Multipoint as CH boiler & kettling

[Egremont]

Weeks ago I floated, for amusement only, the notion of using my existing Thorn multipoint gas water heater to power a new central heating system (with a thermal store). Well, I spent half the summer putting it all in and I'm now testing with the multipoint hooked up (yes really!). I've incorporated much of the hopefully excellent advice from Dr Evil and other folks along the way.

The result?. Well it basically works... almost!. The boiler fires up when it's supposed to and gets the 180L store 'hot' in about 15 mins or so. But I have one problem, boiler 'kettling' (water audibly sizzling / simmering from boiler, which also gets rather hot - albeit it's high summer).

Rough measurements suggest the boiler is processing about 10L/min, and raising the temp by about 30C in one pass. The flow is just like it used to be, but obviously the water-in is warmer than cold mains. The bolier's h ex is some galvanised enclosure containing wrapped 15mm pipes, so is perhaps low water content. I have an open system with the FE tank in the loft and pumps/thermal store on the ground floor, with boiler on 1st floor. The primary pump is a Wilo gold 60 running at top speed (adifferent pump delivers to the radiators). The boiler pipe run is unfortnately (but unavoidably) long, about 12m each way in 3m vertical, 28/22mm pipe, with about a dozen 90 degr bends.

I'm thinking that water needs to be be run through the boiler more quickly, say 30L/min, reducing the temperature differential to about 10C. This purely to quell the kettling. I wanted to ask:

1) What is the typical flow rate through a CH boiler - is it usually more than 10L/min and

2) For whatever reason the Wilo docs suggest a much higher flow (3-4m3/hr =

45-66L/hr) than I'm actually seeing. Assuming the Wilo isn't crocked, or blocked, and I need to increase flow by about 3x, can anyone suggest a suitable new pump?.

The other things I can think to try are raising the FE tank in the loft (by about 1.5M), try to de-scale the exchanger (previous attempts seemingly unsuccessful - BTW who exactly would sell a suitable replacement?)) or add stuff to the water, which will be expensive (esp gel silencer) because of the large volume of water in the direct store. Or (Heaven forbid!) get a new boiler put in...

Egremont.

PS - will I getaway with leaving it a few weeks before putting corrosion inhibitor in?

[Doctor Evil]

There usually a minimum flow, which is usually lower than 10 L/min. But that is pretty irrlevant to your problem.

Firstly check there are no restrictions in the flow and return pipes. Only have "full-bore" isolation valves. On the multi-point, open up any throttles to full. Remove any elbows if you can. Keep it all bends. I assume this is a "direct" store.

It does appear to be running low, so check the obvious.

I doubt that will be the problem.

That should be your first attack. Take off the heat exchager and pour descaler down the heat exchanger

Put full-bore isolation valves on the boiler's flow and return. Fill the store up with inhibitor. If anything is wrong isolate the store and work on the boiler/multi-point.

See above. Do ASAP.

Also put an auto air vent on the highest point just above the multi-point. The kettling may be one or two or all of a number of problems.

[Egremont]

OK Dr Evil thanks for the advice, though I think this might prove to be a struggle...

I asked about typical flow rate mainly to find out whether I'd also likely need to upgrade the pump even with a new boiler, though I'm not there yet!.

Somehow I don't _think_ there are any serious blocks. Can reasonably check pump & boiler inlet filter, thereafter looks like I'd need to break/short the pipework at variuous points and (somehow - don't know how!) check flow at each stage. First thought was the (scaled-up?) boiler h.. ex pipes, then I remembered in it's previous life there was very little difference between mains hot & cold flow - suggests reasonably free flow through boiler. One vague thought - is the thermal store in the loop likely to slug the flow somehow?. Pumped hot water pipes (with shower pump near boiler) follow a similar looped run to/from the store and I've noted flow of 17L/min at the bathtap, cf 30l/min for cold pumped direct to the bath).

The thing about raising the tank is that the minimum operating head for the boiler is supposed to be 2.3M, the height of the F&E tank above the boiler is about 1.5M (depending where you measure). I'm not sure if this is relevant as most of the (dynamic) head in my situation comes from the Wilo pump?, I suspect it would help to raise the tank to some limited degree but perhaps not the first thing to try.

I've already had one unsuccessful go at pouring hot Kilrock & path-clear through the h. ex, but not one bubble!. Maybe I need a proper jig or a bathfull of descaler for this to work properly. I'll probably use patio or brick cleaner first, though nowadays containers don't seem to giive chemical contents so I'd be guessing as to suitability. Anyway working on the theory it's (still) not seriously blocked I might leave this for now and try to get the flow rate up first., though I know any scale is likely to promote kettling.

What valves there are are full-bore, there is an auto air vent at the top of the vertical inlet pipe from downstairs to the boiler. Like I said, I incorporated most of your previous advice, but I'm unfortunately stuck with the elbows in the run. The only 'user' throttle is the winter/summer knob, set to 'summer' to give a bit more flow.

If no obvious blocks come to light, I'm still thinking of a pump upgrade and possibly a new heat exchanger - if anyone can suggest a suitable model/source.

Egremont.

[Ed Sirett]

I may be making a complete fool of myself if this is a Norwegian post.

The symptoms you describe are a predictable consequence of the setup.

The appliance is designed to raise water by 30-40C when pushed through by a pressure of 1-2 bar upwards. Even then the boiler is likely to kettle lightly and more so if older. You are pushing water through with a pressure of 0.6 bar at best, you are starting with water at 30-40C or more.

You best course of action is to find a registered installer, let him send some pictures of this to the Gas Installer Magazine They have a section where pictures of irregular gas installations are published. [1] He can claim his free prize of a combustion analyser (Worth about £250) He might then give you some discount on fitting a heating boiler - a gas appliance _designed_ (and legal) for your application.

[1] Previous notable entries included the home made boiler. the warehouse heater with flue directed into the office to keep the staff warm in there. the boiler connected entirely with metal braid sheathed flexible hoses (including the gas supply). the boiler with no flue at all. etc.etc.

[Doctor Evil]

It is not predictable. on-demand (multi-points) as used to heat thermal stores and skirting heaters in the USA. It is very common. Go to the tagaki web site in the USA and download some of the installation manuals. It gives diagrams of how to do it.

I'm not sure about the 1-2 bar to operate. It obviously runs at around 0.6 bar, and may operate via a flowswitch rather than lifting a diaphragm. The makers minimum pressure would be less than 0.6 bar I would assume.

Kettling should not occur if:

- decent flow

- inhibitor in system

- no scale in heat exchanger.

- no air in system

Not irregular in some parts of the world. Quite regular in the USA. The OP is having the muilti-point as temporary measure. A multi-point, surprise, surprise, is a water heater. Boilers are now referred to as "water heaters". You will fail a corgi test if you call it a boiler. Passing water through an appliance is well, er, er, passing water through an appliance and heating it.

In the past I have seen Ascots used as boilers, and no kettling. Technically, and safetywise, there is "nothing" wrong with the installation. Water is passed through the appliance as it was designed to do. The water just happens to be in a loop, and not fresh so no scale.

His best course of action is to take the steps to eliminate the kettling and once done it will work as well as any "boiler". If the kettling cannot be eliminated then install a boiler, and the temporary nature of the multi-point has ended.

BTW, Chaffateux made a boiler out of a multi-point in the late 1970/80s. It never had any electrical aspect. Just a pump, pumping pressure on the diaphragm to open the gas valve, with a mechanical boiler stat. Worked well.

[Egremont]

Though typically used on the mains, docs say that this boiler can also be fed from a tank supply. The minimum head requred is just 2.29M (BTW how hot would that get?). This boiler will raise 12.5L/min by 25C.

True for this application the input water is already hot, this is why I was looking at getting the temperatue differential down by speeding up flow. If it still kettles then I'm looking at a new boiler!. Long term scaling wouldn't occur as the boiler would only be recycling primary water (inhibited if necessary).

I'm _fully_ aware that this application is unusual, though I didn't know it was illegal(?). At the end of the day water is just being heated up and sent to a storage tank (the thermal store). I thought I'd give this a try and see what happens in test mode, if it can be made to work safely then I wouldn't be in any rush to have a new boiler put in - my central heating usage being intermittant at best.

Egremont (Dumb, but not Norwegian (;->))

[Doctor Evil]

Waater in a tank can be 20C, and way above in loft in summer as the bakes a dark tile roof.

It is not. I know of no regulation that say it is dangerous in any way whatsoever. Multi-points are used for this purpose in many other countries.

[Doctor Evil]

No. If direct iot will not at all. It will give a free flow.

You have a shower pump? Isn't the hot water fed from the mains?

You are right, the head is via the pump.

Use the proper descaler.

Putting two pumps in series raises the head and will be cheaper, as you already have one.

Get inhibitor into the thermal store ASAP, with isolation valves on the boiler flow and return so you don't drain the store and all the inhibitor.

[Egremont]

Before recent works the hot supply was on the mains via the boiler. Now hot is supplied via a tank in the loft via thermal store & shower pump - much better. Sadly space restrictions rule out using two pumps.

It is a genuine concern that this arrangement may (rightly or wrongly) cause a haitus if looked at by an installer during maintenance or, worse, if selling the house. Is there any independant body I can approach to get a definitive ruling on legitimacy?, Rightly or wrongly, I'd have no confidence that it would get an objective assessment from a CORGI installer, who'd have an obvious commercial bias and likely (not sure quite how to put this) a negative 'jobsworth' disposition to boot.

Egremont.

[Andrew Gabriel]

It would be for the manufacturer of the multipoint to say if it was suitable for any non-standard use. However, I think it unlikely they would say anything other than 'No', regardless of the real answer, unless it was likely to lead to many more sales, in which case they might spend some time investigating.

[Aidan]

You'rrrre all doomed. Dooooooomed I tell ye.

There is information on boilers kettling on the Fernox website. The water heater will have collected some scale during it's previous application as a water heater and this may now be causing local overheating. You may cure it by descaling the heater, as per Fernox's recommendations. The pump location is also relevant, it would probably be best on the cooler return than on the flow.

A heating pump will supply a much lower differential pressure than that required to maintain the minimum flow rate; you're probably not getting the specified minimum flow rate to keep the water below boiling. You may also get cavitation in the pump due to inadequate NPSH, so you may later find that it devours pump impellers.

Well you know now. If there were any related problems I think you would find your insurers would refuse to payout. If there were any unrelated problems, I think your insurers would claim they were related and then refuse to payout. You'd have to prove otherwise, which may be difficult given that your money would all be tied up in repairs and alternative accomodation.

Instead of spending time and effort in solving the problems with this "unusual" application, why don't you just buy a boiler? The boiler-related problems will have been solved for you by proper Engineers. It will be cheaper, in time, capital costs, gas efficiency, electricity costs, maintenance costs and repair costs.

What is a Norwegian post? I have Googled but only get a Scandinavian mail service. I'd imagine it must be Dr.Evil complimenting Dr. Evil on his profound wisdom.

Large steps, away from it.

How can Dr. Evil possibly know there is nothing wrong with it? Unless he's inspected the installation, he cannot know. The installation has been carried out by someone who appears to be technically naive, so it is quite likely that there are other problems that he hasn't asked about because he is not yet aware of them.

If this was a "Norwegian post" Dr. Evil would have installed it. Fishy.

There's a thought. You'rrrre doomed. You'rrrre allll dooooooooomed.

[Doctor Evil]

They always say no. Yet in the USA these heaters are regularly used using "fresh water" pumped around a non-ferrous heating system. They don't have any problems

For Ed to say is a hash of an install is totally ridiculous. What goes for Corgi doesn't go for others and vise-versa. An intelligent engineer assessing the risks can see on a thermal store this works totally safe and adequate.

[Egremont]

The manufacturer, Thorn, is sadly no loger with us. I agree it makes no difference - there is no commercial benefit to them in sanctioning this application.

I can understand Ed's reaction, I initially had similar doubts about this when I first raised this a few weeks ago. I've since modified my view somewhat - of course he may not!.

Not really - how come?. And Yes - what is a Norwegian post?.

Egremont.

[Andrew Gabriel]

Well, they are part of Potterton now, and still manufacture a couple of models roughly based on the older Thorn/Main water heaters.

[Ed Sirett]

I meant by Norway that the post was a "troll" to get other posters (such as myself) to declaim the installation. Then watch with amusement the resulting flame war.

For myself, I have decided that drivel baiting should be generally avoided along with bear baiting and bull fighting , once or twice is fun but afterwards it becomes predictable.

[Ed Sirett]

Regulation 8(2) Explains that appliances are not to be installed in such a way as they _might_ constitute a danger. The onus is entirely on the installer when they ignore the manufacturers instructions.

IANAL but I'd bet on the prosecution on this one.

[Ed Sirett]

Regular readers and posters will know that I'm not a jobsworth. There is no way that I'd give a safety ticket for this installation.

Other registered fitters here will have their own view but I'd be surprised if they were to take a substantially different view.

It may well be that heating boilers in other countries work by these means. I.e. they are controlled by the flow of primary water. However it is quite likely that the hydraulic resistance of those units is much lower to allow operation without kettling.

As for the regulatory aspect, IMHO, it's simple case, it's outside the approved use for this appliance.

As for Drivel's: An intelligent engineer assessing the risks can see on a thermal store this works totally safe and adequate.

Normally I don't rise to the bait... but I _do_ have a degree in engineering and he probably doesn't.

Even if all this were approved by the manufacturer. Then you would have a heating system based on a low water content balanced flue natural draught appliance with a permanent pilot. That's about the same as say an Ideal Elite. anyone want to look up the sedbuk for that? I thought Drivel was all for conservation?!

[Dave Plowman (News)]

And surely a domestic multi-point just isn't designed or intended to run for the long periods a domestic boiler is?

[Egremont]

Ed, thanks for your views on this - I've read enough of your posts to know you're indeed not a Jobsworth.

Like you, I don't like getting drawn into circular discussions. However I'll comment that some of the points you raise are at least open to some interpretaton, For instance, does re-plumbing the application constitute an appliance installation (the actual boiler installation was done years ago), what definition of manufacturers approved use have been violated (is there a tighter definition somewhere than just "water heater"), and what manufacturers instructons have been ignored?. .And kettling apart (if this is really an insoluble problem) it's not really clear to me what, realistically, the potential safety problems would be. Anyway it's clear which side of the line you come down on these.

BTW I raised some of the points surfacing (like energy efficiency and potential longer boiler runs) some months ago when this subject first came up (search medway multipoint & diysos).

Egremont

[Doctor Evil]

I see no danger at all. It is perfectly safe. The gas safety controls have been left intact.

If he put a pipe stat on the flow from the boiler this can act as a high limit cutting out the pump, and add to safety. The pump operating activates the burner.

I would not, because if a qualified and experienced gas/heating man gave his view on it technically he would have to say that:

1. There was no safety risk whatsoever.
2. The makers gas safety controls were no compromised (and not even touched by the installer)
3. The installation is common in other countries, with no safety risks.

If you can't figure those three out, get out of the game. Gas/heating engineers were taught to think, not just follow blindly makers instructions. Ring up a technical dept, and if they say over the phone do this, and there was an adverse effect you don't have a leg to stand on, unless in writing. So, asking them sometimes means dickety nothing.