condensing boiler - higher return temp == higher efficiency?

[moggie2002]

Sorry for adding to the many condensing boiler posts, but I'm a little puzzled by the logic behind the efficiency arguments.

I've read about the advised flow return temps for condensing mode, over sizing rads that some people do, temperature differentials on flow return, but don't these requirements or advice offset the efficiency gains somewhat? Oversizing rads obviously incurs some extra upfront expense as well as other possible problems, but what about return temps and the energy required to increase a possibly artificially low return temp up to the flow temp. For example, if a hypothetical system can deliver a return temp that is the same as the flow at, e.g. 70C, then losses in the boiler aside, would it not be better to keep that setup than drop the return temp to 50C just so that condensing can take place, and then use more energy to raise the return temp up to 70. If that is a correct supposition, then what if the return temp was 65C? Might that use less energy to raise to 70C than from 50C to 70C, even with the benefit of heat from the condensate. etc.

Further, how can we achieve the low return? If flow is 70C, all rads are getting hot evenly, there's minimal temperature drop across the rads, and the final return is 60C. Is it really better to create drops in temperature across the rads, giving reduced heat output and therefore potentially a longer running time of the boiler to create a lower return temp, or is the loss of potential output compensated for by having a larger rad that gives similar output from its larger surface area?

Our actual system is an ideal logic+ combi with electric UFH as well, and a network of dallas 1-wire temp sensors around the house with a couple strapped to the main flow and return to monitor that. The system is balanced sufficiently so that all rads (myson decors) get hot, but the surface temp drop from one side to the other is nothing like 10C or more, which to me seems fine as that's surely more heat output. The return temp on the pipe is around 55 with flow at nearly 63 (boiler claims 67), so maybe that's perfect, though the return can rise higher.

So I'm just puzzled about how things really balance out when all factors are taken into account with combis, and that while they are more efficient in themselves, does that really translate into more efficiency in practice?

 

[ianniann]

what about return temps and the energy required to increase a possibly artificially low return temp up to the flow temp

Quite a grasp of physics you've got there. Not! If you were running your pipes through the garden to "artifically" drop their return temperature then that would be inefficient and wasteful. As it is, oversized or fan-assisted radiators, low pump speeds, restrictive flow, or whatever else you are thinking about is not wasting anything. The "extra" heat required by the boiler to heat from the low return temperature to the ongoing flow temperature is energy that has come out into your house somewhere. Very useful it seems to me.

Condensing boilers are inherently more efficient when working with cool water flowing through them. There is potentially about 10% extra energy available, although in practice the difference between "good" and "bad" situations will just be a few percent. So not enough to do stupid things like putting a radiator in the garden to cool the flow down but worth having all other things being equal.

Running the boiler for longer is something to be considered. Running a boiler for excessive periods with a very low flow temperature could possibly be counter-productive. Boilers and pumps use a non-trivial amount of electricity, and running them at very low output (for example at low temperature through conventional radiators) will easily waste you the couple of percent you might gain from the low flow temperature.

Just for reference a return temperature of 55C and an outgoing flow at 63C (or 67C) is too high to gain anything. Condensing starts to occur at about 57C but with any practical heat exchanger design only a small fraction of the available energy is extracted with incoming water at 55C, since it quickly reaches 57C by condensing a tiny fraction of the vapour and then nothing more.

The idea that many people have (and many supposed experts seem to encourage) of a condensing "mode" is flawed. Condensing is not all your nothing. It begins when the return flow drops below the condensing temperature of water (which varies slightly) and then increases as cooler water is able to condense a larger proportion of the vapour. In practical terms for domestic boilers, the cooler the better, subject to it keeping your house sensibly warm and not having to run 24 hours a day cycling every 2 minutes because the radiators aren't shifting any heat. Anything above a return temperature of 57C it makes no difference since there is zero condensing. A good aim for many radiator systems would be a flow temperature of 60C returning at 40C. This will extract most of the available condensing energy and still work reasonably well with conventional radiators. You might need to turn up the temperature in the middle of winter though. Boilers are available that will start out at a high temperature (maybe 70C or even more) when the rooms are a long way below the desired temperature, and then automatically drop the flow to 60C or even less as the room gets to where you want it. The boiler runs efficiently and you don't get a massive temperature overshoot when the thermostat clicks off but the radiators are still roasting hot.

 

[moggie2002]

what about return temps and the energy required to increase a possibly artificially low return temp up to the flow temp

Quite a grasp of physics you've got there. Not!

Thanks for the reply, and LOL. Actually it's better than the post might suggest, but I don't like to boast - "clearly" you might say, so I'll save you the trouble

If you were running your pipes through the garden to "artifically" drop their return temperature then that would be inefficient and wasteful.

Actually we near enough are for part of the run that goes below ground in a cellar with open vents to the outside and winds that come roaring through, and insulation is on order for those pipes. The rest were going to be lagged to, though maybe we shouldn't lag lest we get an even higher return temp to deal with.

The "extra" heat required by the boiler to heat from the low return temperature to the ongoing flow temperature is energy that has come out into your house somewhere. Very useful it seems to me.

True, though not all the losses might be useful or where you need them, like in the cellar.

Running the boiler for longer is something to be considered. Running a boiler for excessive periods with a very low flow temperature could possibly be counter-productive. Boilers and pumps use a non-trivial amount of electricity, and running them at very low output (for example at low temperature through conventional radiators) will easily waste you the couple of percent you might gain from the low flow temperature.

Our Ideal runs the burner and pump all the time that there is demand for heat, only shutting the burner down if it overshoots the set temp by 5C. I wonder whether raising the temp a few degrees higher, and thereafter switching the pump and burner off for some time would save more energy overall. Currently I've got a crontab running the boiler for 10 minutes each half hour, and that seems to work fine from a comfort point of view around the place.

A good aim for many radiator systems would be a flow temperature of 60C returning at 40C.

Will be interesting to find out if I can get there and to see how the rads perform. The temp on flow and return is around that level after an off cycle, but doesn't take long before the return is approaching the mid 50's again once the boiler's fired. The main thing is that at least it's working and the rads get hot.

 

[bengasman]

Your logic is flawed all the way along.
The flow, rad inlet and temperature drop are completely irrelevant from an efficiency point of view.
The only thing that counts is: the lower the return temperature on the boiler, the higher the efficiency.

 

[ianniann]

Operating the burner for only 10 minutes each hour, with the boiler and pump running the whole time, is probably little enough that you're wasting any potential gas savings with more electricity and wear and tear on the system. I take it the boiler is ancient? Most boilers built this century have built-in controls both to maintain the right flow temperature and to modulate the burner down to try and avoid cycling on and off.

If pipes are leaking heat where it is just a waste, then lag them. Screw the return temperatureand screw the 0.1% boiler efficiency it might gain you. A bare copper pipe in an unheated space is a massive waste, doubly so if air is moving over it. Current regs recomend lagging all pipes, indoors and out, hot and cold, but it is much less important from an efficiency standpoint when they are running under your bedroom floor. Underfloor heating, if you like

If your return temperature is too close to the boiler flow temperature then turn down your pump. You should aim for a 20C temperature drop with most condensing boilers, older boilers were design for a smaller drop, but 20C can be difficult to achieve with conventionally sixzed radiators especially if TRVs start turning some of them off. If you turn the pump down a notch, you might find one or two radiators furthest from the boiler down't heat up so well. If so, then first try balancing the system. If you still have cold radaitors then you might have to turn the pump up again. It isn't the end of the world if you don't get a 20C drop, but it's what you are aiming for. It is much easier to configure a system to run at 80C/60C than at 60C/40C, but 60C/50C is better for efficiency than 80C/60C.

 

[moggie2002]

Operating the burner for only 10 minutes each hour, with the boiler and pump running the whole time, is probably little enough that you're wasting any potential gas savings with more electricity and wear and tear on the system. I take it the boiler is ancient? Most boilers built this century have built-in controls both to maintain the right flow temperature and to modulate the burner down to try and avoid cycling on and off.

As mentioned before, the boiler is actually one of the new Ideal logic+ ones that are an unknown reliability wise at the moment, but we decided to be early adopters. Like the rest of the house, the boiler is computer controlled, and in this case is part of a DMX network that controls the lighting, UFH and other odds and ends. So the CH demand is controlled, and therefore the pump cycles too simply because we don't need the boiler on all the time to keep the house comfy.

Current regs recomend lagging all pipes, indoors and out, hot and cold, but it is much less important from an efficiency standpoint when they are running under your bedroom floor. Underfloor heating, if you like

Right. The intent is to lag them all, not least because there's so much cable, with somewhere between 200 and 300 meters of pink cat 5 and probably close to a Km now of twin/earth, and some cables are rather closer to pipes than I'd like.

If your return temperature is too close to the boiler flow temperature then turn down your pump. You should aim for a 20C temperature drop with most condensing boilers, older boilers were design for a smaller drop, but 20C can be difficult to achieve with conventionally sixzed radiators especially if TRVs start turning some of them off.

I'll look at that and I'm sure it's not been adjusted by the plumbers. The pair who fitted the boiler weren't great, and while we've got a really nice guy with a good work ethic doing the plumbing now, he can't do straight pipework (he did admit to one eye being defective), his compression fittings tend to leak, he assumed that the first lot had always fitted flow on the left when in fact they hadn't so we fitted a zone valve the wrong way round, one or two of the pegler terrier II's we have whistle when shutting off possibly because they are on the return (though it shouldn't matter people have mentioned this of those valves), and at least one of his rads off the same run of pipe work has flow/return opposite to the others. He also had the boiler at 2.5 bar last time he made changes so I had to reduce that to stop rattling in the boiler.

If you turn the pump down a notch, you might find one or two radiators furthest from the boiler down't heat up so well. If so, then first try balancing the system.

That was the case first because plumber hadn't balanced, so that was the first thing I fixed.

It isn't the end of the world if you don't get a 20C drop, but it's what you are aiming for. It is much easier to configure a system to run at 80C/60C than at 60C/40C, but 60C/50C is better for efficiency than 80C/60C.

Thanks again, I'll give it a go and see what happens.

I'm not monitoring gas usage yet but it'll be interesting to see the usage over time. Electricity usage has been around 50KWh a day while we only had UFH plus a supply of draughts in the house, but that's lower now that we have CH elsewhere and are starting to fill holes and put the inside fabric, floors and ceilings of the building back together.

 

[bengasman]

....As mentioned before, the boiler is actually one of the new Ideal logic+ ones that are an unknown reliability wise at the moment....

That is one way of putting it.

...I'll look at that and I'm sure it's not been adjusted by the plumbers. The pair who fitted the boiler weren't great...

As was made more than obvious by their choice of boiler make

....and while we've got a really nice guy with a good work ethic doing the plumbing now, he can't do straight pipework (he did admit to one eye being defective), his compression fittings tend to leak, he assumed that the first lot had always fitted flow on the left when in fact they hadn't so we fitted a zone valve the wrong way round, one or two of the pegler terrier II's we have whistle when shutting off possibly because they are on the return (though it shouldn't matter people have mentioned this of those valves), and at least one of his rads off the same run of pipe work has flow/return opposite to the others. He also had the boiler at 2.5 bar last time he made changes so I had to reduce that to stop rattling in the boiler.

He may be a nice guy, but it doesn't sound like a quality installer.
How on earth did you manage to end up with the worst possible boiler make in the country, cowboys to install it, and now a (nice) muppet who will fail to make it much better?
Did you take the cheapest of 15 quotes?

 

[moggie2002]

He may be a nice guy, but it doesn't sound like a quality installer.
How on earth did you manage to end up with the worst possible boiler make in the country, cowboys to install it, and now a (nice) muppet who will fail to make it much better?
Did you take the cheapest of 15 quotes?

Due to the nature of the job we have our trades on day rates. The current plumber was recommended after we decided not to continue with the first plumbers and he seems to know his stuff. He's contracted to some big companies in the area and probably wouldn't be if he was totally rubbish, but they're not going to care or notice if a heating pipe is skewed by a few mm from where it should be or if a rad isn't aligned perfectly, but unfortunately I see those details and it matters.

The boiler was recommended by the first duo, and I did some research here and elsewhere before going with it. That of course showed that Ideal are historically a disaster, but also that things may have changed with their latest offerings, and there were some seemingly genuine positive posts from installers. It's tidy inside, very quiet, and seems to work well, so no real complaints so far other than about one of their tech people who was very defensive about the boiler when I had a simple query, and who was perhaps scarred from their previous years of troubles. IIRC they have a 5 year warranty subject to certain conditions, so I figured I'd give it a chance. We have a C&M boiler in another place that seemed to be a decent make, but the motherboard had to be replaced several times in the first year or two. The last failure when out of warranty was the fan relay and there was no way I was going to fork out for a new board, so I fixed that myself thanks to the relay manufacturer in Holland sending out a free sample, and that seemed a much worse unit in comparison. Only time will tell though on the Ideal.

 

[bengasman]

... He's contracted to some big companies in the area and probably wouldn't be if he was totally rubbish, ....

I can see your logic there, but unfortunately, it doesn't work that way. Big companies looking for subbies are only interested in 2 things.
1. Cheap. You really would not believe how little they pay.
2. Obedience. "Jump!" And don't you dare ask "how high?" before you said "Yes, Boss, thank you Boss."

Quality is of no consequence. If a customer calls up with a complaint, they send another chap, and first one simply does not get paid for the job he did, regardless of whether the complaint was justified or not.
Protest once too often, and you are fired.

I leave it to you to figure out how many good engineers are willing to work under those conditions.

 

[ajrobb]

I think the OP needs to read the FAQ and pay particular attention to 5) HOW TO BALANCE A CH SYSTEM.

You need to use the radiator lockshield valves to restrict flow through each radiator so that the return pipe temperature is 11°C or 20°C (varies with boiler) below the feed pipe temperature. Not only does it improve boiler efficiency, it gets the big radiators heating up quickly.

If the combination boiler has separate HW and CH temperature controls, you should turn down the CH feed temperature in mild weather. This also improves efficiency.

 

[GaytonTonner]

The OP needs some information. Non-condensing boilers were designed not to condense as the boiler innards would rot. This is approx 56C return temperature. Below this condensation occurs.

To prevent this the rads were designed to run at 80C and the return at 70C. Cylinder coils as well. At 10C temperature differential. The boiler heat exchangers were designed to operate at 10C difference. Outside of this and long term damage can occur. Permanently running at 22C difference could result in a cracked heat exchanger.

Condensing boilers operate at higher efficiencies at the lowest return temperature possible. The first simple controlled condensers needed over-sized radiators to drop the return temperature. But a heating system is running at "part load" for 95% plus of run time. They run at full load when the temperature outside is below -3C. This means most boilers are over sized for average run time.

Modern boilers have sophisticated control to monitor the flow and return temperatures. They can detect when the house is up to temperature and lower the burner gas to suit. This means that a condensing boiler fitted to an old system design for 80C - 70C, the radiators do not need to be made larger. The controls ensure the return temperature is as low as possible.

Most condensing boilers can operate at 20C to 25C temperature differential. But some are still at 10C. So beware. Check. But a sophisticated control system will take care of it all for you. The cheap boilers do not have the advanced control systems. Most modulate the burner down to maintain a set flow temperature, not dropping the flow temperature to ensure a dropped return promoting efficiency. These more advanced boilers tend to have integrated weather compensation controls.

Just a note. The efficiencies of a regular boiler can be increased by having a blending valve on the flow and return pipes. This is set to approx 58C, just above condensing dew-point, sending water back to the return at 58C. Set the blending valve and leave the boiler at 10C above. The efficiency of the simple boiler will rise.

 

[mysteryman]

You clearly have to have a temperature drop across a heating system, otherwise there would be no output!

Weather compensation will satisfy all the requirements you need. The boiler will run automatically at the correct flow temperature for the prevailing weather conditions. As stated above, for 95% of the time, you do not need full output, and the weather comp will keep the flow temperature below 53C, and the boiler will be fully condensing.

It is plug and play on the best boilers - and has been the norm on most of the Continent for several decades now.

 

[GaytonTonner]

You clearly have to have a temperature drop across a heating system, otherwise there would be no output!

Weather compensation will satisfy all the requirements you need. The boiler will run automatically at the correct flow temperature for the prevailing weather conditions. As stated above, for 95% of the time, you do not need full output, and the weather comp will keep the flow temperature below 53C, and the boiler will be fully condensing.

It is plug and play on the best boilers - and has been the norm on most of the Continent for several decades now.

Spot on. The Dutch boilers have all this as standard and plug and play Open Therm. Britain is way behind.

Just a note. 53C is the dew-point of the flue gasses. The "return" water temperature is usually a few degrees above this, depending on the boiler's heat exchanger.

 

[mysteryman]

Yes and no.

Keep the FLOW temperature below 53C and the boiler is fully condensing. This will happen automatically for about 95% of the heating season with weather comp.

 

[GaytonTonner]

Yes and no.

Keep the FLOW temperature below 53C and the boiler is fully condensing. This will happen automatically for about 95% of the heating season with weather comp.

I don't know what your "no" is relating to. Keeping the flow below 53C will result in condensing for sure. This is externally attempting to get the flow temperature as low as possible.

You are spot on with weather compensation, a point I picked up on. I have read poster on here when doing searches saying weather compensation is useless, which really does not inspire confidence in installers. Many boilers are easy to set up and all you do is fit an outside sensor.