Gravity fed evacuated tube systems

pre heat swh

Hi Hamish,

As you know I deal with that type of swh, the safety valve should be on the cold inlet of the coil .

Because the coil is closed it is required in case the unit over heats ( never seen it fitted on the hot out side )20091812398564.jpg

hi tony, thanks for the quick reply. What pressure ratings are your safety valves, they are
400&600kpa here, I think this problem is pressure related, the valve releases imediately with no heat around. I'm thinking there is no pressure control valve on the supply line and the incoming pressure is
to high. What do you think, I'll have to check that out. cheers hamish

A closed system, with gravity flow, should not have any significant pressure in it unless it gets hot. And it should not be running at the water supply line pressure. There should be some sort of float valve or manual valve to only allow water in when you need to fill the system. You should not have it connected directly to your water supply.
If you are using a float valve and it is leaking, then you could possibly have the full supply pressure on your overpressure valve, but that same pressure should open the relief valve on your water heater too if that is the problem.
It is most likely that it is opening because of localized high temperature or else is just plain defective.

pressure relief valce

Hi Hamish,

I am not sure about the pressure rating of the relief valve , I do not supply them but it is exactly the same as the type that is fitted to standard electrical cylinder water heater on the cold feed , it has a small lever on the side to test it .

I recommend that the maximum water pressure that flows through the coil should be 3 bar ( if it is to high fit a pressure regulator )

Cheers Tony

Hi tony, Thanks for that, I will check for the pressure reducing valve on the supply pipe. The standard

rating here is 4 bar or 400kpa. It also looks as if the inlet and outlet connections on the geyser

have been mixed up on the coil. Would this make a difference on the coils function.

thanks hamish

Hi Hamish,

The inlet and outlet of the coil are marked but it dosnt matter which way you connect it as it is simply two open ends of a copper coil heat ex changer

I have never head any body having a problem with the safety valve constantly leaking.

4 bar is quite high I think the normal pressure here is about 3 bar

Is the water pressure higher than the valve rating ( do you need a pressure reducer )

Cheers Tony

If it is a Temperature and pressure relief valve it should always be installed on the hot side. Never the cold side and generally right into the tank.
A couple of other questions.
What is the assistant tank. It appears to be an expansion tank with a bunch of other stuff in it but I cannot tell and not familiar with the term.
Second is there a pressure reducer on the house where the water comes in? Many of these have a backflow prevention device which will not allow the water when heated to expand back to the water company ( hence the need for the expansion tank)
If the above are true and you have a pressure reducer on the incoming and the assistant tank is indeed an expansion tank then take all the pressure off the system and pump the expansion tank up as it has most likely lost its air charge, is saturated, and is no longer allowing the water to expand causing the pressure valve to relieve itself.

+1
This is one reason that some models of expansion tank incorporate an air bladder with an external fill valve (tire valve) That way the air charge can never go below a minimum level. If the air bladder fails, you will not notice it until you have lost all of the air and get overpressure events.

Hi Rich,

An assistant tank is a small tank that sits on top of non pressure and pre heat Chinese swhs

Because the main water tank is not a pressure vessel it must be vented

thes assistant tank has a ball float valve to maintain the water level in the main tank with out overflowing

the ball float is set low down in the assistant tank so water can expand into it as the water in the main tank heats up.

It has no blader as in high pressure systems expansion tanks, simply a small vented tank with a ball float valve.

Cheers Tony

So how does the hot water get distributed to the fixtures?

Hi,

The water gets distributed to the fixtures ( house hot taps ) by-

1, Non pressure swh - the swh must be mounted higher than the point of use, when a hot tap is opened in the house hot water flows directly by gravity from the swh tank, at the same time the ball valve in the assistant tank senses the drop in water level in the non pressure storage tank and fills it with cold water
( another option is to have a level and temperature sensor , fill solenoid and controller set up , instead of the assistant tank )

2. pre heat swh- this unit is not gravity feed and can be mounted lower than the point of use, when a hot tap is opened in the house mains pressure forces water through the copper coil heat ex-changer in the non pressure thermal store( main tank ) and to the tap
This system is indirect, and a combination of high and low pressure ie high pressure in the coil and low pressure water in the vented main tank ( which the assistant tank maintains full , when some water evaporates or boils out )

Cheers Tony