Bungalow Renovation - New Heating System Questions

[vulcancontinental]

Go for big radiators - running big rads at a low temperature is much cheaper than running small ones at a high temperature.

Good advice but 'much cheaper'?

Large rads are expensive and intrusive, modulating controls are much cheaper, less obtrusive and offer other benefits.

 

[winston1]

Go for big radiators - running big rads at a low temperature is much cheaper than running small ones at a high temperature.

Could you explain the theory behind this please?

 

[bernardgreen]

Could you explain the theory behind this please?

Lower temperature of water returning to the boiler means there is more condensation inside the boiler of the water produced by combustion and also cooler fumes leaving the boiler. Hence the boiler is operating more efficiently when the return water temperature is lower.

 

[muggles]

Could you explain the theory behind this please?

Certainly

When gas is burned, one of the products of combustion is water vapour/steam. In the traditional standard-efficiency boilers of 10+ yeasts ago, this water vapour and the heat within it was chucked out of the flue to warm the world up.

Modern boilers are able to condense this water out, removing the heat in the process and putting it into the heating system. However, they can only do this properly if certain conditions are met. A difference in temperature between the heating flow and return of 20°C is required, and the return needs to be below 53°C. If the temperature difference (known as Delta T or dT) is reduced or the return temperature is too high, condensing stops and the heat within the water vapour is lost out of the flue, increasing running costs

A heating system sized to run at low temperatures and the correct dT will condense (nearly) all of the time, requiring less gas to be burned to get the same heat output.

As Vulcan has suggested, high-efficiency controls such as those using the OpenTherm protocol will optimise boiler output to further increase efficiency

 

[vulcancontinental]

As flow temp is tied to actual demand with modulating controls standard sized rads are fine which helps if using an existing system. What I was interested in is quantifying the amount of latent heat that can be harvested when condensing. Circa 0.93kw if 100% is captured but in real life much less. Therefore expensive larger rads may hardly see a return based upon a comparison of the cost of material against fuel saved.

 

[bernardgreen]

and the return needs to be below 53°C.

Therefore expensive larger rads may hardly see a return based upon a comparison of the cost of material against fuel saved.

The largest "radiator" is the coil(s) used in under floor heating. In these the circulating water temperatures are in the area of 25°C to 28°C. Well below the 53°C needed for near 100% condensing in the boiler.

But for some reason the majority of gas ( and oil ? ) fired underfloor heating systems seem to use boiler heat exchangers operating at temperatures significantly higher than 28°C and the UFH system has to rely on a thermostatic "blender" valve add heat to the water circulating around the floor coil(s). This circulation in the coil(s) is pumped by a second pump.

Maybe this is due to most "famous name" domestic boilers being designed to fit all systems from small high temperatures radiators to large low temperature under floor heating coils.

 

[winston1]

Certainly

When gas is burned, one of the products of combustion is water vapour/steam. In the traditional standard-efficiency boilers of 10+ yeasts ago, this water vapour and the heat within it was chucked out of the flue to warm the world up.

Modern boilers are able to condense this water out, removing the heat in the process and putting it into the heating system. However, they can only do this properly if certain conditions are met. A difference in temperature between the heating flow and return of 20°C is required, and the return needs to be below 53°C. If the temperature difference (known as Delta T or dT) is reduced or the return temperature is too high, condensing stops and the heat within the water vapour is lost out of the flue, increasing running costs

A heating system sized to run at low temperatures and the correct dT will condense (nearly) all of the time, requiring less gas to be burned to get the same heat output.

As Vulcan has suggested, high-efficiency controls such as those using the OpenTherm protocol will optimise boiler output to further increase efficiency

Interesting. I last seriously studied heating systems 40 years ago when I put a system in my first house. Back then theory was a flow temp of 180˚F (82˚C) and a return of 160F˚ (71˚C), a 20˚F drop which is 11˚C. Radiators were sized assuming an average of 170˚F.

I assume fitting a new condensing boiler to such a system would not work properly as on those figures it would never condense. Yet I'm sure it is often done.

 

[vulcancontinental]

40 years ago there were no domestic condensing boilers, they were standard efficiency cast iron or lead washed copper exchangers with burner beneath the heat exchanger. The reason flow temps were high was to avoid condensation forming on the cast iron if it ran for a time below dew point (50 degrees-ish) and rotting it out or dripping onto a burner. Flow 75, return 65 = no condensation.

A condensing boiler is always more efficient than a non-condensing boiler by a margin, a condensing boiler condensing on the exchanger is more efficient than one releasing its latent heat at the flue terminal but the margin is far, far smaller as are the monetary savings.

The majority of systems are installed with the standard rad flow and return temps as per catalogue so boilers will be run at those temps to achieve catalogue outputs and most systems were installed years ago in to run hot therefore that's why manufacturers make them still. The percentage of U/F systems is miniscule in comparison with radiators.

The benefit of modulating controls are when high temps are required the boiler runs hot, when lower outputs are required the flow temp and the return temp respectively are reduced leading to condensing within the appliance; the rads stay the same size though the customer will sometimes grumble when the rads are cooler rather than baking hot.

10% improvement in high temp operation will be 10% of over 10.7KW of sensible heat from a cubic metre methane; 10% of latent heat recoverable when flue products condense within the boiler represents 0.1kw of the around 0.9kw from the same amount of fuel. Look at the cost of a cubic metre of methane. Look at the cost of an underfloor installation to a radiator system (I'm told and happily believe the comfort factor is higher with U/F; controllability, I'm less convinced).

None of this will save the planet but everything combined does help a bit.

Manufacturers efficiency figures are derived on special rigs with test gas at higher pressures and a standard operating format different to although supposedly representative of the way we run heating systems.

 

[winston1]

Thanks, very informative.

 

[Steelmasons]

Go there Vulc.......certainly a post that stands out above the rest.

 

[Steelmasons]

The largest "radiator" is the coil(s) used in under floor heating. In these the circulating water temperatures are in the area of 25°C to 28°C..

Most condensing boilers have a minimum flow temperature of 35c.....oh dear...

 

[vulcancontinental]

Go there Vulc.......certainly a post that stands out above the rest.

Bit too much information really but I get tired of stupid claims and bullshit googling. It's not only what you save it's the cost of saving it that counts.

 

[Steelmasons]

I get tired of stupid claims and bullshit googling.

Hence why I keep getting banned Vulc.

 

[vulcancontinental]

Hence why I keep getting banned Vulc.

You can always trust the nervous to silence the messenger of truth.

It's ok now steel, I've had my medicine, I'm calm.

I just hope nobody starts off about some boiler made of kryptonite that fixes itself on the never yet recorded occasions it goes wrong and is more efficient after the repair than before.

 

[Steelmasons]

Keep taking the meds as we don't wanna see you get banned..