Problem with shower mixer pressure

[Beemer]

I am trying to adjust a Grohe shower mixer as when turning it to the optimum 38C the volume reduces significantly. The mixer is fed from a twin impeller pump with the cold being very good mains pressure and the hot from a HW cylinder (set at 65C) fed from a two foot high tank in the loft. The shower head is two feet below the loft tank. So to me all that looks fine.

When the hot feed pipes are still cold and the Grohe set to cold I switch on the shower. The pump runs and with the Grohe shower head on max wide spray the pressure is excellent. On immediately turning the Grohe temp to 38C the pressure drops substantially and this is before the hot water has reached the Grohe so its thermostat has not affected the pressure.

My question is how can this system be set up to avoid all or some of this pressure drop?

Beemer

 

[Mike Jackson]

Firstly your installation is illegal as you are not allowed to pump the cold main. You have two ways of dealing with the problem.

1. Take a cold feed from the tank in the loft to the pump, this will give balanced pressures at the shower.

2. Fit a pressure reducing valve on the cold and take it straight to the shower and fit a single impeller pump to the hot only.

Option 1 would be best as long as you haven't killed the pump by running cold mains through it.

 

[PlumbBobBob]

totally agree you cannot pump the mains cold. i would be surprised if you haven't goosed the pump.

 

[Beemer]

Firstly your installation is illegal as you are not allowed to pump the cold main. You have two ways of dealing with the problem.

1. Take a cold feed from the tank in the loft to the pump, this will give balanced pressures at the shower.

2. Fit a pressure reducing valve on the cold and take it straight to the shower and fit a single impeller pump to the hot only.

Option 1 would be best as long as you haven't killed the pump by running cold mains through it.

Mike,

As you will have already guessed I'm not a plumber and I apologise for my ignorance in saying that the cold feed was mains pressure. This is an installation in my house that was installed by a plumbing/bathroom fitting firm about 15 years ago. I think the pressure at ideal temperature has been falling over the years. Today I checked out the design and its as follows:

In the loft there are two large plastic tanks that are coupled together to increase capacity. There are gate valves on the hot and cold tank outlets. They feed the hot water cylinder and the cold impeller of the pump. The hot impeller feed is taken not from the top of the cylinder but about 6 inches down to prevent I was told the pump drawing air from the overflow. All pipework is 22mm.

I'm only guessing that the problem of pressure drop when the Grohe mixer temperature knob is turned beyond 38 is being caused by the mixer itself. I don't know how these mixer valves work. I would have thought that setting the valve to a higher temperature would just allow more hot flow but is it actually reducing the cold instead? As I mentioned in my original post the pressure drop occurs even when the hot water has not yet reached the valve so its not the internal thermostat causing it. The drop occurs even if the shower head is removed so I thought that this makes it less likely that the impeller seals have worn.

Beemer

 

[Mike Jackson]

At 15 years old it could be various things. Worn pump, scaled up pump, scaled up shower valve, blocked filters. You need to work back from the shower checking what flow you are getting at various points in the system. My first point of call would be to isolate the hot and cold and take the shower valve off to check ant inlet filters and check that you are getting a free flow of water to it.

 

[Paulus]

:iagree: with Mike. Take out the valve first and check the filters. After 15 years you are likely to find they are gunged up. If not I would try the filters in the pump inlets next.

 

[Beemer]

Mike/Paulus,

Thanks for your advice. I will check the filters out.

Beemer

 

[Beemer]

Mike/Paulus,

Success! I first removed the Grohe pump filters and there was a significant amount of gunge in the hot filter. Next I removed the mixer and found the the compression rubber tube on the hot side had deteriorated. The mixer hot filter was reasonably clean. With the mixer off I started the pump and an amazing amount of brown algae and rubber bits came flying out the hot side. After this the pressure on both sides was equal and hammering against the far cubicle wall. I reversed the bad rubber compression tube and refitted the mixer. Taa ra! Same virtually the same high spray pressure regardless of temperature.

Out of curiosity can either of you tell me what the 6mm plugged hole is for? It is a vertical tube hole completely through the middle of the Grohe mixer plugged with a tube that has a cross shape throughout its length. When I look down the hole I cannot see any side opening that might be to accept waterproof grease if that is what the hole is for?

My wife has just had a kidney removed so she will be pleased to get more comfort from the "as new" shower.

Many thanks for you advice.

Beemer

 

[Paulus]

Hi Beemer,

Thanks for the feedback. It's always a good thing to keep the Mrs happy.

Sorry I can't help with the purpose of the plugged hole without seeing it. I'm pretty sure it wont be for grease though.