immersion heater hot water problem

sat56

Dont know if this makes a difference but using e7 so only heat water at night. The problem occurs every morning without fail.

The hot water outlet runs horizontally to a T junction which the downward connection runs to the taps, but the other end of the T junction carries on horrizontally then goes vertically up and is open to drop in the top of the cold tank, if that makes sense.

keystone

Thanks for that. First thing to try is to drop the temperature on your HWC. Air released when the water is heated has to go somewhere. If you are using E7 and keeping the heater(s) on all night to give you a full cylinder of hot water to last all day it might just be getting overheated thus generating more air which is then being pulled down the feed pipe to the taps because it arrives there before it gets to and can escape via, the vent pipe. Its late - hope that makes sense.

Cheers

birkee

DVardysShadow wrote: »

There is no major distinction between a 'vacuum' and a 'pressure' pump for liquids. The pump will put a pressure difference across itself - if the outlet side is an open tap downstairs by several meters, the pressure at the outlet of the pump will tend to be at a lowish pressure and the inlet side will be at an even lower pressure. The pressure across the pump for a particular flow rate will be the only given in this problem

The net effect will be that if the cold water tank does not have the head to create an adequate flow into the HW cylinder, then the pump will draw from the vent.

Impellor pumps don't create any vacuum. They require a certain water inlet pressure to function. i.e. 500mm minimum.
http://www.plumbingpages.com/featurepages/Shrpumpshome.cfm

They compress the water in the impellor, and force it to flow in a particular direction. No head of water in supply, no pumping. (Not enough water to compress. the pump has too much air in it, which it can't compress, due to the design.)

There is a difference between vacuum pumps and pressure pumps. The direction they are designed to drive liquids or air.
Look at pump designs for 'lift' (suck) and outlet pressure figures. You'll find large differences.

If someone can tell me they use peristaltic pumps, diaphragm pumps, or venturi effect pumps, in domestic situations, it would be worth knowing.
I've experimented in an industrial situation with all the above types.

keystone

birkee wrote: »

Impellor pumps don't create any vacuum..........

snip

snip

..................... with all the above types.

Interesting I'm sure but am struggling to see how it's relevant to the OPs problem which is air in the pipework. Pumping the o/p from the HWC will only exacerbate that problem.

Cheers

PS As you seem to have missed it - how does a Fortic tank work?

Canucklehead

sat56 wrote: »

Dont know if this makes a difference but using e7 so only heat water at night. The problem occurs every morning without fail.

The hot water outlet runs horizontally to a T junction which the downward connection runs to the taps, but the other end of the T junction carries on horrizontally then goes vertically up and is open to drop in the top of the cold tank, if that makes sense.

Hi,
I would prefer to see the vent rise directly from the top of the cylinder.
Any air bubbles will then go up the vent, not sit in the pipe waiting to be drawn into the service pipe.
You might still need to upsize the cold feed.

GSR.

ormus

thats why that horizontal pipe is kept as short as possible, before the vertical vent pipe to the tank is teed off.
at least, it should be!

keystone

Canucklehead wrote: »

I would prefer to see the vent rise directly from the top of the cylinder.
Any air bubbles will then go up the vent, not sit in the pipe waiting to be drawn into the service pipe.
You might still need to upsize the cold feed.

Agree entirely and that would have been steps 2 and 3 for me as a proper fix. However, browsing the Utilities Forum I found that the OP has a problem with their E7 not working properly leading to the heaters running all day if they are not careful. So I was taking it one step at a time.

Cheers

Canucklehead

Hi

That would be a thermostat problem then.
E7 isn't responsible for that.

Set stat to 60 degrees .If it overheats ( doesn't switch off) replace the stat.
GSR.

birkee

keystone wrote: »

Interesting I'm sure but am struggling to see how it's relevant to the OPs problem which is air in the pipework. Pumping the o/p from the HWC will only exacerbate that problem.

Cheers

PS As you seem to have missed it - how does a Fortic tank work?

Cold water tank sat on top of the hot water tank with a direct feed down. little head pressure compared to loft tank supply. Use where there is no space for a loft tank usually.
You are talking vented as opposed to sealed tanks aren't you?

What else d'yu want me to say?

DVardysShadow

birkee wrote: »

Impellor pumps don't create any vacuum. They require a certain water inlet pressure to function. i.e. 500mm minimum.....

Suppose the pump is pumping to a full bore open pipe, discharging to free air. The outlet pressure is atmospheric pressure, by definition. The pump flow characteristic will dictate the pressure rise across the pump in the direction of flow. As the outlet is at atmospheric pressure, the inlet will be below atmospheric pressure by the pressure rise across the pump. Therefore the inlet of the pump will be at a partial vacuum pressure.

Provided the water source can meet the flow rate, this does not turn into a void in the inlet. If a void does develop in the inlet, of course the pump will not shift water and the pressure rise across the pump will disappear.

All that is being said, with the term vacuum being loosely used, is that the inlet side of a pump can be at a partial vacuum pressure.