Converting a vented cylinder with coil HE to use PHE

[sat312]

This is my first time on this forum and I have a question which I seen asked many times but there has never been a clear answer.

I have a standard Y plan central heating system with a vented indirect water cylinder. The current coil doesn't heat the water quickly so I was looking to replace the coil with a plate heat exchanger.

This is my current setup:

Current

• sat312
• 19 Feb 2012
• 1

Current

what I was looking to do is the following:

Proposed 2

• sat312
• 19 Feb 2012
• 1

proposed2

Is it possible to do this?

Can someone explain how I could reconfigure my current system to allow me to have a configuration for instant hot showers without having to replace my current boiler?

I found this link which is very close to what I would like but I can't see how to apply this to my setup.
//www.diynot.com/forums/viewtopic.php?t=75050&start=15

Any help would be most appreciated.

 

[swbjackson]

I assume you are proposing to feed this secondary heat exchanger from the cold main. This throws up the first issue with your idea, you are not allowed a pump on the cold main.

What you are proposing is a similar idea to a combi boiler. Have a look at the flow rates from a combi. A 24Kw combi will give around 9 l/min at a 35‘C temp rise. For a good flow rate you need to go to 30Kw and above. I'd be surprised if your boiler is as large as 30Kw.

 

[Norcon]

Your diagram appears wrong.
There seems to be a pump on the mains which you can't do.

Whats wrong with the current set up?

 

[Dan Robinson]

Plus a 22mm y-plan doesn't shift enough energy to be worthwhile.

You are going to have mains ho water going into a enter cylinder.

You'll need blending valves to prevent over heating the to of the cylinder - if you ever plumb I in correctly

 

[mehran]

Replacing the cylinder is easier...

But If you did want to use a Phe then you not put it on the mains but put it on the cold feed to the cylinder pumping the heated water into the cylinder out let.

 

[muggles]

Buy a modern rapid-recovery cylinder

 

[sat312]

I would like firstly to thank you all for replying so quickly.

I'm a basic diy'er so please forgive me for my ignorance.

The only problem I have with my heating system is that it's very slow heating up the water, especially when my teenage daughters get infront of the bath queue

Again like for so many cost is a big issue and I cannot afford to install a complete new system. So I was looking at a temp solution that would buy me some time until I can afford a more robust system. Or at least until kids decide to move out

I was looking at first to install something like the showermaster from heatweb.com but they have had a change of management and those products are no longer available.

OK. Having a pump on the mains is a NO, NO.

Therefore can I have the cold in from the water tank?

Or is it best to just replace the water cylinder and if so what cylinder will provide the best heat recovery. Especially from my current Potterton Profile 80e boiler?

 

[Boilerman2]

You cannot put MAINS water throuh the plate and then into the open vented hot water system as per your diagram it will cause a massive overflow from the CWS tank!!

why not fit two essex flanges to the cylinder, one near the top and the other near the bottom, pipe the secondary side of the PHE to these connections using a BRONZE pump, this will give you the recovery you want, using the existing cylinder as a buffer vessel of hot water, however the cost of all of this is prohibitve the PHE will be around £150 and a bronze pump is around £200 so it would be cheaper to fit a fast recovery cylinder instead!

 

[sat312]

Is this what you mean:

Untitled

• sat312
• 19 Feb 2012
• 1

In general would a plate heat exchanger provide faster recovery then the existing coil?

I agree with you and muggles that replacing the cylinder would be easier but cost of a good rapid recovery cylinder like the megalife is around the £1000 mark for 150Ltr cylinder.

If I start looking at spending above the £1K mark then I might as well go for a new boiler, etc.

 

[Nige F]

Get a custom made cylinder - google Kwikcyl - or buy one like it for £75 on eBay.I did , and it had a massive coil , plus it was grade 2 copper

 

[a546345]

I have used a PHE on my CH flow and return (controllable via a 3 port valve) to heat DHW as an experiment.

Some subtleties with doing this with my boiler (a 30kW condensing combi)

1. When calling for heat on the CH circuit, my boiler provides minimum heat for 3 mins (8kW) and only then ramps up to desired temperature. A shower is therefore only lukewarm for the 1st 3 mins.

2. If desired temperature is overshot due to low flow on the secondary then my boiler shuts down for 3 minutes before restarting at minumum heat as per point 1. The shower therefore goes cold for 3 mins!

3. You need a way to signal demand for heat to the boiler.

 

[sat312]

well it looks like a new high recovery cylinder then.

One last question - what is better, copper or steel cylinder?

 

[Dan Robinson]

Think about losing the y-plan too. You are hobbling the boiler by around 15-20% (need to check my maths - in my PHE heated bath ATM )

 

[crashndie]

heres an option out side the box ....fit a second standard cylinder to your current system change the y plan for 3 zone valves and 2 cylinder stats. Use the primary cylinder to fill the new cylinder (hot outlet on the old 1 to cold feed on the new 1) use the hot out let on the new cylinder to feed the taps make sure u vent both out lets to the tank in the loft this is a must . this way you have a twice the storage and 2 coils heating it up...

or just fit a hi heat recovery ive used steel ones in the past never had any problems with.

 

[SimonH2]

Is this what you mean:

Untitled

• sat312
• 19 Feb 2012
• 1

That won't work either.

Follow the flow. Water comes dwon from the header tank, through the PHE, through the pump and into the DHW cylinder- then what ? You'd have to pump water up the vent and overflow it into the header tank - thus heating your header tank.

The cold feed from the header tank must go into the bottom of the cylinder.

The inlet to your PHE should come from the bottom of the cylinder (where the pump is connected in this diagram). The outlet of the pump should go into the cylinder part way up.
At a pinch, tee into the cold feed pipe and hot outlet pipe. While the pump is running, the flow will be reversed between the tees and the cylinder.
But it would be better to have separate connections - at least for the delivery - for reasons that will soon be apparent.
When you've drawn it right, water will come form the bottom of the cylinder, through the PHE and pump, and return to the cylinder.

Ideally, you would balance the flows in the two halves of the PHE to maximise heat transfer and deliver water to the top of the cylinder at the desired temperature. In practice, unless you add modulating pumps with control circuits, or thermostatic valves, you can't do this (without compromising performance) as the conditions are fairly variable. The simplest would be a thermostatic diverter valve rigged so that the output from the pump (in the position shown) returns to the bottom of the PHE until it reaches your desired cylinder temperature.

If you want to leave that out, apparent recovery times will be longer. You might want to put the flow into the cylinder part way up (perhaps 1/2 to 2/3 of the way up), then if the flow rates are low enough to avoid turbulence in the cylinder then you should be able to keep some hot water "ready for use" even when recovering. The cylinder will heat up, and when the water from the PHE is hot enough, it will start rising by thermal syphon to heat the top of the cylinder.

The alternative is to set the flow rate so that with the feed water very cold (possibly only a few ˚ in winter), the delivery water is up to the temperature you want. But this means you will sacrifice significant capacity when the feed water (bottom of cylinder) isn't that cold. That would obviate what you are trying to do.