Condensing exhaust temperature

[ScottishGasMan]

And regardless of whether they are condensing or not, condensing type boilers have high surface area heat exchangers so they pull a lot more heat from flue gases than a standard efficiency boiler even at higher temps.

there are multiple technologies now to enhance the condensing effect that you get such as Weather compensation and priority HW systems on "System" boilers (ones heating a cylinder and radiators) where the boiler will allow a high flow temp to cylinder to reheat it quickly, but not heat radiators at same time, once cylinder is heated it then moves back to heating radiators at a lower temperature to get more time in condensing mode, weather compensation then allows even lower temperatures to feed to the radiators dependant on the outside temperature again keeping the flow temp lower when it can.

Combis can have additional heat recovery units built in now, where the cold mains water is passed though the flue gas to again get much more condensing out the boiler, before the cold water is run through the secondary heat exchanger to get the heat to the taps,

Its certainly not a scam, there are many regulations involved these days to increase the efficiency of units and controls as high as possible, where we fall down in this country is the regulation only says we must fit the equipment, it doesnt mean we use the correct size of heat emitters to benefit from it, we do training for engineers and homeowners very poorly so most benefits can be squashed very quickly by a few settings being left wrong on controls and basic set up.

 

[John D v2.0]

there are many regulations involved these days to increase the efficiency of units and controls as high as possible, where we fall down in this country is the regulation only says we must fit the equipment, it doesnt mean we use the correct size of heat emitters to benefit from it, we do training for engineers and homeowners very poorly so most benefits can be squashed very quickly by a few settings being left wrong on controls and basic set up.

That's very interesting and we are moving more and more to plug and play and intelligent systems, but I'll agree someone unqualified can really mess things up.
I think the new generation of smart controls are getting to the same stage as the car industry when we went to fuel injection systems. The efficiency and pollution were improved no end due to tight regulation, but now you need a computer to diagnose anything. With the advent of greater connectivity perhaps our boiler would start emailing suggestions to us, for example it noticed the lounge heat up is slow, an extra large rad is 200 pounds fitted, then it becomes a marketing opportunity.

 

[fixitflav]

where the boiler will allow a high flow temp to cylinder to reheat it quickly, but not heat radiators at same time, once cylinder is heated it then moves back to heating radiators at a lower temperature

Yes, and a corollary is that if there is a diverter valve (as opposed to two 2-port valves) a mid-position type is not needed. If it's a new system go for a cheaper, more reliable either/or valve. If improving an existing set-up with a mid-position valve, it would have to be wired to work in either/or mode (or the head replaced).

 

[vulcancontinental]

There's a chemical reaction when burning methane, the by products to note are CO2 and H2O (water in a vapour as it's over 900 degrees). It's going to be hard to cool it to below 50-59 degrees in a domestic exchanger they're very small, you need a lot of factors for a good heat exchanger, surface area being one of them. The actual the dewpoint temp depends on the excess air in the flue. The success of the exchanger depends on how much sensible heat the exchanger removes as if you're brining water back to the boiler below 50 degrees a better exchanger will have more of the surface at a lower temperature than a less successful exchanger. Some manufacturers count the air intake cooling the flue gases in a concentric flue resulting in the release of latent heat warming the combustion air adding to the efficiency of the appliance.

The heat exchanger is the important thing, a condensing boiler has the potential to condense not a necessity as the gains are so small. Forget 11%. 10.73KW sensible heat in a cu metre methane 0.93KW latent heat. You can't get all of both but you go for the biggest prize, around 9.8 sensible and 0.3 latent. That's why ali boilers exist and don't need to condense and why the manufacturers of stainless have a hard on for low temp running.

Got a plume you haven't got all the latent heat, people look to the flow and return temp on the boiler but it's the flue and return temp, the closer the better.

 

[ScottishGasMan]

That's why ali boilers exist and don't need to condense

Can you expand a bit on this? Very interesting stuff.

 

[vulcancontinental]

Ali conducts thirteen times better that stainless, it can be shaped with fins and ribs to increase surface area, it can be made in one piece no joints or heat treatment, it can be shaped to improve turbulence and Coanda effect, it expands and contracts more giving less purchase to debris and with a wider bore. Cast tends to be thicker and softer but drawn ali is similar in wall thickness to stainless and can be a harder alloy.

Condense continually on Ali and it'll need two services a year on a really efficient system, stainless will not but it will not capture as much sensible heat unless it is very complicated.

Nothing wrong with stainless as it doesn't form oxides but you need lots of surface area to get the heat and is very difficult to shape to improve tribulation and surface area: look at the Gianonni and Atag, lots of tube and joints and welds or gaskets. Still needs the same servicing as an Ali exchanger does running hot.

The UK is over 90% radiator heating people like hot rads, if you can give them better or equal efficiency cheaper with less of an ecological impact and easier recycling that's a factor in a company's eco profile too.


Look at the complexity of the atag and gisnonni and problems with flow rate and dirt, doesn't mean the exchangers bad but it affects the performance and customer satisfaction.

This is not an advert for Ali just a list of observations, you know who I work for and we use stainless but not yet in the UK.

 

[John D v2.0]

Interesting postp but surely the thermal conductivity of the hex is not relevant due to the fact that it's so thin. The conductivity of the water and flue gas would be far more relevant and how long they spend in contact ie surface area and flow velocity on both sides. Also whether it's a counter flow or not.
In mvhr systems the heat exchanger is plastic, but still achieves over 90% efficiency.

I'm not sure why domestic heat exchanger are too small to cool the flue gases below 55c without great difficulty, in a system designed to condense or a full ufh there would be no problem at all. It's down to the system design rather than any issue with the heat exchanger.

Also you say ali is better because it's self cleaning but later you say it needs a lot more servicing.

Also i have to disagree that the latent heat of natural gas is only around 3% of the total, if you return the flue gas outside at the same temperature it entered you will get much more,. That would of course require a heat pump between intake and exhaust which would of course be more hassle then it's worth!

 

[ScottishGasMan]

the thermal conductivity of the hex is not relevant due to the fact that it's so thin

The hex material still has to absorb the heat from the gas before it can transfer to the water so while I agree on very thin metal it would be negligable, by the time you add up the mass of metal in a condensing heat exchanger theres still alot of metal to heat first so the better it can transfer the more efficient.

Also you say ali is better because it's self cleaning but later you say it needs a lot more servicing.

Also there is a much narrower PH band that the system water can be kept at in a UK heating system when using Ali before it starts to react and corrode (ali likes acidic water where as steel radiators are better being closer to a base PH)

Also i have to disagree that the latent heat of natural gas is only around 3% of the total

I think when they reffer to latent heat they mean the heat recovered purely from the condensing effect rather than total heat lost in flue gas.

 

[Dan Robinson]

Also you say ali is better because it's self cleaning but later you say it needs a lot more servicing.

Read those bits again more carefully. It's not what VC said.

surely the thermal conductivity of the hex is not relevant due to the fact that it's so thin.

But it is.

I'm not sure why domestic heat exchanger are too small to cool the flue gases below 55c without great difficulty, in a system designed to condense or a full ufh there would be no problem at all. It's down to the system design rather than any issue with the heat exchanger.

Because of the volume of combustion gases being pushed through at high outputs, amongst many other things. System design is often very hard to do anything significant about on boiler swaps. Manufacturers and installers are attacking many fronts.

 

[Dan Robinson]

Post overlap

 

[John D v2.0]

The hex material still has to absorb the heat from the gas before it can transfer to the water so while I agree on very thin metal it would be negligable, by the time you add up the mass of metal in a condensing heat exchanger theres still alot of metal to heat first so the better it can transfer the more efficient.

Ah i see so you are meaning the thermal mass is higher and that's the issue due to cycling losses. I think that's a good point although the thermal mass of the water sitting in the primary circuit is pretty huge.

Also there is a much narrower PH band that the system water can be kept at in a UK heating system when using Ali before it starts to react and corrode (ali likes acidic water where as steel radiators are better being closer to a base PH)

With inhibitor the pH will be buffered quite high for the reason you state. Metals are generally poor with acids, hence why the parts of the boiler in contact with condensate are the correct grade of stainless steel.

But it is.

Yes I'll give you that, but in a counter flow heat exchanger changing steel to Ali is going to be negligible in comparison with an increase in surface area.

System design is often very hard to do anything significant about on boiler swaps. Manufacturers and installers are attacking many fronts.

Agree, and you're right, even a straight swap due to the larger heat exchanger surface would drop the flue gas from well over 100c to close to the return temperature. It would be nice if the whole heating system was given an efficiency rating, otherwise how can you compare 2 installers quoting for a band A condensing boiler and radiators.

Post overlap

That's always good to get two points of viewthanks for challenging my comments

 

[Dan Robinson]

Remember that the flame is nearly 2000 degrees.

 

[ScottishGasMan]

It would be nice if the whole heating system was given an efficiency rating, otherwise how can you compare 2 installers quoting for a band A condensing boiler and radiators.

This is very much an issue you see in new builds, they tick the boxes for boilers but use the smallest heat emitters imaginable for cost saving so often to get the house warm your running 80 degree flow temps.

 

[John D v2.0]

Read those bits again more carefully. It's not what VC said.

I've done that and you are right i didn't read carefully at all. actually i just think my real objection is around the Ali vs steel thing, physics is the king, and then the materials work within those rules. Have a look at this quote:

a condensing boiler has the potential to condense not a necessity as the gains are so small. Forget 11%. 10.73KW sensible heat in a cu metre methane 0.93KW latent heat. You can't get all of both but you go for the biggest prize, around 9.8 sensible and 0.3 latent. That's why ali boilers exist and don't need to condense and why the manufacturers of stainless have a hard on for low temp running.

Low temp running on the output gases is the only way to get efficiency

Remember that the flame is nearly 2000 degrees.

At the fundamental level the efficiency with x% excess air is a function of the flue gas temperature. Assuming intake temperature, gas calorific value etc etc are all constant.
So your only aim is to get the flue temp as cold as possible. Whatever tricks of hex design can do that perhaps ali is better suited as I'm not a material scientist.

You mention 2000 and 2000 is indeed a big number and of course every degree below 2000c you go, you get the specific heat capacity of that volume of air back. This is linear so it adds up to a similarly large amount whether you go down to 180c or 80c as i guess is your point.
However at around 55c you hit the dew point and water starts condensing out. As you drop the temperature further the capacity to hold moisture continues to drop albeit at a slower rate and by 10c you've got almost all the water back.

For anyone doubting that condensing the water is significant, think about this: if your boiler is set up better it can produce several of litres of condensate in a day. To work out how much energy came back from condensing it, stick it in a big pan on the hob on a (say) decent 3kw ring on full blast. How long do you think it would take for all the water to get to 20c to 100c? And from that point, how long to get to 101c? (ie all evaporate)

 

[vulcancontinental]

1.46l/cu m of methane is the condensate. Measure it and you'll know how much condensate there is, I have done it but you need to separate the flue from the hex.

If the flue products in the air flow through the hex are warmer than 55 then the only vapour to condense will be that which comes into contact with the cooler metal of the hex and not all of it will nor will all of the surface be, there is a sharp gradient of surface temperature rise after the return.

Run hot and cycle can be more efficient than continually condensing as the anti cycle delays built into boilers keeps them off for say 5 minutes three times in 60. Three times an hour means a reduction in fuel saved over running cooler looking for the latent heat over an hour. Comfort levels will not be as stable possibly but they can't be factored into efficiency.

Low modulation also means the boiler runs continuously but to achieve that greater quantities of excess air are use to keep the burner cool reducing flue temp and efficiency.

Black and white it isn't.