Medway Multipoint & Thermal Store

[DIYSOS]

In a post a few days ago, off-topic I mentioned (in Jest!) using an old 23KW gas multipoint water heater to heat a thermal store for a new CH system. It ended with me thinking I'd ditch the multipoint to make way for the cylinder. Err, well... see below.

First, a question relating to thermal stores. To promote a cool return temperature I'm contemplating that I may (or may not!) need to run my

1300mm/210L store at a lower temperature - say 70 degr rather than 80. To compensate, I was going to get Newarkcyl to add one or two alternative tappings a bit higher up the cyl for the space heating take-off. But I'm working blind, I don't have the foggiest what the existing return temperatures will be, or where to ask for the extra holes to be. Any ideas?

And one other thing, what is the difference between a 'vented' CH boiler and a multipoint - both just heat water don't they (:->).

I just couln't let go of the idea of a free (installed) CH boiler. There

*will* be a thermal store - the only issue now is what & where the boiler will be. If the council don't object on Part L (It's an existing boiler) then I'll hook up the multipoint. I now plan to put the store on the ground floor, under the stairs in the middle of the house. The primary pipes will run to the multipoint boiler along a path that will run close to the (ground floor) siting of any new Condenser boiler - if and when that happens. F&E tanks in the loft. Pumps near to the store (maybe).

At the moment I'm now planning on doing it as Dr Evil suggested - integrated with common primary water to boiler, store and radiators. There might be risk of the Fernoxified, sludgy brown water affecting the multipoit's rubbery flow sensors? but I'll take a chance. And as the store is now downstairs, in the event of a catastrophic leak upstairs at least the cylinder contents wouldn't add to the flood.

One thing that occurred to me is that the water in the multipoint would generally be hotter than if heating from the cold mains. And as the pressure via the F&E will be relatively low this might risk 'kettling"? For this reason I'd like the return temperature to the boiler to be cool. On the other hand, if fitting a condensing boiler (now or in future) then again I'd like the ruturn temperature to be cool to promote condensing. So, I've gone for a somewhat oversized 1300mm, 210L thermal store, Newarkcyl, to aid 'stratification'. Two stats as well - external as these give positioning flexibility.

And if the above is completely mad let me know!.

Thanks DIYSOS.

[DIYSOS]

Well no news is hopefully good news, as it might mean this plan isn't completely mad.

I can see I'm probably barking up the wrong tree trying to recalculate a position for hotter radiator take-off. Even so I'll ask for an extra tapping about 200mm above the normal take-off - gives me an option just in case.

Cheers - DIYSOS

[Doctor Evil]

I'll get back. First, does your multipoint have temperature control? If so what is the highest temp it supports?

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[DIYSOS]

It doesn't have a thermostat, and by implication I assume no temperature control (unless there is a cutout), other than indirect via the cylinder stats. This probably means it could have steam coming out it's ears if the cylinder stat mechanism were to fail... anyway I'd thought perhaps to maximise (within reason) the speed/volume of water through it, to minimise the temperature rise on each pass.

Following taken from an earlier post...

Heat input 30.27KW, heat output 22.7KW, raises 12.5L/min by 25C. Minimum operating head if connected to static tank 2.29M, max pressure 13 bar.

Installation guide is on bax/partsarena...then go Main... goWater heaters... goMedway... go Basic later production... go Installation instructions... go Mersey, Medway automatic & Trent... and there it is (Medway).

DIYSOS

[Kaiser]

If it's a Medway or Mersey it does not have a temperature control. If it's a Medway Super or Thame then it has a temperature control on the front lower panel.

[Kaiser]

6.5 litres per minute @ 50C.

[DIYSOS]

Oh I see a 'temperature control' as in 'knob' (;->)... well it doesn't have a variable temperature control as such though it does have a Winter/Summer knob in the bottom right corner (two modes only, "winter" = less but hotter water flow). It says it's a Medway Automatic on the lower panel. It doesn't have a Thermostat (BTW that would be the "Trent"?), and it doesn't seem to modulate - burners just seem to go on & off.

I believe it would have been installed mid 1980's - one clue is a reference to British Gas (ie post-privatisation) inside. And thankfully it's room sealed with a 12x12 inch hole behind that is the back flue.

DIYSOS

[BTW no relation to TV program of same name!]

[Rob Morley]

AFAIK no relation to me either - so why are you using my email address?

[DIYSOS]

... or to put it another way will heat 12.5L pm @ 25C like I said before. But I repeat not a Medway Super, no temperature selector, nor I believe a cutout.

DIYSOS

[Doctor Evil]

If you want a lower store temperature go for a larger store, as you are doing, but have a plate heat exchanger for DHW take off. These are so efficient they can run with a thermal store at 60-65C.

You are wanting the store below the boiler position and below the highest CH rad. This will mean two coils in the store. One for CH and one for the boiler. Both boiler and CH circuits will need to be pressurised. A system boiler comes complete with vessel. A separate vessel is needed for the CH circuit. The store needs is own F&E tank. Look at:

can install another coil for you. Have two stats on the cylinder to prevent boiler cycling. Or get a cheaper quote for a defined spec.

To use the multi-point with the store, a temporary measure by you I believe. You will need a temporary F&E tank for the boiler flow and return primaries. Run the primaries in 28mm. Have the pump on the boiler return to apply pressure to lift the diaphragm. Have a clamp on pipe stat on the boiler flow set to 80C, this cuts off the pump when 80C is reached. This will prevent boiler temperature overshoot. Have the cold feed from the F&E between the pump and boiler and the open vent teed off the flow pipe.

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[DIYSOS]

I've just had a quick glance before going out and I just wanted to check back...

True the store would be downstairs and the boiler upstairs... all very upside down. However the cold tanks would be in the loft, the small F&E tank could be raised if necessary. The store is not a combination and no radiators will be in the loft.

Is it still the case I need a pressurised system or wouldn't 100% integrated/vented be OK?.

Thanks again,

DIYSOS

[Doctor Evil]

That changes matters a little. The store will be open vented and direct CH and boiler primaries. No coils inside at all. The flow/return primaries is still the same as I explained except no separate F&E tank. Have a full bore isolation valve on the flow and the return pipes to the boiler to isolate when the condensing boiler goes in. Have an auto air vent on the boiler flow near the boiler to remove any air. Also have an auto air vent on the highest point on the rad circuit if possible.

One F&E tank in the loft, above the highest rad, feeding the store on the ground floor. As you want the store at around 65C, best have the CH return at the bottom and flow tapping taken off about 2/3 of the way up. You have two tappings. One 2/3 of the way up and one near the flow at the bottom. This gives you leeway. With high a temperature store you use the two bottom tappings for CH. With a low temp the top. You could connect both and tee into the CH flow. Have an isolation valve on each. Just turn one off and the other on. Then you are covered if things don't turn out as you thought. Simple and cheap.

If you have TRVs on all rads then fit a Grundfos Alpha pump on the CH circuit - no need for a flow switch. A normal pump on the boiler primaries.

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[Andrew Gabriel]

Main added a cutout to heaters later on (not sure which model it first turned up in). It is a standard self-resetting electrical disc type thermal cutout, connected in series with the thermocouple and flame failure solenoid, and positioned on the heat exchanger pipework at the top of the heat exchanger. (The heater itself won't self reset, as the pilot light would need manual relighting after the thermal cutout self-resets.) The earlier units use a bimetalic flame failure sensor, which couldn't have such an electrical cutout added. (Also, the bimetalic flame failure sensor just stops the main burner coming on, it doesn't cut the gas supply to the extinguished pilot light.)

[DIYSOS]

All in all I think things are taking shape. I like the idea of gating the store between condenser and high-temp mode!.

I don't actually see the multipoint as a temporary measure,

[DIYSOS]

I'm fairly sure my model doesn't have the electrical cutout you describe, didn't see anything on the HX pipes when I looked the other day*, plus it doesn't have an electrical connection (;->).

The flavour of cutout I was on the lookout for was something to turn off the boiler if the (water) temperature overshoots it's set limit. I think this is the thermal cutout you describe... according to the docs on baxi/partsarena, the Medway Super had such an "energy cut-off device". For better or worse, my model is too primative to have one of these.

*I had the case open as I wanted to remove the HX to de-scale it, long overdue after 20 years!. I followed my new-found instructions, undid the unions and screws, but when it came to it the flue part of the HX is immovably fixed into the wall-mounted flue housing - it really won't budge. So, all I can think of is wait until if/when it gets disconnected for replumbing. Then, connect into cold in / hot out pipes and somehow run descalant through the whole thing - flow sensors and all (do these contain a rubber diaphram and are they the 'weakest link'?) These will also have to survive 'Fernox' of similar, and anything else coming out of the radiators if my mad plan actually happens.

Wonder if you can still get spares for these things?

DIYSOS

[Doctor Evil]

When using the multi-point have the store temp 75C to avoid condensation inside the multi-point. Adjust the stats when a condensing boiler is fitted.

And

A store temp of 75C, The bottom stat is set to 75C, the top to 60-65C. Heating from cold. The full store gets to 75C and the bottom stat cuts off the boiler. In reality the top will be hotter than the bottom. When the store cools the boiler will be called when the top gets to say 65C. It then continuously heats the whole store until the bottom stat is 75C. Without this the boiler would cycle as the temp around the bottom stat moved up and down slightly. Some use one bottom stat but a Honeywell stat with a user adjusted switching differential.

Go to: http://www.heatweb,.com and down to Standard Heat Bank Options. This is a design tool, and select twin stats on the right. Then the wiring button top right. It gives you wiring diagram. It needs a smal cheap relay to operate properly - £5 inc base from Maplin.

It is also a good idea to fit two stats when using a quick recovery coil cylinder too.

The stats on the cylinder switch the pump. The pipe stat acts as a boiler stat. If the pipe stat fails then the cylinder stat will cut out the pump when the store is up to temp. If both fail and the pump is running, then it will boil over and you will be alerted.

Two sets from the store, each with a Grundfos Alpha. and a programmer/stat operating the pumps. The store is a wonderful neutral point.

It shouldn't be, the water heater, that is what boilers are officially called now (say boiler and you fail a CORGI exam), is already there, still heating water.

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[Doctor Evil]

baxi/partsarena,

There is no flow sensor, a rubber diaphragm that lifts when pressure is applied under, the last time I looked. It may have changed.

Spare are easily available.

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[Andrew Gabriel]

Mine doesn't have an electrical _supply_, this is all driven off the thermocouple. As I said, older ones don't have a thermocouple.

Can't recall what my model is -- it's about 5 years old and does have thermostatic control, and was the middle range of 3 which were sold at the time. My much older one (which did overheat and self destruct) was a Main Medina. The rather long 15mm pipe with lots of elbows which used to feed gas to it had 1974 stamped on it IIRC, but that could predate the Main water heaters as the house did have an Ascot at some point (neighbour still had one when I moved in, and I have a matching filled in flue hole in my outside wall in the same place).

My recollection (hazy) is there are just a couple of screws holding the top of the HX in place in the cabinet, and the flue connection is just a slide fit. I suspect if it won't slide out, you won't be able to take the thing off the wall. These units have all used the same flue for decades -- if a unit dies, the new ones are all designed to hang on the flues of the older versions, so you don't need to replace the flue too.

I descaled my old one a few times in the bath. I used kettle descaler and rather worryingly, the liquid came out bright blue or green (I forget), but it didn't seem to do any harm. The new one has internally coated teflon pipes IIRC, which is supposed to reduce the need for descaling and I haven't had to do it in 5 years -- flow rate still fine.

I suspect so. They were very popular and around for several decades. They still do a couple of models, although one is quite different (electrical with fanned flue).

[DIYSOS]

OK I mis-understood, mine does have a thermocouple but I didn't see anything on the HX pipes, I'll check again sometime in case I missed it.

The flue is jointed such that the whole unit will slide out, but the joint set back several inches from the wall bracket. The HX flue is tight into this bracket - no scope to lift & pull. Maybe with brute force it would give, but I'll go for pumping descaler thru the lot. Interesting to see what colour the residue will be!.

BTW the model in my series with a thermostat was called Trent.

DIYSOS

[DIYSOS]

OK now I understand how two 'stats work - thanks. What a helpful Dr Evil you are!.

inside the multi-point. .

This may well be sound advice... however by design mains water into this boiler (sorry water heater) would be stone cold anyway?.

Thanks all for your input, I think I've got enough now to be going on with. I'll be spending the next week or two doing the bread&butter stuff of installing radiator circuits, by which time I should hear back from the LC. With the pumps + immersion (new circuit) to install, I'll probably raise a part P building notice anyway (or was there some dodge involving 3 pin plugs?).

... and I plan to drop my pseudonym of DIYSOS for any future topics - poor name choice on reflection. There will be a no-prize for identifing me under my new guise (probably my proper name!).

Bye DIYSOS