to operate in condensing mode?

Burning natural gas results in the production of some water in the form of steam.

If you simply allow that steam to go up the flue a fair proportion of the heat being produced by the gas is wasted.

In condensing mode this steam is robbed of it's latent heat (becoming water) by contact with the returning cooler water circulating through the boiler. Generally the cooler the return the better this works.
 

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