Introducing a two port valve to my semi gravity central heating system

[Joe Hardy]

During the severe winter when my CH is on permanently the gravity feed to my hot water tank causes the domestic hot water to become dangerously boiling hot.

I believe I need to install a cylinder stat that operates a two way valve stopping the gravity flow into the hot water tank once the temp in tank is 60 degrees C

Am I correct here - if so what is involved electrically?

The hot water tank is in the loft and my boiler / programmer is at ground floor level in the attached garage - no way of wiring from the loft to the garage

I have the required fused connection unit (power supply) in the loft for the valve motor supply - is it just a case of wiring the thermostat (switch) into the live of the supply (via a junction box) such that power is delivered to close the valve once the water has reached the required temp?

Once the water cools with usage I presume the stat (switch) will open and cut the power supply to the valve - will this cause the valve to open (possibly spring loaded) ?

 

[jackthom]

Is there not already a stat wired in from the cylinder to the boiler which could be pressed into service for this? Much easier to add a motorised valve and it's control wiring closer to the boiler.

 

[Joe Hardy]

Thanks for the reply jackthorn - Unfortunately No

I agree but it's no problem adding the valve into the pipework in the loft - fully boarded out and easily accessible pipework around the hot water cylinder- I will be draining the system to replace an undersized radiator so thought I would solve this issue at the same time - just need to know what I am doing is correct/safe and the right thing to do

 

[jackthom]

Is this a gas boiler system and how does the boiler get switched on to heat the water when the central heating isn't required, during the Summer for example.

The problem is that you can't just shut off the gravity feed while the boiler may still be firing to try and heat the DHW (unless of course the boiler only fires when the central heating is running)

 

[Joe Hardy]

The programmer allows hot water to be separately controlled from the central heating (normal operation for semi gravity systems of the 1970-1990 period) - in the summer we have CH off - HW only on a timed basis say 2 x 1 hour periods (we also have an electric emersion heater in the domestic water tank so we have the option of gas or electric domestic water heating)

Due to the type of system semi gravitational (pumped central heating - gravity hot water) when the CH is on hot domestic water is an automatic by product - when it is on for long periods the domestic hot water is near boiling point and I have to drain it off for safety reason to avoid scalding - massive waste of energy

 

[jackthom]

At times when you only require hot water there will have be some sort of feedback from a thermostat on the cylinder to tell the boiler to run or stop running and to open/shut-off the new motorised valve.

The 2 port valve would get it's motor power switched ON by the cylinder thermostat when there is a demand for heat. Then the valve motors over to the open position and an internal microswitch in the valve switches on the demand to fire up the boiler.

One way or another you will need to run a cable from the loft down to near the boiler. Plus make sure it's at least 3 core to cover any future upgrade.

 

[Nige F]

During the severe winter when my CH is on permanently the gravity feed to my hot water tank causes the domestic hot water to become dangerously boiling hot.

I believe I need to install a cylinder stat that operates a two way valve stopping the gravity flow into the hot water tank once the temp in tank is 60 degrees C

?

In the return from the hot clynder to the boiler - The flow/vent MUST be clear from boiler to feed/expansion tank.

 

[Joe Hardy]

Nearly - when in HW only mode (and within the timed on period set in the programmer) the thermostat on the gas boiler itself starts the firing up of the boiler maintaining a set temperature in the gravity loop pipework to the hot water domestic cylinder and shuts the boiler down when the set temp is reached i.e. heats it up to a temperature set value then stops until the temp drops then starts again. This is not a problem when we are in HW mode only as we can restrict the on/off times via the programmer such that the domestic hot water never gets too hot.

when we are in CH + HW mode the room stat brings on the central heating boiler either on a timed period OR permanently when it is -4 or lower outside - the latter is when we have a problem because the domestic hot water has no time to cool down and gets hotter and hotter - under this condition (and this is the only problem condition).

I was thinking a two way valve in the gravity feed leg of the loop to the domestic hot water tank would stop this constant heating by gravity once the temp in the hot water tank had reached 60 degrees C - using a cylinder stat to open/close this valve.

 

[Joe Hardy]

Nige F

If you can imagine that the flow pipework to my domestic water tank forms a 'T' such that the first leg of the tee is the flow from the boiler the second leg of the tee is into the hot water tank with the third leg rising above the domestic hot water tank and into the expansion tank

I intended to install this valve in the second leg such that the vent leg to the expansion tank is free of obstruction no matter if the valve is open/closed.

 

[Joe Hardy]

Sorry jackthorn - misread your last post

Fair point - yes the thermostat will allow power to the motor - I was hoping that the valve would be spring return such that once temperature dropped the power circuit would be broken and the valve would 'spring' back open - should it stay shut the boiler would heat up the gravity leg to the required temp and then the boiler via it's own stat would shut off the boiler and we would be left with no hot water - unless this valve springs back open automatically on loss of power - is such a valve available

What is puzzling me is that their must be hundreds of thousands of CH systems like mine in operation as they were the norm for 30 years - am I the only one who has this problem?

 

[jackthom]

My own house had a similar system many years ago but gravity feed systems are gradually being replaced by fully pumped ones which allow better independent control of DHW and Heating, plus faster DHW recovery with the right sort of cylinder.

You are correct that the 2-port valve will spring back to the closed position by itself when power is removed by the cylinder thermostat. Although you may feel you can keep a tight control over how often the heating is allowed to run on it's own, it is bad practice to have the potential situation where the boiler is still running and quickly cycling ON/OFF because the valve is closed and there is nowhere for the heat to go.

I dare say the older cast iron battleship boilers won't be too upset when this happens but it makes me uneasy.

 

[Joe Hardy]

I would like to have a safe acceptable solution which is why I posted in the first place - and I really appreciate your input here - what I need to understand is how the boiler internally works because in the scenario I have described CH + HW + boiling domestic water - the boiler will virtually always be on to cater the demand for CH with HW as a side show so it won't going on and off every minute.

In my boiler there must be two separate loops one pumped for CH and a second none pumped circuit (22mm dia judging by the pipework I can see) which is virtually just a column of water that is heated by gravity.

This poises the question that when I have drained the system in the past have BOTH loops been drained? and how are BOTH re filled without air locks - the CH system is easily bled and returned to normal but I have never bled the 'gravity loop'

 

[flameport]

What is puzzling me is that their must be hundreds of thousands of CH systems like mine in operation as they were the norm for 30 years - am I the only one who has this problem?

No, it's a standard arrangement.
Download the Sundial C Plan Reference Guide from here: http://www.honeywelluk.com/products/Systems/Sundial-C-Plan/ - link at the right side of the page.

 

[Nige F]

You can still get a Drayton Cyltrol valve which is non electric and was the usual control on gravity primaries back in the day - £100 + though.

 

[jackthom]

You don't need to worry about the gravity loop having an airlock as it can always bleed itself up the vent to the feed and expansion tank in the loft.

TBH I think what you are planning, either with a 2 port valve or using the Drayton Cytrol, is probably safer than having uncontrolled heating of the cylinder when the CH is working overtime in winter. Just make sure whatever you choose doesn't intercept either the vent from boiler to F&E tank or the cold feed from the F&E tank to the boiler.

It should be on the Return Tee where the pipe comes out of the cylinder. Be very careful cutting etc just in case the cylinder connections are old and a bit fragile.