Flow and return temperatures for optimum efficiency

[John D v2.0]

Ideally the flow would be as far below the dewpoint temperature as possible, not the return. That's why I emphasize the flue temperature not the return or flow; the flue temperature is the giveaway on how successful cooling of the flue gases has been or put another way, how good the heat exchanger is, something people seem to overlook.

That's very interesting, given the increase in heat pumps trying to get energy from the outside air at 5c at 80% humidity. You'd have thought a stream of flue gas at 50c and 100% humidity would be an attractive target for a heat pump.
I wonder how long before boilers start having flue gas heat recovery based on a heat pump and boilers really do become "more than 100% efficient." In terms of energy in Vs heat into the circuit.

 

[old duffer]

And with high modulating boilers, those over 1:10 the ignition fan speed (power input) on a relight is quite high and with the system demand so low the boiler immediately shuts down again, micro-firing. Installers have been told by manufacturers tech people this is normal operation. A lot depends on the heat exchanger.

Look for less than 8 degrees flue to return as an indicator, some are in excess of 20 degrees.

It's all dynamic, no perfect plateau.

Stuck at home yesterday I was experimenting with OpenTherm disconnected; solely weather comp. To get an idea of an operating cycle look at this video, couldn't be any fresher!

40kW combi, 30kW to heating servicing a heating load of around 8kW at -3 degrees. Boiler is 1:8 modulation. I watched the cycle five times with hardly any variation at all.

why do you have such a huge combi in such a small property?

 

[Harry Bloomfield]

why do you have such a huge combi in such a small property?

Maybe for provision of hot water?

 

[old duffer]

Maybe for provision of hot water?

a combi is not the solution if that is indeed the reason. Continuous water heater is the only answer if there is high demand volumes for multiple showers/baths but such a small property would not have these anyway.

 

[Harry Bloomfield]

a combi is not the solution if that is indeed the reason. Continuous water heater is the only answer if there is high demand volumes for multiple showers/baths but such a small property would not have these anyway.

No, but it's usual to size a combi's output, to heat the water demand, rather than the space heating demand.