Sealed heating system design/conversion - Cue Ed or Andy H

Ed's excellent sealed system FAQ covers the principles of operation plus what to do when something goes wrong, but it doesn't tell you how to design a system from scratch or how to convert a vented system (apart from mentioning a couple of pre-requisites).

Is there a FAQ or similar anywhere which goes into things like:

  • How to determine the volume of a system
  • How to decide on the size of pressure vessel required
  • Any rules about the location of the pressure vessel relative to boiler, pump, zone valves (if it matters)

According to my calculations, you need a pressure vessel whose capacity is about 10% of the water volume. Does that sound reasonable? [I'll expand(!) on that below]

So you need to be able to estimate the water volume. If you know how much pipe there is of each size, that bit's not too difficult. Then there's the radiators. Are there any rules of thumb for litres per square metre for typical single-panel radiators? [I'm sure I could make a stab at it by estimating the cross-sectional area of all the 'tubes' - but it would be nice if I didn't have to]. That leaves the boiler and the heating coil in the HW cylinder. Are there any typical values for these?

I'm sure there's scope for a FAQ here if anyone cares to write one (if there isn't one already).

[Back to my calcs. How does this grab you? Assume you have a 10 litre pressure vessel, and prime it with air at 0.7 bar (1.7 bar absolute). Then you pressurise the system with water to 1 bar (2 bar absolute). ISTR from the dim and distance past about the Law of either Boyle or Charles or somebody along the lines of the volume of a gas at constant temperature being inversely preportional to the absolute pressure. If this is correct, our 10 litres of air at 1.7 bar will reduce to 8.5 litres at 2 bar. If we now heat up the system so that the pressure rises to 2 bar (3 bar absolute) the volume of air will reduce further to 5.67 litres. The difference between 8.5 and 5.67 (2.83 litres) represents the amount of expansion water which has entered the vessel. So, for a pressure rise from 1 bar to 2 bar, a 10 litre vessel can accommodate about 2.8 litres of water.

My research on the volumetric expansion of (tap) water suggests that it expands by about 2.5% between 4 decC and 80 degC. So a 100 litre system would expand by about 2.5 litres - which could be nicely accommodated by a

10 litre vessel, giving a pressure rise of just under a bar. (Actually, the temperature rise will be less than this since the system will be a room temp rather than 4 decC when cold. But the air in the vessel probably warms up up a bit, which I have ignored - so maybe the two tend to cancel.) Are there any major flaws in my reasoning?]
Reply to
Set Square
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Here was my plan of action for that (going from conventional boiler to combi, original tanks in the loft):

Boiler Replacement Procedure

Determine pipe arrangement Determine gas pipe sizes Calculate quantities required Establish pipe routing for CH Add desludging agent to existing system Order boiler and enough fittings and pipe to make sure all eventualities are covered!

Prep

Ensure hot water at full temp Turn off boiler

Loft Work

Tie up ballcock in CH header Drain system - ensure 3 port valve is in centre pos to ensure heating loop is drained. Cut CH feed and return in loft and loop back. Include gate valve on a tee in loop Cap pipe ends to prevent seepage

Kitchen

Take static gas pressure reading and ensure loss is zero Turn off gas Pull fuse from boiler FCU Disconnect boiler electrics and remove programmer Disconnect CH feed and return Disconnect gas feed Open boiler Remove from wall Remove old flue fill old flue hole with bricks and mortar Using template mark out positions Core bore new flue exit Fit boiler mounting frame (remove tiles as required) Connect CH feed and return Remove existing gas feed pipe to under worktop Cut new hole through worktop for DHW feed and return, 22mm gas, condensate Install 22mm gas feed from existing ¾" pipe Turn off water Cut in new DHW feed and return and connect with service valves Install filling loop Mount boiler on tray Install new flue Install condensate drain

Loft

Tie off cistern ballcock Drain remains of hot water Disconnect hot water output from cylinder and cap off Disconnect input mains water from cistern and CH header tank Disconnect cold shower feed from cistern and connect to rising main

Bathroom

Remove bath panel and identify CH stat wire - join to existing down wire and disconnect from remainder. (Leave remainder available for pulling through new wires later!)

Kitchen

Connect boiler electrics and stat Carry out any pre checks as per install manual Purge gas meter and pipework Carry out gas soundness test Commission boiler as per manual Flush as required Treat with corrosion inhibitor Remove tanks etc. and cylinder from loft.

Job Done!

All in all getting on for four days work!

If in doubt, err on the side of too large for your expansion chamber, since too small is a problem too big is not.

Sounds ok.... here is a table from the install manual for my boiler:

Assuming pre charge pressure between 0.5 and 0.75 bar

System volume Expansion vessel (litres) volume (litres)

25 1.8 50 3.7 75 5.5 100 7.4 125 9.2 150 11.0 175 12.9 190 14.0 200 14.7 250 18.4 300 22.1 For other system volumes multiply by the factor across 0.074

Not obviously.

Reply to
John Rumm

Thanks for that. It looks like 10% is a bit conservative and that 7.4% is ok.

Your todo list looks pretty comprehensive - and goes further than I envisaged. I plan to keep the existing boiler and stored hot water system, simply replacing the F&E tank with a pressure vessel and associated bits.

Some rules of thumb for estimating radiator/boiler/cylinder coil capacities would still be nice - as would guidance on acceptable locations for the pressure vessel.

Reply to
Set Square

Some rules of thumb for estimating radiator/boiler/cylinder coi

capacities would still be nice - as would guidance on acceptable locations fo the pressure vessel.

Put the expansion vessel on the return to the boiler in a positio where there are no controls between the vessel and the boiler like zon valves or gate valves, so that it is effectively always in open serie with the boiler.

You can put the pressure guage filling loop and blow off valve al together in the same place if that is feasible. Check ebay component like that come up cheap once in a while

-- Paul Barker

Reply to
Paul Barker

See;

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's at least one other document on the RWC website about sealed systems, in the section about 'Training Guides'. These are Acrobat document and, for some reason, I can only get them to open during office hours.

Reply to
Aidan

Thanks. A useful reference!

Reply to
Set Square

Is that mandatory? It would be much easier to put it in the airing cupboard - using the pipes which currently go to the F&E tank, which connect into the flow pipe just before the pump and zone valves.

So there *would* be an open path to the boiler - but it would be on the flow rather than return side. is there a problem with that?

Reply to
Set Square

I'd go along with your reasoning. If another poster were (able?) to argue as well he might have more credibility.

Questions about converting to /designing a sealed system do come up from time to time but question about sealed systems and their maintenance are almost daily. Hence the syllabus of the FAQ. There is a greater need IMHO for a 'choosing a heating system' FAQ, 'choosing a boiler make' FAQ and 'diagnosing gas boiler faults' FAQ.

Those three questions must now be pushing 20% of the threads in one form or another.

When converting expansion vessel can often go in the loft where the old F&E tank was and connected to the vent pipe (but some boiler makers, Keston and Potterton at least, want it on the return so watch for that).

Usually the most frequent hassle is that the most convenient place for the filling loop (airing cupboard where rising main and circuit pipes are close) is the least convenient for the relief pipe. The releif valve usually has a place for a pressure gauge to screw in. The filling loop does not. You end up having to add another pressure gauge near the filling point.

A useful guesstimator for system volume is 10 x radiators = system litres. My local plumbers' merchant basic tells people to buy nearly 1 litre /kW of boiler - this is probably rather excessive vessel sizing - mainly because modern boilers a modulating units and are often well over-sized.

Reply to
Ed Sirett

Many thanks for your reply. [Hopefully my degree in Physics - albeit 40 years ago - wasn't totally wasted!]

The installation manual for my boiler (Baxi Solo 70/4 PF - MkI) says that the vessel should be connected close to the *inlet* side of the pump - which is right where the F&E tank is connected at present. This gives me the option of the airing cupboard, or the attic in place of the F&E tank. My only concern with the latter is that it would be more difficult to get at to check and re-charge the air pressure if needed.

The manual also says that the relief valve should be close to the boiler on the flow pipe. I would rather put everything in the airing cupboard - still on the flow pipe, but about 10 metres from the boiler (as the pipe runs). Do you see a problem with this?

My airing cupboard would be the most convenient place for the filling loop. There would be no problem getting the relief pipe to the outside world. Are there any rules for locating the spout, relative to where people may be standing? Incidentally, the boiler manual says that the filling loop must be fitted at "low level" - but the diagram shows it between the pump and the hot cylinder! I presume it means low level with respect to the floor, not in absolute terms?

That gives rather conflicting results! I have 10 radiators (albeit including a couple of large ones) so that gives a system volume of 100 litres. Using my 10% rule, the expansion vessel would need to be 10 litres. [Someone quoted another source at about 7.4% of system volume]. However my boiler has a max output of 20kW (26kW input) which indicates about 20 litres. This sounds a bit excessive to me.

Reply to
Set Square

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