Expansion vessels - Do I need one?

[wifeiswitteringagain]

I have a vaillant gas condensing open vent boiler which I am plumbing in at present. In the short term I plan on running my underfloor heating off it just to dry the floors out prior to fitting timber floor. Thats all it will do for the next 3-6 months. The water volume in the ufh is approx 50 litres. (Long term I will have a heatrae sadia system fit cylinder installled which comes with an expansion vessel, safety blow off and auto bypass etc. My question to anyone who can advise me is; Do I need to buy and fit an expansion vessel just for the ufh short term (3-6months). i will obviously fit a safety blow off and an auto bypass valve. Thanks.

 

[Gasguru]

You appear very confused

You mention a Vaillant open vent boiler ie not a sealed system so therfore no expansion vessel would be required.

Since this sounds like a new system I would however advise you not to go the open vented option. Sealed systems offer several advantages especially with underfloor heating. Perhaps you mean you are going to add sealed system components to make the system sealed?

Are you refering to a Megaflo cylinder? These are unvented and do not normally have an expansion vessel fitted. All the safety components come with the cylinder.

Be aware boilers and unvented systems are notifyable to building control.

 

[chrishutt]

I have a vaillant gas condensing open vent boiler

If the boiler is "open vent" it means it is suitable for a system fed by a feed and expansion tank. If you do not have this then you will need to set up a sealed system instead. This must include an expansion vessel. The expansion vessel for the unvented cylinder that you propose to fit later is not connected to the heating circuit so is not relevant.

 

[wifeiswitteringagain]

Hello there and thanks for the quick response. Sorry but i didn't explain my sitaution very well. The boiler is an open vent boiler but i will use it in a sealed un-vented situation which is fine as it states in the manual. I must obviously fit an auto by pass and a safety relief valve. I bought this boilel because its the only Vailant without a pump as I will have an unvented Heatrae Sadia 'System fit Megaflow' cylinder (when we can afford) which comes with all the safety bits and bobs and a pump etc. as you probably know. The 'System Fit' pre-plumbed cylinder comes with a 12l vessel for the dch. I just wondered whetehr its necessary for me to fit an expanison vessel in the shrt term for approx 50 litres of ufh water running at the lower temp. I dont really want to buy one if I can get through without. Thanks. Steven

 

[chrishutt]

You must have an expansion vessel, even for small volumes. Why are you so keen on the system fit Megaflo?

I've just checked on the system fit Megaflo and it seems that it includes an auto by-pass valve but not an expansion vessel. Which makes sense, really. So you will need to buy an expansion vessel anyway.

 

[wifeiswitteringagain]

Chishutt,
So I'll have to buy one then. How many litres do you reckon? Is there a formula for working out the size of the exp vessel?

The systemfit megaflow looks like a good buy since its pre-plumbed and has all the valves etc on it. I hate shopping!! Why do you ask - have you got any alternatives???

Thanks

Steven.

 

[wifeiswitteringagain]

I forgot. No the boiler doesnt have a bypass. The Heatrae Sadia website doesnt mention the exp vessel but the hardcopy brochure does!!, so I rang them yesterday and they told me it comes with a 12litre vessel for the dch.
Sounds like I'll have to buy one then. More expense. I'm a Yorkshireman tha knows!!

 

[chrishutt]

- have you got any alternatives???

Most people would have bought a system boiler which would have had the expansion vessel, pump, pressure release valve and auto by-pass incorporated. Some even have the control system for the cylinder circuit built in.

Then later you just add a standard unvented cylinder, preferably to match the boiler - e.g. Vaillant unvented. The two are then properly integrated and covered by the same manufacturer's guarantee, which makes life simpler if anything goes wrong.

As for expansion vessel volume, 8 litres is pretty standard, unless you have a very large system. there is a formula for working it out, but it's not really critical.

 

[wifeiswitteringagain]

Isn't hindsight a wonderful thing. My plumber freind suggested the Vaillant / Megaflow Systemfit combination due to his good experience of the two. Perhaps i should have joined this forum when making the boiler decision. Its too late now - the boiler is on the wall and its wired and the flue is in. The only option I do have is which cylinder to go for. Any ideas? Is the vailant one any good?
Can you give me any advice on the size of exp. vessel pse. In the long term I will connect it up to the 2nd floor cast iron rads as well, of which the volume I haven't a clue.

 

[chrishutt]

You seem to be making things very complicated for yourself. If you don't go for the system fit Megaflo then you'll have to put together all the sealed system components yourself. Not difficult if you know what your doing, but you don't (yet).

Which reminds me, you shouldn't be fitting an unvented cylinder yourself - they are potentially dangerous. you should engage the services of someone with the "unvented ticket" as it is known, meaning they will have undergone a brief training and assessment in unvented cylinders.

 

[wifeiswitteringagain]

Yes, I'm very good at making things complicated, but in my defence I'm taking avdice off a Corgi registered heating engineer. The reason why I'm going for the system fit is because it will be pre-plumbed therefore quicker / simpler to install. I will have my corgi freind check it all out prior to lighting it up. Thanks.

 

[Gasguru]

Whether the cylinder comes with a vessel or not you need one for the heating side. Personally I like to sit a large 35 litre vessel (complete with legs) on top of the cylinder. The pressure doesn't fluctuate much then on these small systems. There're cheap from BES.

You also need a safety valve, pressure gauge and filling loop. A bypass is fitted to the cylinder.

The clock supplied is only a 2 channel, you need 3 channels (U/F, rads, hot water), so either get a 3 channel clock or add a single channel clock.

You will also need another zone valve for the U/F heating.

I assume the U/F heating manifold comes complete with mixing valve and its own pump.

The wiring center supplied will need modifiying to cope with the U/F.

The wiring is similar to Honeywell S Plan (Switched live to the boiler and pump live from boiler to the pump on the cylinder).

The U/F clock live should go through the U/F room thermostat to the U/F zone valve motor connection. This same feed from the U/F stat should also go to the U/F pump. The U/F zone valve switch contacts should then feed the boiler switched live as in the other 2 zone valves.

If the pump valves on the cylinder are the ball type replace them with gate type valves...many of these packaged units are poorly put together often using cheap pusfit fittings.

And I would double dose the inhibitor, the last thing you want is any corrosion and sludge in the U/F pipework.

 

[wifeiswitteringagain]

Thanks for all that Mr Gasguru. Very informative. I've got the BES catalogue so I guess i'll need to put in an order for a vessel. OK on all the clock notes, yes i was going to buy a single channel clock for the ufh and use the 28mm spare tapping on the cylinder (which is supplied capped) and fit the zone valve to it. Yes the ufh manifold has its own pump and mixing valve etc - all installed. It also has its own wiring centre with all the stats controlling the zone actuators.

I will need to study youre wiring advice and will bear in mind your comments ref. the pump valves.

What inhibitor would you use in the ufh /rad system?

Another question I have is whetehr to fit a pump to the primary to feed the ufh manifold. The ufh has its own pump and since there is only 2m between the boiler and manifold I would expect the ufh pump to do the job in the short term (until I fit the megaflow) Whats your thoughts on my plans pump wise?

 

[Gasguru]

What inhibitor would you use in the ufh /rad system?

Double dose with Sentinel X100 (remember you also need the cleaner, I would fit quality full bore isolating valves to the radiator and U/F circuits, its easy then to know your cleaners is in the rads where most of the gunge is).

Another question I have is whether to fit a pump to the primary to feed the ufh manifold. The ufh has its own pump and since there is only 2m between the boiler and manifold I would expect the ufh pump to do the job in the short term (until I fit the megaflow)

So initially you will only have the boiler and underfloor heating. You will need the expansion vessel, safety valve, pressure gauge, filling loop.

The U/F heating probably works on the "injection" setup ie a thermostatic mixing valve "injects" boiler flow water into the U/F flow manifold until the the manifold is up to say 40 centigrade. Then the thermostatic mixer just allows circulation around the loops (using the U/F pump until the loop temperature drops again). Therefore you must have an additional pump to circulate water from the boiler to the U/F setup and also an auto bypass valve.

 

[wifeiswitteringagain]

Thanks for your help Mr Gasguru. Youve helped me make up my mind and I've just ordered a pre-plumbed 210litre Megaflow cylinder. Cant afford it but neither can i afford to be buying all aforementioned bits and fitting them, then having to redo it later and being left with a load of leftover bits!

I read up on the Megaflow cylinder and I've done the right thing getting the open vent boiler. The Megaflow installation insructions state that the boiler shouldn't be a system type, but an open vent type with no pump / controls. I not as daft as you probably think I look!!