Oso vs Megaflow

[meldrew]

I'm having a new h/w and c/h system fitted and the old copper cylinder system removed. Also boiler replaced as it's donkey's years old (Kingfisher, about 160btu I think, a big old beast)

I'm having a pressurised system basically 'cos everyone tells me the showers are far more powerful, and I need the airing cupboard space so want to lose the old copper cylinder. The new system will supply 3 bathrooms.

The old boiler is being replaced with two wall hung BoschWorcester Greenstar 29 condensers in tandem (my plumber says this arrangement will work better than a single boiler - any views? - and I want a 300litre-ish pressure cylinder.

I have the choice of oso and a megaflow so wondered - does anyone have views about the relative merits - especially reliability - of these two systems?

Thanks

 

[chrishutt]

There's not much to go wrong with unvented cylinders, so reliability isn't really an issue. Both are stainless steel, so shouldn't ever corrode. Safety valves, etc. will need an annual check, in same way as boilers.

The only point to make is that Megaflos normally use an internal air space to accommodate expansion, whereas most use an external expansion vessel. The internal air space needs to be recharged at intervals, but the procedure is quite simple and you can do it yourself. In any case you can use the megaflo with an external expansion vessel as an option.

 

[blahblah]

Know nothing about putting boilers in tandem but total 58.6kw- thats a lot of gas on full rate. Any other appliances off of the meter? And where are you putting a huge uv cylinder to save space???

 

[ChrisR]

Some warning bells ring:
Are you sure your mains water supply is up to the job?
What size is all the pipework?
What showers do you have?
How far is it from your gas meter to the boilers?
What size gas meter do you have - U6?

Both Oso and Megaflo have internal expansion.
One difference is the pressure reducing valve , M is set to 3.5 iirc and Oso 2.5. Will this matter? Oso can be up to 5 I think.

 

[meldrew]

Know nothing about putting boilers in tandem but total 58.6kw- thats a lot of gas on full rate. Any other appliances off of the meter? And where are you putting a huge uv cylinder to save space???

Yes it is a lot... I'm paying GBP105 a month on gas at present.... I dunno where it all goes, when you say any other appliances off the meter I suspect half the bloomin' street is running off my meter.... Trouble is its a biggish draughty old house, vicwardian over 3 floors.

I'm hoping all this work I'm doing to the house (inc insulation) will improve things also I am told that the efficiency of these condensing boilers (SEDBUK A?) will make a noticeable difference to the bills compared to the old floor stander. When that kicks off it's like a howitzer!

The uv cylinder goes in the garage (or what was a garage ) beside the new boilers

 

[blahblah]

Its a serious question. With the proposed boilers at almost 60kw thats your normal supply virtually gone- and on a short feed. What about cooker, fires? You may end up needing a second supply and meter, new mains supply and pipe work galore!

 

[meldrew]

Some warning bells ring:
Are you sure your mains water supply is up to the job?
What size is all the pipework?
What showers do you have?
How far is it from your gas meter to the boilers?
What size gas meter do you have - U6?

Both Oso and Megaflo have internal expansion.
One difference is the pressure reducing valve , M is set to 3.5 iirc and Oso 2.5. Will this matter? Oso can be up to 5 I think.

Thanks for this, I'll go and find this stuff out and post again tomorrow. Anything I can do to avoid a very expensive mistake will help, especially since I will have run out of money by end of the whole building project!
PS What is iirc?

 

[ChrisR]

If I remember correctly, iirc.
Check your expected minimum mains pressure with your water provider. Open all your mains cold taps - ie kitchen, garden + any others, and time how long it takes to fill a bucket to get a maximum flow reading.

I often find that I go to do a quote to fit an unvented cylinder where someone has set their heart on one, only to find that nobody bothered to check the mains, and that it's way under what's needed. In some cases another plumber puts one in, then has to get out of a fix.

More often than not, an old existing pipe from the road has to be replaced. Newish ones are often OK.

 

[silverback]

definately check your incoming water mains pressure and flow rate, are you having a full new heating system installed, you may find that if sized correctly you may not need such a high output boiler, if you are keeping the old system get it cleaned and get your plumber to use the whole house method to size the new boiler, if he is a half decent plumber he will know what that is, again you may find that your boiler is oversized for the heating requirements, there is no benefits to be had by over sizing the boiler, it will reduce the system efficiency.

I can never understand why plumbers recommend two boilers in a domestic situation, if a boiler is availiable with the correct output use it, new boilers will modulate so even a 60kW boiler will not run at full capacity, if not required to do so, in my opinion there is no benefit to having two boilers in tandem, it costs more to buy, you have two boilers to maintain and there is twice as many things to go wrong, the only time two boilers should be used is to acheive an output that cannot be reached with a single boiler.

 

[DP]

If the CH load can be spilt into living or bedrooms, it will make a difference to gas consumption by creating three zones (all independently controlled) i.e. bedrooms, living rooms and HW zone.

 

[silverback]

You can zone these areas with one boiler, you do not need a boiler for each area, zoning will improve efficiency.

 

[PTH]

I cant see any reason for putting two domestic boilers in tandem - to do it properly you will need sequencing control which most domestic boilers are not compatible with. Why not just put a larger boiler in if you have such a big heat load - You can get wall hung boilers up to 120kW now, and in condensing format. Look at commercial boilers on the normal manufacturers websites. One large boiler will be cheaper to install than two smaller ones, less space, and if you use a modulating one, it will probably be cheaper to run.

That's a huge heat loss for a house - It would be interesting to see the calculations!

 

[Paul Barker]

Chris has called it on the unvented. Victorian house? very likely you have a half squashed led mains, you're going to be very disapointed in three bathrooms with no cold storage. You might need to store a good deal of water and pump it to your unvented cylinder. In my 4 story detached victorian house of large proportions I use a small 30,000 btu old style boiler to heat a garden flat and a 40,000 btu Ideal Mexico heats the main house two floors. The top floor is not heated. Those two boilers cope admirably. Because we have a big house we have excellent gravity fed hot water. Our single 7lb led mains isn't given more work to do than it was designed for, and can still do it's job with no inconvenience to us whatsoever.

Our gas bill is quite high but nothing like your figures. If the boiler required much work I'd put in my favourite combi and turn the massive airing cupboard into a downstairs toilet/possibly shower room and make a small airing cupboard with a rad in it somewhere else in the house. There are so many oversized bedroom is victorian houses.

I do fit unvented, the oso is my preference because it has more usable capacity and costs less. You can flood it and use extrenal expansion for more capacity.

You'll probably have a trickle of water out of your bath taps if the wife puts the washing machine on and your son is having a shave in one of the other bathrooms.

I'm not really oposed to tandem boilers, after all when one brakes down you have still got heating and hot water, and the cost of two small ones is the same as one big one. The whole house sizing in your case has either gone wrong or you have a house four times the size of mine which I doubt, up north the big houses are big indeed, rich industrialists from West Yorkshire had these houses built for weekends taking the spa waters. The money was up North in those days.

 

[ChrisR]

I don't see anything wrong with using two boilers. If you set the thermostat on one lower, it'll only come on when the system water is lower temp - ie when demand is greater, so most of the time you'll be using one boiler within its range rating. Of course you can change the stats from time to time. Also - the obvious benefit - when one boiler goes wrong!!! Running the house on one boiler will be OK in all but extreme conditions.

Bear in mind that the coil on the HW cylinder will take 25kW, so if you need LOTS of HW in the winter you will hit the maximum output with your two boilers. Two reasonable showers running together will empty your 300 litres in 10-15 minutes.

A Keston 55kW I put in mostly sits switching on and off at its lowest possible output, and yes, it has gone wrong. People with houses that need lots of energy seem to shout loudest when that happens, too...

In my humble opinion the "Whole house method" for working out a boiler size is very sad. It implies that plumbers can't sit down for 20 minutes and work out the losses properly, with all the different factors taken into account. Surely it's worth it when you're spending thousands on a heating system?

By the way, DO check that whoever plans to install your two boilers is competent to wire them up, ie so that one can be completely isolated with the other still working! It will almost certainly be worth zoning the house too, with programmable thermostats for example.

-----------------
This post crossed with Paul's, just above. I agree that you're going to be very lucky if you find your mains pressure/flow adequate, even with a new mains pipe. It's a bit of a hassle replacing the internal mains pipes, too. Which means you'll probably end up with stored water and a pump, or two.
House heat losses can vary enormously - depending for a start on whether they're detached or not! Also remember that the design's outside temperature for a new system might be minus 2 or lower, with all (yes all, it's personal) the rooms at 21 or more. Northerners might manage with unheated bedrooms, but we don't DO cool in Surrey!

 

[silverback]

I knew someone would come out with that, if one boiler breaks down you can still use the other for heating & hot water, what a stupid way of doing things, would you buy and run two cars in case one breaks down, then you can use the other, there is absolutely no advantage to having two boilers, if there is one that is available with the correct output