Boiler house condensing boiler-any advice as regards brands?

BUFF

Moss5 said:

https://www.heatgeek.com/condensing-boilers-efficiency/

• Condensation: releases 2,500,000 J/kg latent heat

That's a theoretical maximum of 690 Watts of energy for every litre of condensate created.

That is like free electricity from an air-source heat pump.

 All of the efficiency calculations are based on the boiler operating in the condensing mode at all times. (Flow 60, Return 47).

My radiators are sized for a 36-year-old system with flow temperature 75.

If I want to see what it feels like at flow 60, I can turn the boiler down to 60.

Feels like a cold house.

I also have a ‘Dunsley’- style mixing tank to enable me to choose heating from a multi-fuel back-boiler. To take advantage of the condensing boiler would require me to fit larger radiators/under-floor heating and remove the integration with the coal/log burner.

Hopefully, I won’t live long enough to see the pay-back on that disruption.

"it's the same air & therefore same dewpoint?"

But gas contains water. 

I still haven't seen evidence that it requires a 47C or lower flow to condense as you stated " Return temperature would need to be less than 47C for an oil boiler to enter dew-point and create condensate.." I would be delighted if you could link something.
I actually run return substantially lower than that to gain additional efficiency but it isn't required for the boiler to enter dewpoint & start condensing. As I linked Grant appear happy at  55C-40C for condensing to occur.

With lower flow/return temps you have 2 choices to achieve your required temp. - run the system longer (not necessarily a bad idea as running longer lower tends to be easier on the boiler & therefore increases reliability/life) or increase radiator output (usually by changing them).

Natural gas does contain water but at very low levels. So, of course does the air that it's burned in. I understand that there is a roughly a 1.5x  water production factor in terms of energy of fuel consumed between gas & oil combustion. My understanding is that the exhaust will be at 100% relative humidity so I guess that, as coffeehound suggests, it must be the different make up of the exhaust causing a different ability to hold water & therefore dewpoint?. Again, if you could link to something, I would be grateful?

Moss5

"I still haven't seen evidence that it requires a 47C or lower flow to condense as you stated"
https://www.heatgeek.com/condensing-boilers-efficiency/
Water content of fuel is not sloshing around in the bottom of the tank (but it can be), it is H and the O.
I stayed in a Spanish villa in winter. the only form of heating was a portable gas heater, no flue.
within an  hour the windows/patio doors were streaming with water. 

BUFF

Interesting, thank you. Although it does show that condensation is possible above 47C for oil.
As does the below .

coffeehound

Interesting info.  I wonder if LPG comes under 'manufactured gas'