Negative head pump - no hot water

Pepiz

So about a year ago I renovated the bathroom and got rid of the electric shower by installing a negative head pump next to the cylinder (feeding only the shower). The outlet from the cold tank is just above the cylinder approximately 1.2 m above floor level so that's why it needed a negative head pump. The cylinder has two side immersion elements (no boiler) locations but only the lower one is installed.

Now a little bit on the renovation.. The contractor I contacted was a cowboy. So the installation was done as if a positive head pump was going to be fitted with 1/2" pipes from the cylinder/cold feet and to the shower. The pump only feeds the shower. The guy installed a positive head pump, turned it on, no water coming out unless you dropped the shower head. After many discussions and some research on pumps he replaced it with a Grundfos 3 bar negative head pump from DPL at his own cost. Grand.

6 months later, no hot water comes out of the shower while there is hot water in the cylinder. After closing the valve in the shower mixer the pump stays running for some time (5-10 min max). This used to happen maybe 1/6th of the time but it got to the point that it happened almost every time.

During this time I also had the high level cut off thermostat in the element go off requiring me to hit the reset switch. This was a new element installed when I did the renovations.

First one plumber came out and said Grundfos wouldn't stand over the pump because of the installation to 1/2" pipes and that I needed to swap the pipes (tiles would need to be removed for this) and see if the pump would work.

A month or so later another plumber came and said that it would be sufficient to replace the pipes feeding the pump only but that he couldn't guarantee that the pump would work, he discussed the problem with Grundfos techs and they indicated it was likely that PCB was gone but again couldn't tell me for sure that would fix it. Not knowing if the pump was damaged I got it replaced by a reconditioned all new parts Stuart Turner 1.5 bar negative head pump.

During testing while the plumber was still there he realized the thermostat on the element was faulty (allowing current through even at the lowest temperature with a tank full of hot water, water too hot according to him). This was replaced. And everything appeared to be solved.

A week later no hot water in the cylinder, the high level cut off needed resetting again but the thermostat seems to work so unsure as to why it tripped. I reset it and the cylinder heats up again.

The next morning I turn on the shower and, similar to my initial problem, no hot water comes out. Water comes out when I lower the temperature but its all cold, its coming at high pressure from the pump that immediately turns on as it used to just doesn't pump hot water. As before, the pump runs after the valve is closed for some time. And as before it works again once it shuts off.

I have no idea what is causing the hot water not to flow into the pump or the pump or no to pump it when turns on. I also have no idea if the water is in fact getting too hot or the high level cut off is faulty.

In the time between the initial problem and pump replacement I had to do some magic to try to get it to work when I needed it and found that switching off the pump at the isolation switch for a while worked sometimes and also that opening a tap in the sink or somewhere helped shut down the pump (not immediately but did help I think) when it stayed running after it was closed. When the hot water IS pumped and I turn the valve off the pump shuts off immediately as it should.

These aren't fixes it's just information that might help figure out WTH is happening.

This is driving me insane.

whizbang

Is there any kind of a leak upstream of the pump?
Where does the feed to the cold side of the pump come from? - is there a separate tank connection for the hot/cold?
How big is your tank. - Is it running dry?

It sounds to me like the pump is pulling the cold water out of the bottom of the cylinder, maybe air in the cylinder is causing the overheat trip.
Is the flow sensor on the hot side of the pump?

K.Flyer

When you say No Hot Water, do you mean No Water at all, or Water but its Cold.

Pepiz

whizbang wrote: »

Is there any kind of a leak upstream of the pump?
Where does the feed to the cold side of the pump come from? - is there a separate tank connection for the hot/cold?
How big is your tank. - Is it running dry?

It sounds to me like the pump is pulling the cold water out of the bottom of the cylinder, maybe air in the cylinder is causing the overheat trip.
Is the flow sensor on the hot side of the pump?

No leaks upstream that I am aware of, the shower is very close to the pump and it would've been noticeable IMO.

Cold feed comes from water tank directly above. I can get pictures later.

Tank is roughly.. 900 mm x 450 mm. I will measure later. When the fault occurs I sometimes go check the pump and the tank is always hot with water in it.

Flow sensor I am not sure where they are fitted. The pump has a bell type thing that I believe senses the loss in pressure in the pipe and turns on the pump, again the pump IS turning on, just not pumping hot water.

K.Flyer wrote: »

When you say No Hot Water, do you mean No Water at all, or Water but its Cold.

No water when my mixer is set to hot temperatures, if I reduce the temperature cold water will start coming out. So both I guess.

K.Flyer

Pepiz wrote: »

[...]
No water when my mixer is set to hot temperatures, if I reduce the temperature cold water will start coming out. So both I guess.

It could be that there is a blockage in the shower valve on the hot inlet, or a blockage / airlock in the pipe feeding to the pump.
If the flow sensor on the pump was faulty the pump should still activate on the pressure switch.
You mentioned that the hot water thermostat was giving trouble. If its over heating the water and you have a thermostatic shower valve, the thermostatic part of the shower could be shutting off the flow.
Either way 1/2" pipework supplying the pump is not doing you any favours either.

whizbang

Tank is roughly.. 900 mm x 450 mm. I will measure later. When the fault occurs I sometimes go check the pump and the tank is always hot with water in it.

I presume this is the cold water storage tank, not the hot water cylinder ..?

Pepiz

K.Flyer wrote: »

It could be that there is a blockage in the shower valve on the hot inlet, or a blockage / airlock in the pipe feeding to the pump.
If the flow sensor on the pump was faulty the pump should still activate on the pressure switch.
You mentioned that the hot water thermostat was giving trouble. If its over heating the water and you have a thermostatic shower valve, the thermostatic part of the shower could be shutting off the flow.
Either way 1/2" pipework supplying the pump is not doing you any favours either.

With a blockage in the shower valve would the hot water work half the time? The pipes feeding the pump were replaced when the pump was replaced and they are 3/4".

The high level cut out thermostat keeps tripping, what could be causing this? The thermostat was replaced and is working (no power on the element side when the temperature setting is lowered).

whizbang wrote: »

I presume this is the cold water storage tank, not the hot water cylinder ..?

Apologies. I meant hot water cylinder.

whizbang

Do you ever get air coming out with the water from the taps ?

With a blockage in the shower valve would the hot water work half the time?

Yes.

high level cut out thermostat

- this is part of the same immersion element that was replaced ?

Pepiz

whizbang wrote: »

Do you ever get air coming out with the water from the taps ?

Never from the taps, from the shower after working when it wasn't working previously and not until it doesn't work again.


- this is part of the same immersion element that was replaced ?

Yes it is, this was replaced over a year ago at this stage.

UPDATE:

The current situation is:

- the high level cut out trips (I get a cylinder full of hot water).
- This causes no hot water to come out of the shower (unless I open the hot tap in the sink, which seems to get water flowing and the hot water comes out after some air being purged).
- Hot water comes out of the shower just fine until I run out of it in the cylinder, I then usually reset the high level cut off, this trips once the tank is full and the whole process starts again.

It seems from this that there is something preventing the regular thermostat from reading the correct temperature inside the tank and stopping heating the water before the temperature is too high, together with that water that is too hot or whatever happens at that pressure/temperature makes hot water not flow to the pump.

Could the sleeve into which the thermostat slides in be covered in scale inside the tank causing it not to sense the water temperature correctly? then the water overheats, trips the high level cut off and this somehow makes it difficult for the pump to pump that very hot water or it causes air to come into that section somehow?

Next I will let the element heat the water some amount and lower the temperature setting in the thermostat to a very low value and check if it is stopping the electricity supply to the element.

Is this thermostat sleeve being covered in something a common fault? This is again complete guesswork from what seems to be happening.. I am open to ideas.

K.Flyer

Is there any chance of posting a few pictures of the pump and how it is plumbed to the cylinder.

If the water is cold in the cylinder, does it flow freely through the pump and out through the hot side of the shower valve without interruption?

What pipework is connecting the pump to the cylinder, plaatic or copper?

Where does the hot pipe from the pump connect to the cylinder.