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How do calorifiers work?


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Specifically, how do they not get cold quickly?

 

I run my engine for a while, water comes out of the tap boiling hot. But that hot water must surely be replaced by cold water coming in to the calorifier. So the new cold water must make the existing hot water in the calorifier colder. Is how it works?

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Heat will be preserved by insulation. Hot water will rise to the top (take off point), cold entering the bottom of a vertical calorifier.

 

Over to mtb, he's an ex-pert:)

Edited by rusty69
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13 minutes ago, welly said:

Specifically, how do they not get cold quickly?

 

I run my engine for a while, water comes out of the tap boiling hot. But that hot water must surely be replaced by cold water coming in to the calorifier. So the new cold water must make the existing hot water in the calorifier colder. Is how it works?

It's like a mini version of the type of hot water storage tank you may have had at home. Hot water from the engine is piped through it and heat transfers from the hot water being piped through it to the water contained within it.

 

They are normally insulated so the hot water inside will retain its heat for some time. Eventually though if the engine has stopped and you have drawn off the heated water it will go cold.

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On our boat the water stays hot for 24 hrs, unless you draw it all off.  It gets really hot which shows thet thermostats are doing their job. It takes about an hour to get a tankful.  Best to have a navy shower.

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I have av thermostatic mixer valve that feeds the hot taps info bathroom.  It mixes the very hot water from the tank and cold water to produce a safer and more useful temperature. 

 

The hot tap in galley is fed direct from tank.  Handy for the kettle etc. plumbing.jpg.f1b92fdfa2ac7e7473f36022734ead79.jpg

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12 hours ago, Sapphal said:

On our boat the water stays hot for 24 hrs, unless you draw it all off.  It gets really hot which shows thet thermostats are doing their job. It takes about an hour to get a tankful.  Best to have a navy shower.

Yeah, mine also stays hot for 24 hours. I was just mulling this over when I was 4 pints in on a Sunday evening. Bit of a boozy "how do magnets work?" thought. And navy showers are my showers of choice, both on land and on water. I had a long cruise this weekend from the bottom of the Hanwell flight to east London and I don't think my hot water has been quite as hot as that ever. So its stayed hot very nicely.

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The above posts don't really answer the question "why does the water stay hot".

An answer is that hot  water from the engine fed coil in the calorifier rises to the top of the tank where your domestic system takes its feed from.

As you draw hot water off, cold water enters from the bottom. Because hot rises and cold doesn't there's not much mixing of hot and cold, thus the tank stays hot for some time and only gets colder because the insulation around the tank is not perfect.

 

I have a larger than normal "cauliflower" and it's vertical, so there's good separation between the hot and cold layers - for a horizontal tank (seem more popular nowadays), there's more mixing and methinks they don't stay hot for so long. 

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7 minutes ago, OldGoat said:

The above posts don't really answer the question "why does the water stay hot".

An answer is that hot  water from the engine fed coil in the calorifier rises to the top of the tank where your domestic system takes its feed from.

As you draw hot water off, cold water enters from the bottom. Because hot rises and cold doesn't there's not much mixing of hot and cold, thus the tank stays hot for some time and only gets colder because the insulation around the tank is not perfect.

 

I have a larger than normal "cauliflower" and it's vertical, so there's good separation between the hot and cold layers - for a horizontal tank (seem more popular nowadays), there's more mixing and methinks they don't stay hot for so long. 

Should that be "hot water heated by the engine fed coil in the calorifier........"

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11 minutes ago, OldGoat said:

The above posts don't really answer the question "why does the water stay hot".

An answer is that hot  water from the engine fed coil in the calorifier rises to the top of the tank where your domestic system takes its feed from.

As you draw hot water off, cold water enters from the bottom. Because hot rises and cold doesn't there's not much mixing of hot and cold, thus the tank stays hot for some time and only gets colder because the insulation around the tank is not perfect.

 

I have a larger than normal "cauliflower" and it's vertical, so there's good separation between the hot and cold layers - for a horizontal tank (seem more popular nowadays), there's more mixing and methinks they don't stay hot for so long. 

Mine is indeed a horizontal 'cauliflower'. Looks like there's some discussion of horizontal vs vertical here.

 

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2 hours ago, OldGoat said:

The above posts don't really answer the question "why does the water stay hot".

An answer is that hot  water from the engine fed coil in the calorifier rises to the top of the tank where your domestic system takes its feed from.

As you draw hot water off, cold water enters from the bottom. Because hot rises and cold doesn't there's not much mixing of hot and cold, thus the tank stays hot for some time and only gets colder because the insulation around the tank is not perfect.

 

I have a larger than normal "cauliflower" and it's vertical, so there's good separation between the hot and cold layers - for a horizontal tank (seem more popular nowadays), there's more mixing and methinks they don't stay hot for so long. 

I would say that is true, our tank is horizontal under the bed.  It is well insulated, uit once you have run a bath from it then the remaining water is only warm, which I put down to the cold water coming in.  So I don’t think you get good separation between hot and cold.

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Another factor must be what happens when the engine is switched off. The hot cauliflower will then exchange heat with the engine cooling pipework, once the latter becomes colder than the 'heated' water. The engine then acts as a great big heat sink.

 

In our boat we have a cock to isolate the engine circuit from the cauliflower, meaning that the water stays hotter for longer. The only drawback is when I forget to open the cock again when starting the engine, but I usually remember sooner or later.

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16 hours ago, MJG said:

 

 

They are normally insulated so the hot water inside will retain its heat for some time. Eventually though if the engine has stopped and you have drawn off the heated water it will go cold.

You've raised a point which I was wondering about quite recently. Does the engine heat water only when it [the engine] is running?

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2 minutes ago, Athy said:

You've raised a point which I was wondering about quite recently. Does the engine heat water only when it [the engine] is running?

I suppose there will be some residual heat in the cooling circuit which will dissipate into the calorifier contents, after that no, I'm not sure how it could?

 

The other way of course to heat the water in a calorifier is by use of a 240v immersion heater. The Dog House had one which had never been coupled up to the mains on the boat.

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15 minutes ago, Machpoint005 said:

Another factor must be what happens when the engine is switched off. The hot cauliflower will then exchange heat with the engine cooling pipework, once the latter becomes colder than the 'heated' water. The engine then acts as a great big heat sink.

 

In our boat we have a cock to isolate the engine circuit from the cauliflower, meaning that the water stays hotter for longer. The only drawback is when I forget to open the cock again when starting the engine, but I usually remember sooner or later.

It’s often recommended to fit a NRV in the engine/calorifier circuit for that very reason. 

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Thanks, WE and MJG. I wondered if the heat from the engine (and our engine is basically a half-ton of iron with a few moving bits inside) would continue to warm the water up for the hour or so that it stayed hot after being switched off, or whether the water needed to circulate (as I assume it does in a water-cooled engine while it's working) to be heated up. It sounds as if it needs to circulate, then.

Edited by Athy
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22 minutes ago, Athy said:

Thanks, WE and MJG. I wondered if the heat from the engine (and our engine is basically a half-ton of iron with a few moving bits inside) would continue to warm the water up for the hour or so that it stayed hot after being switched off, or whether the water needed to circulate (as I assume it does in a water-cooled engine while it's working) to be heated up. It sounds as if it needs to circulate, then.

If there is no valve (NRV or a handraulic version like mine) then the Laws of Thermodynamics say that the hotter body will warm the cooler body (by convection) until the temperatures are the same, then the whole lot will cool down together.  

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Unless the engine to/from calorifier pipe work routing is such that it inhibits the thermo-syphon flow but the you may be getting into easy air lock territory. Sometims reversing the engine connections on the calorifier minimises such circulation.

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55 minutes ago, Machpoint005 said:

If there is no valve (NRV or a handraulic version like mine) then the Laws of Thermodynamics say that the hotter body will warm the cooler body (by convection) until the temperatures are the same, then the whole lot will cool down together.  

But generally speaking, if the hot body (calorifier) is higher than the cooler body (engine) it should not circulate.  My boat has a horizontal calorifier on the starboard swim and doesn't lose any heat to the engine (except a very minor bit by conduction along the feed pipes). I did buy a flap valve to put in the circuit but wasn't needed.

 

Here's an idea to get to know how much hot water you have:   Get a digital min/max thermomenter with a remote probe (used to get the external temperature).  Now about halfway up the side of the calorifer, carefully remove a square of insulation foam (don't use a sharp knife) and put the external probe in the hole next to the calorifer skin. Glue or tape the bit of foam back on top.

 

It will vary from setup to setup, but for instance on mine if the "external" temperature reads 48 degrees or higher then there is plenty of hot water for two navy showers.  In the summer it will often show this in the morning after cruising the day before even after washing up the night before.  It saves you heating more hot water than you need.  As the probe is halfway up the tank, the water at the top is actually hotter than 48 degrees; you just need to get familiar with your own system.

 

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1 hour ago, Machpoint005 said:

If there is no valve (NRV or a handraulic version like mine) then the Laws of Thermodynamics say that the hotter body will warm the cooler body (by convection) until the temperatures are the same, then the whole lot will cool down together.  

Excellent - a scientific explanation that I understand! Have a greeno.

So, the engine does NOT need to be still running to continue heating the water. From memory, its operating temperature is about 55 degrees Centigrade, so id the water in the calorifier is at a lower temperature, the engine, even though idle, will continue to raise the temperature of the water until it [the engine] cools down. Then, because the calorifier tank is insulated, the water will remain warmer for longer than the engine does.

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8 minutes ago, Athy said:

Excellent - a scientific explanation that I understand! Have a greeno.

So, the engine does NOT need to be still running to continue heating the water. From memory, its operating temperature is about 55 degrees Centigrade, so id the water in the calorifier is at a lower temperature, the engine, even though idle, will continue to raise the temperature of the water until it [the engine] cools down. Then, because the calorifier tank is insulated, the water will remain warmer for longer than the engine does.

Only if the pipe routing, valves and pipe size allow for thermo-syphoning/ gravity circulation. If it wont then no exchaneg of heat can take place.

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1 hour ago, Tony Brooks said:

Only if the pipe routing, valves and pipe size allow for thermo-syphoning/ gravity circulation. If it wont then no exchaneg of heat can take place.

Or to put it another way, the phenomenon can be prevented by pipework system design. The First Law still applies though!

1 hour ago, Athy said:

Excellent - a scientific explanation that I understand! Have a greeno.

Ahem -- that's because it's an engineering explanation, but thanks anyway!

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52 minutes ago, Machpoint005 said:

 

Ahem -- that's because it's an engineering explanation, but thanks anyway!

Engineering is science and, as such, something which I generally don't understand.

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4 hours ago, Machpoint005 said:

If there is no valve (NRV or a handraulic version like mine) then the Laws of Thermodynamics say that the hotter body will warm the cooler body (by convection) until the temperatures are the same, then the whole lot will cool down together.  

Cue Flanders and Swan:

 

'"Heat can't pass from a cooler to a hotter.

You can try it if you like but you'd far better notta,

'Cause the hotter in the cooler gets acooler as aruler,

And that's a physical law"

 

ETA I've sometimes wondered if getting a classfull of Year 10s dancing around the lab, singing this might be an, ahem, different way to teach Physics.

 

Edited by Victor Vectis
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