New to forum - ex plumber (with a question!)

[Ric2013]

Okay, so I turned my water heating off on Friday night for some reason, after it had heated my cylinder. Today, the water was lukewarm and I fired up the boiler. It used 0.889cu m of gas, or 9.8kWh to heat the cylinder.

Bouncing idea about, if we say a conservative estimate of double the heat required, we would expect to see a proportional increse in gas used. Doubling the heat requirement has resulted in only 30% extra gas consumption.

Would it therefore seem reasonable to say that, in all likelihood, the inefficiency is quite possibly that the system requires a certain amount of heat to heat the primaries and cast iron heat exchanger?

This theory could be tested by testing the gas required to heat hot water, when the boiler is already hot from having been used for heating, if winter ever comes, that is!

 

[CHRISX]

Okay, so I turned my water heating off on Friday night for some reason, after it had heated my cylinder. Today, the water was lukewarm and I fired up the boiler. It used 0.889cu m of gas, or 9.8kWh to heat the cylinder.

Bouncing idea about, if we say a conservative estimate of double the heat required, we would expect to see a proportional increse in gas used. Doubling the heat requirement has resulted in only 30% extra gas consumption.

Would it therefore seem reasonable to say that, in all likelihood, the inefficiency is quite possibly that the system requires a certain amount of heat to heat the primaries and cast iron heat exchanger?

This theory could be tested by testing the gas required to heat hot water, when the boiler is already hot from having been used for heating, if winter ever comes, that is!

Hello again Ric2013,

I notice that You have not commented on my previous question about whether you have lived in your Home since the Boiler was installed and whether it is likely that your Heating system has always had Corrosion Inhibitor correctly dosed within it ?

Also as I mentioned even if it has had the Inhibitor a Boiler of that age is bound to have significant Limescale within its cast iron Heat Exchanger because the Inhibitor is not a `Preventor` re. Corrosion & Limescale.

IF there is a significant layer of Limescale within the Boiler Heat Exchanger the burners are having to `heat through` the Limescale to heat the water that is flowing through the Cylinder coil / Heating the Hot water.

In that scenario there would be a significant amount of additional Gas used to `Heat the cast iron Heat Exchanger and Primary Flow & Return water` and consequently the Water in the Cylinder.

Here is a quote from Fernox about Limescale within a Boiler Heat Exchanger:

QUOTE:

While the presence of limescale within a heating system is not as visually apparent to the average homeowner as it is in everyday kitchen appliances, its existence is just as prominent and harmful. Limescale deposits within a heating system can cause blockages in pipework and the boiler’s heat exchanger, significantly impacting on the system’s efficiency. This forces the boiler to work harder than necessary to produce the desired temperature and can eventually lead to the premature failure of system components.

In fact, the presence of limescale in a boiler greatly inhibits heat flow within the home and is one of the main causes of spiralling fuel bills.

With limescale conducting thermal heat at a rate 400 times less efficient than the copper piping, even a 1/16 of an inch deposit of scale around a boiler’s heating elements can increase fuel costs by up to an alarming 15%.

END OF QUOTE

I could easily imagine a Boiler from the 1980`s having far more than 1/16th of an inch of Limescale - probably more than 1/8th of an inch even if there has been Corrosion Inhibitor in the system all of the time.

I could be wrong about this but I don`t think that 1/8th of an inch of Limescale would equate to only about a 30% increase in Gas to heat the water flowing through the Heat Exchanger - it could be much more than an additional 30% ?

I don`t think that the inefficiency would just increase in multiples of 15% for every 1/16th of an inch that the Fernox quote mentions ?

I wonder why Fernox are using `1/16th of an inch` as a measurement ?

Here is the Fernox webpage where the Quote came from:

HARD WATER, SIMPLE SOLUTION - Fernox UK

Chris

 

[Ric2013]

Chris,

Sentinel claims it protects against both corrosion and limescale, but you are right that the boiler pre-dates me by 20 years, during which period we have no idea how much inhibitor (if any), or what kind, was in place. I would imagine Fernox is similar. In any case, the boiler kettles, so that would suggest scale is present.

I accept your point that inhibitor is probably not 100% effective (if I understand you correctly) and I have a lot of scepticism with regard to claims made by inhibitor firms.

I hope you can see that the 30% I am referring to above is not the 30% you are referring to. They are two separate 30% figures.

The point I was making is that, even if we accept the boiler efficiency has fallen to 25% or whatever due to limescale (though I am highly sceptical the efficiency could fall to that level before the heat exchanger burns through), then, by doubling the amount of heat required by my DHW cylinder (by letting it run cold), we would expect the gas input required by the boiler to double, whereas it has actually only increased by 30% respective to re-heating a largely warm cylinder. This suggests to me that my original 7.5kWh to reheat a fairly hot cylinder is not only down to boiler inefficiency.

Regarding the 1/16" claim, if a sixteenth of an inch provides x insulation (resistance to heat transfer), then an eighth naturally provides 2x insulation. Couple, though with the insulation of the iron itself, and each doubling of the scale thickness should, theoretically, increase the insulation by a little less than a doubling.

The point I was making is that the primary circuit is composed of about 11 litres of water in the primary circuit and boiler, (and some in the cylinder coil) and around 40kg of cast-iron in the heat exchanger. As such, the primary circuit requires the equivalent of however much heat is needed to heat 15l of water from 20 to 80 degrees, before it is heated through. Which doesn't explain a lot, given that it's little over 1kW, though it's worth testing this theory.

In any case, I've cancelled work as suffering from flu, but I'm getting bored and will probably whip the immersion heater out at some point tomorrow and see what state the inside of the cylinder is in. Can always do a few minutes at a time, eh?

 

[Ric2013]

Well, the hardest thing was having enough strength to coil the poxy hose back up after I'd drained the cistern. Realise now there was no need to drain the cistern, but obviously thought processes not at maximum levels!

The cylinder was also empied. There is a thin coating on the coil, but it looks to be relatively minor, though I appreciate that the thickness of scale is hard to assess. I think I'll dump some citric acid solution in the cylinder and see if I can dissolve some of the limescale, and hope this won't wreck the cylinder! Apart from some none-too-impressive pipe bending inside the cylinder, it seems relatively in order.

Also, a drained cylinder allowed me to test the heat required to heat the primary circuit. Gas input required is 2.28kW before the boiler thermostat switched off. This is interesting. While it could mean the boiler is <50% efficient, assuming my guestimates of heat required by the primary circuit are accurate, it also means that, in summer, the boiler has to use as much gas as, burned efficiently, would be needed by the DHW cylinder (heating the whole cylinder through 15°C requires 2kW) just to get the primaries heated.

 

[CHRISX]

Well, the hardest thing was having enough strength to coil the poxy hose back up after I'd drained the cistern. Realise now there was no need to drain the cistern, but obviously thought processes not at maximum levels!

The cylinder was also empied. There is a thin coating on the coil, but it looks to be relatively minor, though I appreciate that the thickness of scale is hard to assess. I think I'll dump some citric acid solution in the cylinder and see if I can dissolve some of the limescale, and hope this won't wreck the cylinder! Apart from some none-too-impressive pipe bending inside the cylinder, it seems relatively in order.

View attachment 34919

Also, a drained cylinder allowed me to test the heat required to heat the primary circuit. Gas input required is 2.28kW before the boiler thermostat switched off. This is interesting. While it could mean the boiler is <50% efficient, assuming my guestimates of heat required by the primary circuit are accurate, it also means that, in summer, the boiler has to use as much gas as, burned efficiently, would be needed by the DHW cylinder (heating the whole cylinder through 15°C requires 2kW) just to get the primaries heated.

Hello Ric2013,

The Cylinder looks fine, how old is it ?

As You know a `badly scaled` Cylinder would have not only much more Limescale on the Coil but also there would be Limescale debris in the bottom where it had broken off the Coil.

It is very interesting when the results of `experiments` like you have just done are available because we very rarely get to see the figures from a physical `Test`.

How much water do You calculate is contained in the Primary pipework and the Boiler Heat Exchanger ?

Obviously the Boiler Heat Exchanger Limescale level is unknown regarding how much additional heat is used to `heat through` any scale.

Chris

 

[Ric2013]

Hi Chris (and whoever else is reading),

The cylinder was there in 2006. How much older than that, I do not know.

Well, yes, I am surprised there is not a collection of limescale flakes at the base. Not bad for its age. I've taken out cylinders that are perhaps 50 years old, but this was not even a tenth as much limescale.

Thank you. I find it interesting to see how things work in real world situations, and I'm glad you appreciate the sharing. It would be interesting to compare how other systems of this kind are functioning. I do have a couple of friends/family/customers who might let me run the experiment, but I digress.

The primary pipework is around (2 x 7m of 22mm) 4.5l, and heat exchanger water content is another 6l. Round the total to 11l and it shouldn't be far off.

Given that the F&E was not covered and therefore subject to increased evaporation when I bought the place, I would imagine there is very likely to be quite a lot of limescale in the boiler.

 

[Ric2013]

Well, 500grammes of citric acid powder dissolved in water and then filled to the top of coil and heated through did indeed remove the limescale, so it must have been quite a thin layer. As expected, this made little difference to performance.

 

[CHRISX]

Hello Ric2013,

I would still be looking at the Boiler being heavily scaled.

I feel that the calculated excessive amount of Gas to heat your Cylinder / Hot Water can only be attributed to it having to heat the water in the Primary loop through a layer of Limescale.

If as I suspect there is a significant layer of Limescale within the Boiler Heat Exchanger that would be absorbing a significant amount of heat from the burners before the heat is actually heating the water that flows through the Cylinder coil.

I cannot see anywhere else that the amount of `Heat loss` that You previously described could be lost - it cannot be being lost from the Primary Flow & Return ?

Chris

 

[Chuck]

I've never done a decent heat calc but when the old Mexico finally dies I will do because having a boiler that's oversized is very inefficient due to cycling.

You've probably thought of this, but...

While you're waiting for your boiler to die, make a note of the average external temperature and how much fuel it consumes (daily) during a spell of cold weather. This information will give a reassuring sanity check for your calculations.

 

[Welder]

There is software now that does a certified heat calc for you at £10 or less per pop. No need to do any measurements yourself. I'll look it up and post.

For any heating installers it looks a no brainer cos it makes sure everything is sized perfectly so minimal cycling and maximising reliability...