central heating

[steven70]

Hi..Is the HW cylinder normally connected to the boiler (via motorised valve/isolating valve/pump/isolating valve) with a non-return valve at the cylinder, ie flow can only enter cylinder at this juncture and not flow back towards boiler? My system is 15yr old, pumped CH, gravity DHW, Baxi back boiler..self-contained ground floor flat. I ask because I dont know if this pipe is blocked at the cylinder inlet (outlet??) or if it is as described above.

regards..steve

 

[Diyisfun]

The water flows from the boiler to the hot water tank via a heat exchanger & then back to the boiler. Thats how the hot water gets hot.
There is a valve (motorisd) situated in that loop that supplies the heating, depending on how the valve is wired.
Im not a plumber/heating eng, but I think thats what you are asking

http://www.diydoctor.org.uk/projects/domestic_hot_water_systems.htm

 

[D_Hailsham]

Hi..Is the HW cylinder normally connected to the boiler (via motorised valve/isolating valve/pump/isolating valve) with a non-return valve at the cylinder, ie flow can only enter cylinder at this juncture and not flow back towards boiler? My system is 15yr old, pumped CH, gravity DHW, Baxi back boiler..self-contained ground floor flat. I ask because I dont know if this pipe is blocked at the cylinder inlet (outlet??) or if it is as described above.

Does your motorized valve look like this?

If so you have a fully pumped system; not pumped CH, gravity HW.

The reason so many are confused is that the only reason hot water comes out of the tap is the difference in level between the cold water tank, i.e. gravity. The plumber/heating engineer uses the term "gravity HW" to mean the way the water is circulated from the boiler to the heating coil in the cylinder. In pre-pump days, circulation was due solely to the fact that hot water is less dense than cold, so it rises up the pipe from boiler to cylinder, where the heat is extracted. The density then increases, so the water drops down the return pipe back to the boiler.

There are four connections on a normal indirect HW cylinder:

1. On the very top: this is the Hot water out to the taps etc;

2. At the very bottom (often hidden at the back!): this is the cold water feed from the tank in the loft;

3. About halfway up: this is the feed, via pump and MV, to the coil in the cylinder

4. About 30cm below no 3: this is the return from the cylinder to the boiler

The fact that you have a non return valve suggest that the system has been converted from a gravity HW to a fully pumped. It was put in to prevent reverse circulation.

 

[steven70]

many thanks for replies. Am I correct in saying that the hot water flows from my hw cylinder to the taps because of the approx. 4-5ft difference in their respective levels? (I have already said I live in a ground floor flat) the cold water F and E tank/cistern is only about another 2ft above the cylinder which is fed pumped hot water from the Baxi back boiler which heats the water using gas as the energy source and NOT a coil in the cylinder. I thought all Baxi's worked this way, but as you can see I've only ever made this one post and I'm willing to learn. I thought that by keeping my question short I could get a succinct reply, but I'm already into explanations and wishing I wasn't such an optimist! (better being a pessimist...then you never get let down) Please 'D_Hailsham', explain to me what the boiler(4) is for, if there is a coil in the cylinder(3). Am I missing something here? Also why would the boiler pump hw via pump and MV to a coil in the cylinder? Am I mixing up an immersion heater coil with a coil inside the cylinder that holds water. I'm a 'newbie' to all this and confused! Help!

 

[Goldspoon]

A coil is very simple - hot water travels from your boiler to your radiators but on the way to the radiators the pipe travels through your cylinder and out again (pipe is shaped like a coil whilst it passes through cylinder). Thus the pipe heats up when the rads are on and the extra effect is that water in the cylinder is heated up and can be used for washing etc.

Water in the central heating pipework and water in the cylinder never come into contact with each other (one is dirty and one is clean).

An immersion heater is a small heating "stick" stuck through the side of the cylinder that gets hot when switched on and also heats up the water in the cylinder if and when required.

Try searching about on web for central heating
pic here of just one type of system
http://www.diydoctor.org.uk/project_images/boilers/Conven_s.gif

 

[steven70]

grateful thanks for prompt reply Goldspoon. That clears up bit of confusion in my effort to get a clear picture of my hw/ch system ie the 'coil' in the hw cylinder is actually a coil shaped pipe which holds pumped hw from the (in my case) gas fired Baxi back boiler? However am I right in thinking that because of the motorised valve I can have 3 different options ie 1) HW 2)CH 3)HW and CH ? and this means that the hot water for the radiators does NOT pass through the HW cylinder if I only want the CH on?

If this is so would it be safe to use mains water to flush thru from the HW cylinder side of the MV if I kept the pressure down low as I've noticed the pipes/water is very dirty. Also would it be OK to do the same for the cold water feed or is this a no-no?!!

I was tempted to do the above yesterday but thought all the muck would would contaminate the cylinder!!but maybe I would cause problems elsewhere if I proceed as above

 

[Goldspoon]

ie the 'coil' in the hw cylinder is actually a coil shaped pipe which holds pumped hw from the (in my case) gas fired Baxi back boiler?

Correct.

 

[Goldspoon]

Okay I'm not an expert here... learning at the moment so if any experts want to jump in!. I believe though that there are two types of 3 port valve:

a) Diverter - can be HW or CH but not both
b) Mid position - can be HW or CH or BOTH

However am I right in thinking that because of the motorised valve I can have 3 different options ie 1) HW 2)CH 3)HW and CH ? and this means that the hot water for the radiators does NOT pass through the HW cylinder if I only want the CH on?

 

[Goldspoon]

My first post is a little confusing, sorry.

Basically dirty stinking 'orrible water not suitable to wash in or drink travels from the boiler and meets the T shaped valve. If only CH is on it heads through rads and back to boiler for reheating (does not go through coil in cylinder). If only HW is on it heads through coil in cylinder (coil gets hot and heats up nice clean water in cylinder itself) and back to boiler for reheating (does not go through rads). If valve is mid position then water can happily turn right and left at valve and go round both circuits at the same time and back to boiler. So you are correct in that if CH only is on then water does not travel through coil/cylinder.

and this means that the hot water for the radiators does NOT pass through the HW cylinder if I only want the CH on?

 

[D_Hailsham]

Okay I'm not an expert here... learning at the moment so if any experts want to jump in!. I believe though that there are two types of 3 port valve:

a) Diverter - can be HW or CH but not both
b) Mid position - can be HW or CH or BOTH

Correct, but that could be confusing the issue for the OP

The Diverter is used in the W Plan, the Mid position in the Y plan