Short Cycling Boiler

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Background
Conventional Ideal FF380 80,000btu Boiler 13 Rads (No TRV's) and Programmable Thermostat System just balanced - temp drop of 11/12 degree C across all rads and boiler. Pump speed on minimum.
Problem:- The boiler will fire continously while the house is warming up from cold in the morning but for a short period just before the room stat is satisfied the boiler will short cycle, Fire for 2 mins off for about the same duration. This will happen until the room stat switches the boiler off.
It only tends to happen in the morning when warming the system up from cold. When demand is triggered during the day/evening to top up the temp it seems to fire continuously until the room stat is satisfied.
Obviously this is the boiler stat turning the boiler off but I wouldn't of thought this would happen as the flow temp from the boiler is 74 degree C and the return about 62 degree C.
Is the problem the amount of time it takes to heat the house up in the morning?
Is it a problem or am I concerning myself about something that isn't a problem
If it's a problem is there a solution?
Cheers
Nige
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The solution is to use a boiler that has a modulating burner. Then, it just turns down the heat when the flow temperatures get too high. I suspect the difference between morning and evening is that the house gets up to temperature before the flow temperature goes too high. This is because it is warmer in the evening than the morning. Deeper into winter, you'll probably find it does it in the evening too.
I wouldn't worry about it. It is slightly inefficient, that's all. It is why they developed modulating burners.
Christian.
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I suspect that the boiler is a bit too powerful for the job it's doing - and is simply cycling on its own stat to maintain the set flow temperature. If this is a problem, you could try de-rating it a bit by reducing the gas pressure. [If you look at the installation handbook it will probably have a table showing heat output vs gas pressure].
If you do this, of course, it will be a bit slower to heat up on really cold days.
As others have said, a modulating boiler would vary its output automatically - but it's a bit drastic to throw away an otherwise perfectly good boiler!
Roger
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This may be the boiler stat faulty or a slow running pump. I assume the house is still cold and the thermostat is calling for heat.
- The heat appears not to be getting out of the boiler fast enough - the boiler stat is faulty, - the boiler is way too large.

So all is right when the room stat is in control. It may have a boiler anti-cycle function so all is fine when it is mainly in charge.

Needs a little more observation and seeing if the boiler is too big.

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Could the boiler stat be set too low?
Sparks...
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Good point. the stat should always be near to or at the top of the range.
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Does this apply to (back boiler) gravity fed systems too?
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range.
All non condensing boiler should be on high temperatures to avoid condensation occurring in the burner box.
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How do I know if I have a condensing boiler? It was in the house when I moved in, and there are no instructions for it.
Bob
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It probably isn't. Does it have a make or model? What does the flue exit look like and what route does it take? If it is short (i.e. straight through the wall), it will plume noticeably if condensing.
Christian.
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On Thu, 27 Nov 2003 09:17:22 -0000, "Christian McArdle"

I would have thought the whole point of a condensing boiler is that the exhaust contains little or no water vapour. A condenser will have a permanent drain, although this could of course be coupled to the pressure relief plumbing and not be immediately obvious.
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This would only happen if the flue is long enough for all the water to be removed, or the return temperature is very low. If the return temperature is around 60C, then the boiler fundamentally can't cool the flue gases any lower that this. Those gases would then still contain water that will only condense out when the temperature drops further, either within the colder flue (if it is long enough) or upon exposure to colder air (at the terminal).
Any condensing on the flue or outside air is wasted energy. That is one reason why greater efficiency occurs at lower return temperatures. More condensing can then occur upon the heat exchanger itself, where the energy can be usefully recovered. (There are also thermodynamic reasons for greater efficiency at low temperatures that apply to all boilers, not just condensing types).
Christian.
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On Thu, 27 Nov 2003 11:45:48 -0000, "Christian McArdle"

I don't think that that is completely it, Christian.
The major heat recovery in terms of delivering heat to the heat exchanger water is from the change of phase of water from gas to liquid - latent heat of condensation.
The lower flue gas temperatures result in less loss of heat outside in the form of hot air, CO2, water vapour etc., and anything that can help with that is a bonus as well in that the heat will dissipate inside the property. For example, having a longer flue inside the house on a condensing boiler will result in the flue gases being cooler as they leave the building and more water will have been collected. However, that will be heat transferred other than into the water.
Obviously the heat exchanger temperature has an impact on that as well.
I just took a look at the display on the front of my boiler. It's currently showing 6 degrees outside, the flow temperature is 61 degrees, the return 45 degrees. It also gives me the fan rpm from which I can look up the heat input - approx. 7.5kW - and the pump is running at 35% of max. There is very little plume at all.
I can press a test button and deliberately force it to run at a 50% of full power rate (approx. 14kW). There is still very little plume.
I only have a very short flue - literally the elbow above the boiler and straight through the wall.
.andy
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Maybe I didn't explain very well.
The water condenses out of the gases because the gas temperature has reduced to the extent that the air is fully saturated. As the gas temperature reduces, it can contain less water, so the water condenses out onto the heat exchanger, raising the temperature of the heat exchanger as it does so.
If the coldest part of the heat exchanger (which is the same as the return temp) is 60C, the gas is capable of containing far more water than if it was 30C. Therefore, less water comes out for condensing, and the heat exchanger benefits from less boost.
This is completely different from simply the energy lost by expelling higher temperature air. You would even gain this energy advantage even if you subsequently reheated the air to the same temperature. The difference would then be that the exhaust gas would have lower humidity, water would go down the drain and the system would be more efficient, but obviously less efficient that had you not reheated the exhaust.
Although the plume starts happening when the boiler gets cool enough to condense, if the return temp is very low, then the pluming may reduce again, simply because the heat exchanger becomes efficient enough to remove most of the water from the gases, which then don't have much further in temperature to fall.
Christian.
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On Thu, 27 Nov 2003 13:45:51 -0000, "Christian McArdle"

Agreed. I rather suspect that the contact time and area for the gases with the heat exchanger has an impact as well.....

.andy
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There's a law of diminishing returns, though. Once the gas temperature is within a few degrees of the return temperature, there's little point in hanging around.
Christian.
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On Thu, 27 Nov 2003 11:32:18 +0000, John Laird

It very much depends on the design and operational conditions of the boiler.
The important thing to realise is that the extra energy recovered when condensing is due to the latent heat of condensation. This is the change of state of water from the gaseous phase (steam) to the liquid phase (water). Heat is given up as a result of this change, just as extra heat is required at boiling point to turn water to steam.
It's important to note that water vapour (that you see) is *not* steam - it is fine droplets of liquid water. By the time that you see the plume, the energy release has already occurred. Thus from the perspective of efficiency, it doesn't matter whether the water vapour is collected inside the boiler or goes out through the flue.
Some designs of boiler and heat exchanger appear to be better than others in terms of how much pluming happens, but it does not appear to significantly affect the efficiency. If you look at the SEDBUK tables, most recent boilers are in the 90-91.3% range and the variation is fractions of a percent between them and not statistically significant. There are still a few older models that are in the mid to high 80s range, but are older designs.
It is possible that in some of the older designs that the heat exchanger(s) has/have not condensed so much of the steam to water vapour and that some is escaping in gaseous phase through the flue, and condensing to water vapour as soon as it hits the outside air. Since there are so many models bunched together in efficiency terms at the top 90-91% range, it suggests that they are reaching the limits on design and physics for the technology.
There are certainly different burner designs. Most of the good quality products have a downward facing burner and stainless steel heat exchanger, to improve performance and deal with the acidic condensate. The older ones tended to have more conventional burners and heat exchangers and a second heat exchanger after the main one.
There are also implications in the burner design with the amount of pollutant emission, especially of NOx gases.
I have a German made boiler from a company called MAN Heiztechnik (division of the same company that makes trucks and diesel engines). This is towards the top end on efficiency and one of the lowest NOx emitters in the industry. The burner and heat exchanger design is cylindrical with the burner being a wire mesh arrangement sitting at the centre of a stainless steel chamber with pipes around the periphery. The condensate falls to the bottom at the back and passes to a trap system for disposal. Very little water vapour leaves the boiler flue, even under quite high firing conditions.
There are photos at http://www.man-heiztechnik.de/index.php
Click on the second thumbnail picture with the flashing red "test" logo and then at the resulting Micromat EC product page, the link under the photo marked "Mehr fotos und details". There is a gallery of pictures and the heat exchanger is shown as the third and the burner as the fourth. The manufacturer won a best on test award in August for this product even though it was originally introduced in 1995.
.andy
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Soon, it looks as if 86% efficiency will be the minimum. This precludes all non-condensing boilers. This creates problems with the plume in flats and the likes, so advanced research is under ways with prototypes using Zeolithe heat pumps which run on natural gas for the provision of domestic heating and hot water. These units are currently floor mounted and resemble a typical boiler in appearance. Zeolithe heating appliance's use less energy and more environment-friendly than electric heat pumps and gas boilers. It provides considerably higher output levels than the current conventional and condensing boilers. Carbon-dioxide emissions are reduced by approximately 20 to 30%.
Zeolithe heat pumps can be toned down to eliminate a plume and still beat current condensing boilers in efficiency. Have the gas powering them condense and efficiency rises again.

The burner is a "radiant" burner. designed to primarily give off radiant heat. In normal bottom mounted rail burners the heat transfer is mainly via the products of combustion passing through the heat exchanger, the reverse.
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On 26 Nov 2003 01:42:27 -0800, snipped-for-privacy@virgin.net (Nigel Percy) wrote:

During the morning warm-up, the flow temperature will probably be at its highest after the longest "burn" - I assume from what you say about later demand that the periods are much shorter ? 2 mins on/2 mins off is not really a great problem in terms of lost efficiency.
However, I note you say you have a fairly powerful boiler and a lot of radiators. I have a similar configuration, but my boiler pump is set on max. Anything less and the boiler will "trip" more often. The rads on my system vary from about 1/2 turn open to almost fully open to achieve an acceptable balance. (They all have TRVs too, however, although I did the balancing when they were all demanding heat.) Anyone care to comment on whether boosting the pump a little might help the OP ?
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(Nigel Percy) wrote:

I mentioned that. I would first turn the stat right up and see how that works. No joy, then increase the pumps speed, assuming it is not at full speed anyhow. Ensuring that the boiler is matched to the house is another point. If too large then see if the burner can be turned down via the governor.
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