Hot water cylinder fitting queries

[Matthew]

Leaky HWC has prompted me to fit a new one. I've got a fairly standard setup, boiler to provide CH and HW via indirect cylinder - but the pipework in the airing cupboard is a disaster so I'm going to rip everything out and reinstall it. As such, just have a few queries that I hope some of the good folk here can answer...

• Probably going to go for a bog standard cylinder to replace existing. However, I might consider smaller rapid recovery type. If I do, does anything need to be done differently?

• Looking at cylinders on offer, most of them seem to come with 2 tappings on the side (that is, the other side than the heat exchanger). Current cylinder just has lower one for cold inlet, so what's the higher one for?

• My system at the moment has two motorised valves to divert water to cylinder or rads. Any reason not to reinstall with a single 3-way valve?

• Between the pump outlet and the motorised valves, there seems to be a bypass back to the return to the boiler. It has a gate valve fitted. Is this bypass required and if so, how should the gate valve be set?

• Return from heat exchanger also has a gate valve fitted between cylinder outlet and joining up with rest of circuit. Any special reason for this to be there?

• Where should the thermostat be positioned on the cylinder? It's about a third of the way up at the moment, is that right?

Phew, that's quite a list. Thanks in advance for any help anyone can lend...

Matthew

[John]

Check your boiler output is going to give you enough to heat it up in a reasonable time, otherwise a larger (prelagged) cylinder with an appreciable stored amount may be better for your circumstances.

Have you counted tanks in the loft? For an indirect system you should have two - a large one to supply cold water into the bottome of the cylinder for heating and supply to the hot taps, and a small one which is the feed and expansion tank for the heating system. Lets hope your ols system didn't have a single feed primatic (spit). For heat to transfer into the cylinder from the boiler you must have a route (somewhere) for hot water from the boiler into the primary heating medium (coil of pipe) and out of it back to the boiler. I take it you have looked up indirect hot water/heating systems and read the faq s for the group

A very good reason is the level of reliability. Single (Y-plan) systems have an inherent design "fault" where the valve motor can and does remain energised for very long periods rather than the two valve (S-plan) systems where the valve is only energised during periods calling for heat If you opt for a high recovery rate cylinder you may be able to use a single diverter valve rather than a Y-plan mid position valve in which case starting from a cold cylinder ALL the heat from the boiler will go to the cylinder i.e. none to the radiators during the time it takes for the cylinder to heat up. This often leads to com-plaint by inhabitants who expect rads to start to show heat within minutes of being switched on, not after a quarter of an hour

Its very neccessary if you have thermostatic radiator valves. Modern systems installed to standard require this valve to be automatic response i.e. opens progressively in response to TRVs closing so as to maintain a minimum circulation through the boiler at all times. Some boilers have an automatic bypass built in to the boiler but not many.

To adjust the flow through the cylinder coil and prevent it "robbing" the rest of the system. The coil can transfer a certain amount of heat and there is no point pumping an excess of water around it to the detriment of the rest of the system.

Close enough - you haven't done ANY reading up on this subject have you?

You have opened a can of worms

[Doctor Evil]

Use a quick recovery coil cylinder suing a Part L is false economy. It will heat up without any boiler cycling. Iwould suggest using a shower coilo cylinder. This has a coilo at the top of the cylinder in which only a shower runs through. It heats the cold water mains instantly providing high pressure showers (no power shower pumps are needed). Some makers call it a shower coil and others a power coil. Most makers will istall one for you.

The piping in the airing cupboard goes as such. This will reduce pump noises and much neater.....

The boiler is downstairs, the F&E tank in the loft. The flow from the boiler will go directly to the overflow over the F&E tank.

- In the airing cupboard tee off as high as possible near the ceiling on this 22mm pipe.

- Bend down to the cylinder beneath.

- Tee off, to top of cylinder coil

- This pipe then enters the DHW 2-way valve.

- a bend from the tee at the cylinder and insert the CH 2-way valve

- half way down the pipe from the top ceiling tee to the 2-way valves tee insert the pump.

You will have a straight run either side of the pump reducing water noises. Aso, it will make the system self venting. Any air in the DHW coil will rise up through the 2-way valve past the pump and to the vent. Same with the CH side.

To stop the pump sucking in air, take the 15mm cold feed from the F&E tank and tee it into the pipe from the open vent pipe tee to the pump (the tee near the ceiling). Keep it just before the tee, in other words the two tees together. The pump pulls down on the cold feed from the F&E tank and open vent, with virtually the same suction on both

- Self venting.

- Low noise

- no air sucked in.

Draw it out. It is one simple way of preventing air accumulating and reducing pump noises. The pump will also be above the cylinder and easy to get at.

Use fixed font.

============ Vent || ||¦ ¦ ||¦ F&E tank ¦ || ----------- || ¦¦ || ¦¦T (cold feed from F&E) T||===== Horiz. pipe near linen || || cupboard ceiling T = Tee || C|| || || || C|| Pump in centre of || (P ) long straight firmly || C|| clipped (C) to wall || || || C|| || ||T || ZV====ZV= -> To DHW coil flow |||| (ZV = Z valve) |||| || Flow to CH rads || From boiler flow (Combined fill, vent & flow)

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[Matthew]

John, thanks v much for your reply, much appreciated.

exchanger).

Ah, sorry, I meant on the other side of the cylinder? My current one has two tappings on one side for the coil and one at the bottom of the other side for the cold inlet. Many that I've seen (eg

another tapping above the cold inlet. What's this for?

systems have

Ah, that explains a lot, thanks.

I have TRVs so I guess I need this. A search here has shown me a Honeywell automatic bypass valve which perhaps I need. However, currently there's just a standard gate valve - presumably this would be set partly open at the moment? If so, wouldn't I be losing output from the boiler when I'm using CH/HW, cos some of the water is going straight back the boiler?

Again, make sense, thanks.

Sorry! I think I did read up on this on this group a while back, but if I remember, it provoked much debate and talk of multiple thermostats which sounded too complicated at the time..

Anyway, thanks v much for your help.

[Matthew]

Dr Evil, thanks for your reply, especially very helpful ASCII art! That all makes perfect sense and is pretty much what I have there at the moment, with a few exceptions:

• Currently my tee off from the vent/flow pipe is about as low as it can be. In light of your comments about straight runs and noise etc, I'll position it higher, thanks.

• I've also got an air seperator installed at the T, which seems to do pretty much what you said re the cold feed, but just with a larger (28mm?) pipe section. Is this required, or will just joining them together closely as you've described be enough?

• Also have bypass valve as described above. Will attempt to source this.

Anyway, thanks very much again for your help...

[John]

It can be used to connect a circulating system so the hot water is circulated (by a pump) out of the top outlet, round the house hot draw off points and back to the cylinder. This allows "quick" hot water at the taps rather than having to wait for it to arrive from the cylinder when you use a tap.

SNIP

A little but it really isn't an issue.

Ah! Drivel polluting the replies. I suggest you use a killfile.

You are welcome.

[Fred]

I thought a system with a circulating pump has the return at the top of the cylinder. This is to keep the hot water at the top and prevent the mixing of hot and cold water within the cylinder.

[John]

Think about it a bit further - whats the point of drawing cool water from lower down the cylinder pumping it round the secondary loop to cool a bit further, then push it back in to the hot water at the top? In one scenario you could have used "most" of the hot water in the cylinder leaving the only remaining hot above the side connection so your circulation pump now forces cold water round the loop to the taps and any excess cold water is mixed with what bit of hot there is left to cool it more.

Perhaps you are confusing primary and secondary circulation loops. With a pumped primary its possible to push heated boiler water in at either end of the coil, particularly "pancake" flat coils as found in some high recovery cylinders

[Matthew]

One more thing about the automatic bypass valve - where's the best place to put it? Not sure yet if I'll go for 2x2ports or 1x3-port valve

- if it's the latter (trying to save some money), presumably CH output from valve can tee off to the bypass one way and the rads the other? Or if I use 2-port valves, does it matter if it goes before or after the valve (goes before at the moment)?

[Ed Sirett]

Probably not.

A bubble free tapping for supply to a boost pump, saving the need to add an Essex/surrey fitting later.

Plenty see FAQ.

Probably yes, if all is well leave alone.

To balance the water and space heating demands so that water heating does not kill the radiators.

Yes.

[Christian McArdle]

Possibly a flange for a shower pump. Alternatively, if large, a hole for a side mounted top up immersion.

Yes. 3 way valves are less reliable, more difficult to wire up and don't easily support sub zoning of the heating system.

An S-Plan system usually needs a bypass. It is best to use an automatic bypass instead of a gate valve. However, this can only safely by done if you have a boiler with an overheat cutout. A boiler that is suitable for sealed pressurised operation (even if run open vented) will be OK. If you have an old boiler only suitable for open vented operation, you must keep the gate valve.

Adjusting a manual bypass is a black art. You basically want it open enough so that cold water can quench a malfunctioning boiler, and so that noise is reduced when running the heating with most of the TRVs closed. However, you want it closed enough so it doesn't short circuit the heating.

This is a balancing valve. Personally, I prefer hot water priority, so leave this fully open, although if you don't have a rapid recovery coil, you should balance it properly with all the radiators to avoid unnecessary cycling and cooling of the rads.

It's usually preinstalled, 5cm or so above the bottom of the heat exchanger. Any lower and it will never switch. Any higher and capacity is wasted.

Christian.

[Christian McArdle]

The 3 port does not usually need a bypass, as the valve is incapable of blocking flow. You would only need a bypass if all your rads had TRVs, which is only allowed if you have flow switch boiler interlock or similar.

This does mean that 'Y' plan is cheaper (one less zone valve and one less auto bypass). However 'S' plan is more reliable and more flexible.

The bypass valve must go before the zone valves. Otherwise, it will be entirely ineffective. It is designed to allow water to flow through the system when the zone valves are off.

Christian.

[Matthew]

Christian, thanks v much for your reply. Based on your and others' comments, I'll go for 2x2-port valves, seems a better option.

So, the existing gate valve bypass is there because all my rads have TRVs. Does this imply that I can use an automatic bypass valve instead? The boiler is a Potterton Profile 50e if that means anything.

One more thing I don't quite get...

I'll put it there because that's where it is at the moment, so no problem. However (and sorry if I'm being dense) isn't it there to maintain flow when the CH is on but all the rads are closed due to TRV setting? In which case the valve would be open and it wouldn't make a difference which side of it the bypass was on?

Thanks again, Matthew

[Ed Sirett]

The bypass serves two functions in this case.

1) To let water circulate when all TRVs close and heating is on. 2) To let water circulate when both zone valves have closed and the boiler still needs to dissipate some residual heat. The Pott. Profile controls the pump directly and so will operate the pump until the boiler has cooled down.

Upgrading the bypas to a smart one is OK but if the existing units is functioning leave well alone.

[John]

It should go before as this will give a flow path for the boiler water in the event you have a boiler with a pump run on facility to cool down the heat exchanger after firing and also provides a similar option in the event you have a shorted limit switch in one of the zone valves so the boiler is "called" even though the valves are closed

[Matthew]

controls

Aha, makes perfect sense now. I'm quite good at the actual physical fitting of stuff and rebuilding the existing setup is no problem, but I'm much happier if I actually understand everything that I'm doing.

Unfortunately, there is no leaving anything alone! Almost every joint in the airing cupboard has leaked at some point, and the cylinder itself is sitting very precariously on top of the old hot air system :( Everything's gotta go (even have to put in a new floor), so I'll try an auto... presumably I start with the lowest resistance setting and increase it until I get the right temperature at the rads?

Anyway, thanks again for everyone's help! I'm feeling much better equipped to tackle this now.

[Christian McArdle]

You should note that the boiler requires an "interlock". This means the boiler must entirely shut down when there is no call for heat.

The two usual methods of doing this are:

1. Using a room thermostat. Any rooms with the room thermostat must NOT have a TRV, as this will shut off the radiator before the room stat can operate, making it ineffective.
2. Using TRVs on all radiators in the zone. A flow switch is used to sense when no water is being consumed, which shuts down the boiler.

The first method is by far the most common. However, you say that all rooms have TRVs. If this is the case, then you must have a flow switch to shut the boiler off. You probably don't have this, meaning that when all the radiators turn off, the boiler will continue to short cycle, wasted energy.

Christian.

[Fred]

This is a new one on me and would be grateful for a few more details. I have a friend who doesn't have an obvious place to put a thermostat and this method sounds ideal.

Does the pump have to run continuous or is it a bit smarter than that?

[Christian McArdle]

One method is to have the pump running continuously, but this is wasteful of electricity and pump wear. The other is to have some more complicated electronics that pulses the pump every ten minutes or so, to see if it needs to restart. If any flow occurs, then the boiler will fire and the boiler output will keep the pump on. Obviously, the bypass needs to be before the flow switch...

Christian.

[Doctor Evil]

Use a Grunfoss Alpha pump and a flow switch. The Alpha is not expensive and also reduces noise in the system. The Alpha reduces speed to increased pressure as the TRVs close down. Once flow is below the flow switch setting the boiler is switched off. When TRVs open up the Alpha automatically increases speed and gives more flow and opens up the flow switch and brings in the boiler.

This is ideal for condensing boilers as an auto by-pass is not used. These reduce condensing boiler efficiency and it allows hot flow water directly back to the boilers return. Must be avoided at all cost.

This method is also ideal for TRVs on all rads.

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