Hot water from boiler wrong direction through Hot Water Cyl

[AF]

Been in our house 6 months.

Boiler always cycles on then off a minute later when heating hot water cylinder, but not when heating radiators

Discovered recently that hot water from boiler enters Hot Water Cylinder at the bottom and leaves through the normal inlet pipe. It has probably been this way for several years. The thermostat at the base of the cylinder appears to be inop and has no effect on the boiler.

What effect is the reverse flow having on my hot water heating?
Is it causing the excessive boiler cycling?
Should I get the reverse flow rectified?

 

[ArtfulBodger]

It is considered better practise to have the flow on the bottom of the cylinder and the return above. Doubt if this is causing your problem, unless someone has fitted the pump back to front!!

 

[croydoncorgi]

It is considered better practise to have the flow on the bottom of the cylinder and the return above

Not if you want a gravity hot water system to work properly!!!

OK - on fully-pumped it WILL work either way round but I never actually read a recommendation to put the flow at the bottom.

 

[doitall]

Used to doitall the time CC, in fact mine at home is that way.

Gravity works that way as well provided the cylinder is above the boiler its a piece of cake.

 

[ollski]

The cycling is probably down to the cylinder stat....replace it and see is the easiest way to do it.

 

[Stoday]

Gravity works that way as well provided the cylinder is above the boiler its a piece of cake.

Not properly it won't. It will only heat the top part of the cylinder, not the full capacity of the HWC.

For those who were asleep in their physics lessons, hot water rises above cold water (above 4 deg C).

The thermostat won't work unless it's above the cold return to the boiler.

 

[doitall]

Gravity works that way as well provided the cylinder is above the boiler its a piece of cake.

Not properly it won't. It will only heat the top part of the cylinder, not the full capacity of the HWC.

For those who were asleep in their physics lessons, hot water rises above cold water (above 4 deg C).

The thermostat won't work unless it's above the cold return to the boiler.

Rubbish

 

[croydoncorgi]

The 'thermosyphon' principle of gravity hot water depends on the hot water in the primary flow being lighter than the colder water in the return. So there is a continuous flow upwards from boiler to cylinder, where the primary water is cooled as it flows into the return pipe, and then flows back down to the boiler due to the effect of gravity.

BUT with the Flow entering through the bottom coil connection and the hot water cylinder completely cold, the primary water may be cooled BEFORE it reaches the top connection. It then tries to run back down the flow pipe OR you will get a thermal inversion - cold water in the pipe sitting on top of hot water (people who've been to San Francisco will know about the same thing applying to warm and cold air!). Either way, the thermosyphon will be slow starting up and therefore the water temperature will not recover as quickly as it should.

 

[doitall]

When two systems are in equilibrium, they share a certain property. This property can be measured and a definite numerical value ascribed to it. A consequence of this fact is the zeroth law of thermodynamics, which states that when each of two systems is in equilibrium with a third, the first two systems must be in equilibrium with each other. This shared property of equilibrium is the temperature.
If any such system is placed in contact with an infinite environment that is at a certain temperature, the system will eventually come into equilibrium with the environment—that is, reach the same temperature. (The so-called infinite environment is a mathematical abstraction called a thermal reservoir; in reality the environment need only be large relative to the system being studied.)

FIRST LAW OF THERMODYNAMICS
The first law of thermodynamics gives a precise definition of heat, another commonly used concept.
When an object is brought into contact with a relatively colder object, a process takes place that brings about an equalization of the temperatures of the two objects. To explain this phenomenon, 18th-century scientists conjectured that a substance that was present in greater quantity in the object at higher temperature flowed into the object at lower temperature. This hypothetical substance, called caloric, was thought to be a fluid capable of moving through material media. The first law of thermodynamics instead identifies caloric, or heat, as a form of energy. It can be converted into mechanical work, and can be stored, but is not a material substance. Heat, measured originally in terms of a unit called the calorie, and work or energy, measured in joules, were shown by experiment to be completely equivalent. One calorie is equivalent 4.186 joules.
The first law, then, is a law of energy conservation. It states that, because energy cannot be created or destroyed—setting aside the later ramifications of the equivalence of mass and energy (see Nuclear Energy)—the amount of heat transferred into a system plus the amount of work done on the system must equal the corresponding increase of internal energy in the system. Heat and work are mechanisms by which systems exchange energy with one another.

 

[croydoncorgi]

Do I get the tiniest impression that someone around here is attempting to abstract the urine???

 

[doitall]

Do I get the tiniest impression that someone around here is attempting to abstract the urine???

No

 

[doitall]

Hot water will always rise, and the old gravity ladder system was based on that basic principle, that the water would rise to the top of the building and return via legs into a common return.

It is well know that by introducing the flow into the bottom of a cylinder, and for that matter a radiator works the same way that more heat could be stored in the vessel, modern cylinders have the coil very close to the bottom.

Gravity circulation can be achieved from any pipe that continuosly rises, heat entering the vessel will start to loose its heat on contact with the cooler water to be replaced with yet more hot water until the equalibrium is reach, (the cylinder is the same temperature as the flow from the boiler)

 

[ChrisR]

Heat transfer is more efficient with the input at the bottom. You can prove it mathematically very easily. NB Megaflo's et al are configured that way for that reason. You also get a more even temperature distribution - less stratification.
Many applications are better served by stronger stat though (IE where the water at the top is a lot hotter). If you only ever want full-temp water and not a whole tank of half hot. In a commercial kitchen, say.

 

[felix]

There is no urine extraction. Doitall is absolutely correct, including the bit about energy being turned into mass and back again. No, I'm not making this up. You need 1.02MeV to make mass as I'm sure Doitall knows already. It happens regularly where I work - but this is getting way off the point.

Any heat exchanger works best if you take the hot stuff in at the hottest end of the stuff you're trying to heat up - or the cold stuff in at the coldest end of the stuff you're trying to cool down. The boiler water should enter the cylinder coil at the top, where the cylinder water is hottest, and leave at the bottom. As a bonus, this flow direction is also best for promoting gravity circulation.

 

[ChrisR]

Not so. Heat transfer is not proportional to temperature difference but a polynomial, so it's disproportionately better if the difference is greater. If you integrate over the length of the transfer element you get a bigger total. Which is why high recovery-rate cylinders operate that way round.