Power consumption heat pump

Verdigris

I don't think you understand how condensing boilers work.

wittynamegoeshere

Outside air is mixed with the gas.  If that air is colder then more gas will be needed to reach a given temperature.

What part of this do you disagree with?

I'm suggesting that the headline efficiency figures may not include a realistic value for the outside temperature, and this may result in over-optimistic figures.

Verdigris

It's the temperature of the return flow that determines how much of the latent heat of evaporation in the exhaust gas is absorbed back into the heating system. The cooler the return flow, the steeper the temperature gradient, the mor eht e water is pre=warmed before it goes past the burner.

Simple physics.

Verdigris

I suggest you find out how a condensing boiler works before you further embarrass yourself.

Verdigris

The incoming air is mixed with the fuel (gas or atomised oil) and ignited. The temperature of that air makes very little difference, except that colder air is denser and will deliver more oxygen and give a cleaner burn. When the fuel is burnt, in a conventional boiler, it is exhausted to the atmosphere along with any heat that hasn't been absorbed by the circulating water. In a condensing boiler the hot exhaust gas is passed over a heat exchanger, through which the return water of the heating system flows, giving a pre-heat to the water before it passes over the burner. The cooler the return water is, the more heat it absorbs from the exhaust gas and the higher the boiler efficiency rises.

wittynamegoeshere

The point I'm making is that the difference between the flue and outside air temperature will be greater if the air is colder.  Let's start by agreeing or disagreeing on this point.

coffeehound

wittynamegoeshere said:

The point I'm making is that the difference between the flue and outside air temperature will be greater if the air is colder. 

I see your point, but the fact is the specific heat of air is about 1 J/kg/℃ whereas that of water is 4200, meaning that air takes very little energy to heat up.  So the OAT is basically negligible in greater scheme of things

tux900

coffeehound said:

wittynamegoeshere said:

The point I'm making is that the difference between the flue and outside air temperature will be greater if the air is colder. 

I see your point, but the fact is the specific heat of air is about 1 J/kg/℃ whereas that of water is 4200, meaning that air takes very little energy to heat up.  So the OAT is basically negligible in greater scheme of things

You are forgetting (or not aware of?) the pre- and post-burn purging of the combustion chamber - this significantly affects overall efficiency given the cooling of the hex and casing. This is one reason why short cycling is so undesirable. 

coffeehound

tux900 said:

coffeehound said:

wittynamegoeshere said:

The point I'm making is that the difference between the flue and outside air temperature will be greater if the air is colder. 

I see your point, but the fact is the specific heat of air is about 1 J/kg/℃ whereas that of water is 4200, meaning that air takes very little energy to heat up.  So the OAT is basically negligible in greater scheme of things

You are forgetting (or not aware of?) the pre- and post-burn purging of the combustion chamber - this significantly affects overall efficiency given the cooling of the hex and casing. This is one reason why short cycling is so undesirable. 

Vaguely aware of a purge -- how long does it last?  

[Deleted User]

QrizB said:

DeletedUser said:

Is there a nice little graph somewhere that displays ASHP v GSHP electricity required at various outdoor temperatures to bring a room up to 21 degrees?

No, there are far too many variables. That's why a heat pump quote needs to start with a proper energy survey (not just the tick-box exercise of a EPC).

Really? Other than those i've listed, what are the other variables?