Central Heating header tank location/water head

Just surveying positions for the header tank...
(Please don't say sealed system - I am committed to a vented system)
Now, most of the CH is downstairs and I have a nice position for the header tank such that the base (and thus water head) is 150cm *above* the ground floor ceiling, and about 130cm above the upstairs floor.
The pumps[1] will be 30-40 cm below the ceiling
[1] Boiler circuit, rads, HW plate exchanger all have sperate pumps into/from a thermal store. The header tank feeds the thermal store - basically just a 150 litre copper cylinder with extra tappings.
Am at any risk of lack of head possibly leading to pump over problems?
There will probbaly be 2 rads upstairs, short profile, so tops > 50cm under the bask of the header tank.
Second question - is it usual these days to have a seperate feed to CH from the header tank and a seperate vent pipe hanging over the top - or a single feed in 22mm - with downward slopes of course, all the way?
I *could* get the tank right up to the ceiling upstairs, but it's a bit awkward for other reasons - this is a hipped single story bungalow with dormers upstairs. Hanging the tank inside the top of the dormer is tricky.
I am hoping to fit it in a cubby hole at the top of the stairs unde rthe hipped roof which puts it more or less over the thermal store.
This will meet the boiler's typical demands for 2m head minimum given the chose position for the boiler.
Cheers!
Tim
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Have you checked the boiler manufacturer is ok with this - some require a sealed system for warranty purposes.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Dave Plowman (News) wrote:

Hi Dave,
I'll choose a boiler that is compatable. Viessmann are fine - read one of their manuals. They do however stipulate a 2m head at the boiler which I took into account.
Pretty sure Worcester-Bosch are OK too.
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I've got a Viessmann, and that's why I asked, as I'm pretty sure it said no open header tank. But it's not a combi but a 'system' boiler so comes with internal pressure vessel anyway.
Can I ask why you don't want a pressurized system? I'm a bit of a dinosaur about such things but happily changed my system when fitting the Viessmann, and have no regrets. Indeed it is much easier to bleed - the old system was a bit of a pain at initial fill with airlocks.
I'll dig out the installation instructions.
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Dave Plowman (News) wrote:

There's another range that is fine for open vented.

Because I already have the thermal store cylinder and my design is for an open vented system (ie the store will not cope with pressure).

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Ah - right.

Is there a restriction on how much head you can have to it also? There can't be *that* much difference between the two.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Sun, 30 Dec 2012 01:09:15 +0000, Tim Watts wrote:

Think about the static water levels in the tank and pipes. The water level in the open vent pipe will be the same as that in the tank. You only need enough "pressure" to lift the water up to the base of the loop over the top of the tank to get pump over. The head doesn't really come into it.

Presumably the water in the thermal store is the primary water shared by the boiler, CH and DHW heat exchanger. I'd fit the feed and expansion as per normal to the tank via their own tappings if possible. ie feed to the bottom and expansion always rising from the very top of the cyclinder. A single pipe would have to be connected at the very top to vent any air/gases evolved in the store. When the store cools it'll draw relatively cold water from the header tank into the top of the store. It won't stay there, being denser, it will sink setting up circulation that will tend to destroy any stratification in the store.
Note that the feed and vent pipes are now not in any pumped circuit so pump over (or draw down) might not be such a big issue but I'd still make the vent loop over the top of the haeder a "sensible height".
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Dave Liquorice wrote:

OK. So are there some good general rules about this somewhere? Is it more usual to put pumps in flow or return pipes?

Yes - exactly that.

This is a good point - thank you. I have a top centre tap for feeding the HW plate exchanger - this will also be the vent. Radiator taps are at 1/3 - 2/3 heights (as return water is still warm). HW Plate exchanger return will be at the bottom as I expect fairly cold return temps.

Almost - the top pipe will be in the pumped circuit for the HW plate - but will experience suction rather than +ve pressure. The head at the point will be about 2m.
I could tak ethe exapnsion pipe for a little circuitous route to the top of the dormer and back down (another metre gain) if that is likely to help?
Cheers - and thanks for the thoughts :)
Tim
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On Sun, 30 Dec 2012 12:53:57 +0000, Tim Watts wrote:

There are but I can't remember 'em well off the top of my head. Have a dig about on't web but no doubt you'll finmd lots of conflicting information...

The return is cooler but will then tend to make the entire circuit "low pressure" thus might have a tendancy to draw air in

I thought that might be the case. Just remember to keep the vent pipe rising at say 15 degrees or so. No "nice neat" horizontal section to take it from the center of the tank to the edge and up the wall to the header.

And leaves you a little bit of hot water at the top of the tank to feed the DHW exchanger.

Might be, does the plate exchanger data sheet have anything to say about return temps? I assume there will be a thermostatic mixer valve on the actual HW side to limit the HW temp to something sensible. The store should be operating at 70 to 80C which is far to hot for taps... Thermo-dynamics say return temp will always be higher than the temp of the actual HW leaving the plate exchanger, so it may only be 10 or 20 degrees lower than the top of the tank.
The 300l thermal store here has tapings at inches from the store base, solar coil 4 to 10, CH 13 to 37, HW 42 to 64, oil boiler 13 to 64 and wood burner 4 to 55. So the CH cannot cool the half of the store used by the HW. The solar heats as much of the tank as it can with maximum cooling to that circuit. The wood burner heats most of the volume of the store, with a bit of safety margin should it get things to boiling point. The oil boiler only heats the section of tank used, the thermostat for boiler control is a few inches below the 37 inch level(*).

Not for draw down when the DHW pump starts. What happens when the CH or boiler pumps start is another matter they might lower or raise the pressure in the store. I'd expect them to lower it if in the flow and raise it if in the return, assuming they are close to the store...
I think you'd be OK with 18" or so from highest water level in header tank to the bottom of the vent loop, but don't quote me. B-)
(*) Don't fit it above the the top CH tapping our you'll wonder why the rads are cool, the heating calling for heat but the boiler not firing...
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Hi Dave,
Dave Liquorice wrote:

That's what I assumed. Some things are fairly clear cut and some parts of CH design are not quite so obvious...

True... One of those less clear cut areas - except the boiler loop, where it is sometimes explicitly stated in the installation diagram.

Yes indeed. As I said, I think I can take that up the top of the dormer side wall and back down. I'll slope it down from the peak both ways and do it in 22mm (it's not a long run, probably 5-6m all over).
I am wonder as to the merit of one or two 3-port air traps in the rad circuit - the air port can vent into the same pipe.
The boiler circuit will clear itself into the cylinder - short and simple, ditto the HW plate circuit.

That was a deliberate design choice :) Always need some HW. In fact version 1 of the CH will be HW only and electically heated - my tank contains 3 immersion heater bosses - 1 at about the 1/4 from base level[2] and 2 more above at different radial entry points.
[2] There's a solar coil (sensible future provision) which prevents the immersion bosses going lower. If I had solar, this would be correct to leave a cold zone there for the solar to deal with.
I suspect I *may* get localised boiling if I trie to run all 3 at once - but if 2 work (6kW) then the 3rd can be regarded as an in-place spare. At this point I will sign up for and Economy 7 type tariff and charge the store overnight.
Rads will be added on piecemeal and the boiler will be added last when the system is debugged.
If I do not use the solar coil, I may add a diverter on so the boiler can take water from the bottom tapping so it can heta 100% of the store.
The core control logic will be 2 tank stats (2/3rds and 1/3 level) and relays and a time clock. Later I hope to add some computer control that will override the relays and provide finer control and weather compensation. If the computer fails, it can be turned off and the relays will provide a basically sane operation. I love computers, but I do not trust them!

I'm going to try the 3 port mixer on the *feed* to the plateX - it may or may not work well. If it works, it will reduce stirring in the cylinder considerably. I'll lash this in plastic and see how it behaves before I commit to copper.

Yes - and I will have mixer valves on the rad circuit. I may also have a lower temp UFH circuit too for 2 locations.

Sounds nice. I would have liked to have had a solid fuel input too - but I believed (maybe incorrectly) that I would need a gravity circuit between it and the stove - and I can;t do that as the cyclinder needs to be on the ground floor. I will see how those tapping heights compare to mine. From memory I have a 150l tank (biggest copper cylinder I could find for a sane price). I have one base tapping, one top, 2 at 1/3 and 2 at 2/3 for CH and UFH. And 2 coil connections.

It's probably a safer bet - unless I test it in plastic hose first so I can see exactly what it does.

Thanks - useful number - that would require my circuitous route as the header tank will be up against the roof. Yes I did oversize my header tank to cope with the expansion of about 200l of water! I got a fibreglass tank rated for 100C as it may see very high temperatures and it will be over my daughters bed! (Rembering that unfortunate case of the baby being killed by a boiling over system). There's not much chance of my system boiling as I would need failures in the tank stat, the immersion stat and the immersion safety thermal fuse. But better safe than sorry...

I *think* (hope) I placed my 2 stat probe tubes at sensible heights!
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On Sunday, December 30, 2012 1:09:15 AM UTC, Tim Watts wrote:

Why so? Open vents are fine, they work, I had one for a long time. I'd go for a sealed system if possible, they're better.
On Sunday, December 30, 2012 11:37:42 AM UTC, Dave Liquorice wrote:

Ain't necessarily so.
What is critical is the location of the cold feed connection and the open vent connection. There is a loss of pressure/head as water flows along a pipe, or through a fitting due to the frictional resistance. There is a loss of energy & this appears as a loss of pressure (Bernouilli, etc.). If the difference in the head at the two connections is high enough when the pump is running, then water or air will flow down one pipe and up the other.
In the olden days, the cold feed was on the return side of the boiler and the open vent on the flow side. This was when boilers were cast iron with big waterways, little hydraulic resistance (and so little head loss across return & flow) and solid fuel boilers did boil occasionally. Steam had to escape up the flow pipe, make-up water had to enter with the return water.
Then someone invented efficiency, cast-iron boilers were no longer made and waterways got much smaller. A 'traditional' arrangement of OV & CF ensured pumping over, especially where a cast-iron boiler was replaced. People complained them new-fangled high-efficiency boilers didn't last as long; funny, that.
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On 30/12/2012 01:09, Tim Watts wrote:

That sounds similar to the system that I had until recently.

Ours was probably 2m below the ceiling in the feed pipe before the split to the zone valves. (the feed dropped from the ceiling - it travelled up from the boiler at the back of the house into the eves roof void = along across then down)

No indirect coil I take it?

If you take your expansion pipe from the suction side of the pump, and then tee the F&E pipe into that close to where it joins the main circuit, then you should eliminate the main problems of sucking air into the system, and pump over.

They will probably still be a PITA to bleed, but if your pipe runs don't have any inversions then you should be able to avoid air locks.

You need the vent pipe hanging over the top as a safety precaution (stick a tall loop in it if you can to make the differential pressure required to pump over higher)
## # # | F&E | # |_____| # # # # # # # # # ###### # ############| P -> |######

Higher it is, easier to bleed the rads etc, although harder to get at the tank for servicing etc.
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Cheers,

John.

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Hi John,
John Rumm wrote:

No...

That's an interesting design - I never thought of that.

No inversions on the 1st floor. There will be loads of inversions on the GF as it's solid so all drops from above (essentially there is one radiator bus circuit and *everything* comes off it.

OK - thanks :)

This position is at the top of the stairs - inside a cupboard I will make in the wall under the roof space (stairs come up right at the apex of hips and ridge to one side of the dormer centre line). It's actuall quite convenient from every aspect.
Sounds like it is a goer then...
Cheers,
Tim
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On 30/12/12 01:09, Tim Watts wrote:

I think most of your questions have been answered already, and you seem to have most of your issues in hand anyway.
Your system is very similar to mine, apart from mine uses an in-tank extra-long coil in place of the heat exchanger for hot water (so stored HW would still be available during a power-cut).
Mine has been absolutely trouble free, behaves ideally, and has been returning excellent economy (no gas here, so modern oil-fired pressure-jet Rayburn).
Such tips as I can give you:
Header tank - as high as possible - makes filling/bleeding easier (chapel conversion here, so about an 8m head overall, and 2.5m to the next highest point - the upstairs rads).
Use a conventional vent pipe in addition to the feed pipe to the header tank (there's a formula for calculating how high above the tank the vent pipe should rise). The vent pipe is a critical safety feature - imagine if the feed pipe froze or became blocked by debris.
Pumps for rads pump from the store, to the rads (2 circuits here). Pump for boiler pumps from store to boiler.
Those CH circulation filters (like the Fernox TF1 - recommended) - they're good things, and a convenient place to add water treatment.
Add lots of drain-off points.
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