Warm Water From Cold Taps

[endaxi]

Hello All,

Late Edit... In case this is your first look at this thread it deals with hot water flowing from cold bathroom taps for in-excess of 30 seconds until they run cold again and is specific to an un-vented cylinder, in this case a 15 year old Megaflo CL210 that was recently partially drained. Never had such a problem with this previously. For the most up-to-date appraisal of the problem please see post #63 on page 5 of this thread. Otherwise, if armed with a large mug of coffee, a plate of biscuits, a comfy chair and an interest in this problem, then please read-on...

Perhaps you can help with some ideas as to whether I have a plumbing or a heating problem. I have a plumbing and drainage contract with one company and a boiler and CH/HW contract with another.

Up until a couple of days ago all was normal with my hot and cold water but when the Hot Water is set to ON I now have very warm water running from the upstairs cold taps for about 30 seconds before they run cold. Leave for an hour and the same happens again. I can get a basin full of water warm enough to wash in from the cold tap. We normally have the HW set to ALL DAY.

Here are some clues as to what may have gone wrong....

We had a brand new mixer tap fitted in the kitchen a few days ago by the plumbing people. Since then the water pressure from the kitchen tap seems higher. I have regulated this a bit by using the in-line isolator valves just before the flexi pipes to the tap. I also notice that when I shut the tap off the pipes rattle a bit sometimes. This also happens every time the downstairs toilet cistern finishes filling and the valve shuts off. Although this has always been the case since we had a new valve fitted a few years ago, it just seems to be a bit worse now.

I contacted the plumber who says he only changed the tap so could not have changed the water pressure. Fair enough.

I notice that NRVs are fitted at the inlets to the mixer tap so probably no problem there.

When he fitted the tap he had to cut down the copper pipes to fit the new flexi pipes which were longer than the old ones. He turned off the cold water mains under the sink and asked me to run the hot taps in the utility room and downstairs toilet to relieve the water pressure whilst he worked.

After the hot water had been running for a few minutes I got concerned and switched the Hot Water to OFF on the timer switch. It seemed to me that the hot water tank would be draining down but not getting replenished with cold water. Anyway, he completed the job and left.

A day or so later we noticed the warm water running from the cold taps.

We have a year old Worcester Bosch Greenstar 24Ri 'Heat Only' boiler in the utility room and a pump in the airing cupboard. I noticed the pump was running even though the boiler was not heating. I know there's a few minutes over-run but on investigation I found that with the HW set to ALL DAY the pump was running continuously whether the boiler was firing or not. When I turn the boiler stat down low the pump just keeps running. The only way I can get the pump to stop is by switching the HW to OFF at the timer. The pump then over-runs for a few minutes longer and stops.

If I switch the CH to ON with the HW to OFF the pump behaves normally. ie if I turn the wall stat up the pump starts and the boiler fires. When I turn the wall stat down the boiler shuts off and the pump over-runs for a few minutes then stops.

So, it seems I have two issues...

First, the pump in the airing cupboard runs all day whether there is a demand for the boiler to heat the hot water or not. I have now changed the timer settings so the HW only comes on for an hour or two a few times a day. I'm fairly certain that previously the pump would stop after the boiler shut off, even if the timer was still set to ON.

Second, I feel that the water pressure is a bit higher since the new mixer tap was fitted although I can't see why this would be.

Having spoken to the plumber who said it's nothing he did, I have now arranged for the heating people to come next week to investigate.

I'm trying to get an idea as to what the most likely thing is to be causing the problem as I have visions of the plumbing people and the boiler people both saying, ''Its not our problem guv''.

Any help or ideas would be appreciated.

Edit.... it may be worth mentioning that it is a sealed system so no tanks in the loft. Also, my feeling that the water pressure is higher maybe just my imagination so could be a bit of a red herring. The plumber draining down the hot water for so long may be more of an issue than my perception of the water pressure.

Many thanks.

Endaxi

 

[HERTS P&D]

Faulty 2/3 port valve and incoming stopcock opened to full flow.

Andy

 

[endaxi]

Faulty 2/3 port valve and incoming stopcock opened to full flow.

Andy

Thanks Andy. The stopcock was certainly opened practically all the way so I had already closed it down a couple of turns and it made no difference. Not sure how much it should actually be opened. As for the valve....I presume that's the Honeywell valve in the airing cupboard you're referring to. I'll mention that to the heating engineer if he's out of ideas.

 

[endaxi]

I've done some experimenting...

When I switch the Hot Water to ON the boiler fires, the Two Port Honeywell Valve to the cylinder coil opens and the pump starts running. Seems OK so far. When the hot water demand is met the boiler switches off but the Honeywell stays open and the pump keeps running. That means when I had the Hot Water switched to 'on all day' the water was being pumped constantly through the cylinder coil whether it was being heated by the boiler or not. It's only when I switch the hot water to OFF at the timer that the Honeywell closes and the pump stops running (after a few minutes over-run).

I would have thought that the Honeywell should close as soon as the demand is met and only open when the boiler fires again.

Would this problem cause me to get warm water from the cold taps or have I got two different faults?

Any advice welcome.

Thanks.

 

[endaxi]

I've done some more experimenting and am now a bit worried so hopefully someone can shed some light on this.

I have an un-vented cylinder tank and when the water in the tank is hot and I turn on the cold tap in the bathroom hot water comes out of the tank through the cold water inlet pipe for a while and then the pipe goes cold again (as it should be). It seems that the pressure in the tank is higher than the cold water inlet pressure so forces its way out of the tank until the pressure drops.

The plumber who fitted the new mixer tap in the kitchen last week switched off the cold water at the mains inlet and turned on two hot taps downstairs to relieve the pressure whilst he cut into the pipes. The hot water ran continuously from the two other taps whilst he cut into the hot pipe feeding the mixer tap.

Could air have got into the hot water tank and is now expanding when the water gets hot and so creates excess pressure in there?

Any help most welcome.

 

[oilboffin]

I'd get someone to look at that cos you could be in eager of legionares disease breeding in the pipes cos when cold water is warmed above 25c the bugs can breed and don't forget it still kills 8 out of 10 people who catch it.Bob

 

[endecotp]

legionares disease ... still kills 8 out of 10 people who catch it.

More like 1 in 10:

https://www.cdc.gov/legionella/fastfacts.html

Or 4 out of 92 in the last serious UK outbreak, in Edinburgh in 2012. See also:

https://en.m.wikipedia.org/wiki/List_of_Legionnaires'_disease_outbreaks

 

[dilalio]

You do need to get a g3 engineer in as you have mentioned you have a UV. There could be one or two of several issues occurring here and they need professional appraisal.

The safety devices on the UV should do their job if the heat and pressure increases before the engineer attends. You could manually operate the PRV on the cylinder Temp&pressure relief and on the inlet control group to check that the discharge pipes can safely convey any excess to outside the property (you can trace these to an outside discharge point hopefully).
You could also isolate the new kitchen mixer tap completely by shutting off the isolators and see if symptoms persist elsewhere.
If they do then you could be getting crossover at another mixer (ie shower valve).
If you don't, then crossover could be occurring at the new mixer - manufacturing faults are possible, especially if its a cheap unknown valve brand.
One or both of the expansion vessels may also need recharging but I would expect you to see let by at the relief valves if this were the case.
Continual pump running could be a fault with the programmer, thermostat or wiring, or a combination.

For now, isolate the kitchen mixer and see what happens.

 

[endaxi]

I'd get someone to look at that cos you could be in eager of legionares disease breeding in the pipes cos when cold water is warmed above 25c the bugs can breed and don't forget it still kills 8 out of 10 people who catch it.Bob

More like 1 in 10:

https://www.cdc.gov/legionella/fastfacts.html

Or 4 out of 92 in the last serious UK outbreak, in Edinburgh in 2012. See also:

https://en.m.wikipedia.org/wiki/List_of_Legionnaires'_disease_outbreaks

I thought this would only be a problem with a vented system that has a tank in the loft. I managed to find some Megaflo FAQs on line that says that ''if the cold water is warm for a short period of time before returning to cold then it is more of an inconvenience than a major problem''. I suppose it depends what a 'short period of time' is.

Thanks for the replies.

 

[endaxi]

You do need to get a g3 engineer in as you have mentioned you have a UV. There could be one or two of several issues occurring here and they need professional appraisal.

The safety devices on the UV should do their job if the heat and pressure increases before the engineer attends. You could manually operate the PRV on the cylinder Temp&pressure relief and on the inlet control group to check that the discharge pipes can safely convey any excess to outside the property (you can trace these to an outside discharge point hopefully).
You could also isolate the new kitchen mixer tap completely by shutting off the isolators and see if symptoms persist elsewhere.
If they do then you could be getting crossover at another mixer (ie shower valve).
If you don't, then crossover could be occurring at the new mixer - manufacturing faults are possible, especially if its a cheap unknown valve brand.
One or both of the expansion vessels may also need recharging but I would expect you to see let by at the relief valves if this were the case.
Continual pump running could be a fault with the programmer, thermostat or wiring, or a combination.

For now, isolate the kitchen mixer and see what happens.

Hi dilalio,

Thanks for the reply.

There's no water flowing through the tundish so it looks like the safety valves are not having to operate. Both the PRV in the Combination Valve and the T&P valve are connected to the tundish. That should mean the air gap/tank pressure and water temp are both ok shouldn't it? The hot water is at a reasonable temp and the boiler thermostat seems to control it ok. The new mixer tap has NRVs in the supply and I can't feel any crossover as the hot pipe gets hot and the cold pipe stays cold.

I can however feel the cold inlet pipe to the Megaflo go quickly from cold to very warm when I turn on a cold tap so it looks like the hot water is backflowing from the cylinder and exiting the cold taps. That's why I'm wondering if another PRV/NRV at the inlet to the cylinder would cure the problem. Of course, that doesn't tell me what the problem is or why it occurred in the first place.

BG are coming tomorrow and the original plumber on Wednesday so I can put all your thoughts to them. If they can't solve it then I'll have to get a Megaflo G3 person in.

Thanks again and any more thoughts are very welcome.

 

[MeldrewsMate]

The new mixer tap has NRVs in the supply and I can't feel any crossover as the hot pipe gets hot and the cold pipe stays cold.

I can however feel the cold inlet pipe to the Megaflo go quickly from cold to very warm when I turn on a cold tap so it looks like the hot water is backflowing from the cylinder and exiting the cold taps. That's why I'm wondering if another PRV/NRV at the inlet to the cylinder would cure the problem

Have the hot water cylinder PRV checked and cleaned. It is probable (though not stated) that the cold water supply to the outlets is "balanced", i.e. taken fron the pressure limited outlet port on the cylinder's inlet PRV (pressure reducing valve, not to be confused with pressure relief valve). If the non-return valve (NRV) has failed expanded hot water, already at a higher pressure than the cold output of the PRV, will flow backwards from the base of the cylinder and into the balanced cold supply. Once the excess pressure from the expanded hot water has been exhausted (and 20 seconds seems reasonable) further supplies of cold will flow from the cold mains to the balanced output. This seems to fully explain your symptoms.

As for the pump overrunning. Is the hot water 2-port HONEYWELL valve oriented in a vertical pipe? There's a possibility that there's a small leak causing the cardboard (!) insulator around the microswitch to partially short circuit it, and supply reduced potential (voltage) to the boiler. I got the impression you used to have the DHW on permanently until recently, so this might explain the reason you appeared worried that you'd run out of hot water when the plumber was changing the kitchen tap. Your reference to 'air gap' lead me to believe you have a Heatrae Sadia Megaflow. It also MAY be the case that your boiler temperature stat has been set too low (and you have since lowered it further) Please set it to about 70 C before checking that the same symptoms exist.
BTW, your DHW valve should spring return smartly to the closed position (compare it to the heating valve) when DHW demand is satisfied, or off.

MM

 

[endaxi]

It's worth reiterating that there's an old type Heatrae Sadia 95 605 869 valve set that I think has a NRV incorporated in the airing cupboard in the cold mains supply before the pipe splits to go to the Megaflo inlet and the bathroom taps so we never get hot water coming from the kitchen tap... only from the taps in the main bathroom, en-suite and downstairs toilet.

Seems that the cylinder pressure exceeds the cold water inlet pressure from the combination valve for a while when a bathroom tap is turned on. Maybe just not enough to cause a safety valve to open.

 

[endaxi]

Have the hot water cylinder PRV checked and cleaned. It is probable (though not stated) that the cold water supply to the outlets is "balanced", i.e. taken fron the pressure limited outlet port on the cylinder's inlet PRV (pressure reducing valve, not to be confused with pressure relief valve). If the non-return valve (NRV) has failed expanded hot water, already at a higher pressure than the cold output of the PRV, will flow backwards from the base of the cylinder and into the balanced cold supply. Once the excess pressure from the expanded hot water has been exhausted (and 20 seconds seems reasonable) further supplies of cold will flow from the cold mains to the balanced output. This seems to fully explain your symptoms.

As for the pump overrunning. Is the hot water 2-port HONEYWELL valve oriented in a vertical pipe? There's a possibility that there's a small leak causing the cardboard (!) insulator around the microswitch to partially short circuit it, and supply reduced potential (voltage) to the boiler. I got the impression you used to have the DHW on permanently until recently, so this might explain the reason you appeared worried that you'd run out of hot water when the plumber was changing the kitchen tap. Your reference to 'air gap' lead me to believe you have a Heatrae Sadia Megaflow. It also MAY be the case that your boiler temperature stat has been set too low (and you have since lowered it further) Please set it to about 70 C before checking that the same symptoms exist.
BTW, your DHW valve should spring return smartly to the closed position (compare it to the heating valve) when DHW demand is satisfied, or off.

MM

Just popping out for a bit. I'll read this when I get home.

 

[MeldrewsMate]

I didn't need to read that, I'll let you get on with it.

 

[MeldrewsMate]

Then I found this in a seperate thread...

Hi all,

Forgive my second attempt at explaining a problem but my first thread died an early death. I'll try to be more concise this time.

I have a 15 year old Megaflo CL210 fed from a gas boiler and after a partial drainage of the tank to fit a new kitchen tap now have hot water backflowing from the tank out to the cold water bathroom taps for about 30 seconds before they run cold again.

The cold water mains enters the airing cupboard and into an old type Heatrae Sadia 95 605 869 combination valve. When it leaves the valve it enters a T Piece where one side goes straight to the Megaflo inlet and the other to the cold water bathroom taps, I believe.

Should there be another PRV/NRV between the Megaflo inlet and the T piece to stop this backflow to the cold taps? I never had a problem before the cylinder drainage but a NRV between the tank inlet and the cold taps would stop the backflow, right?

Educating myself on the internet and found that the Megaflow should be serviced annually by G3 qualified engineer. Always get boiler serviced but never had the Megaflow serviced. I should get that done soonest.

Finally, I believe the Megaflo has an airgap inside. My system also has an expansion tank so I'm a bit confused with that.

Would it help to recharge the air gap? Nothing flowing through the tundish.

Grateful for any advice.

Thanks.


It makes it a lot easier to remotely diagnose if you'd given the info in THIS thread. Don't try to recharge the airgap, nor recharge the (red?) expansion vessel, they're OK as the system pressure holds up at 1.7 bar (this thread) and there's no discharge from the Megaflow CL210 (previous thread)

I am not convinced there's a parallel universe, so I'm goingto assume the two threads refer to the same thing.