Combi Boiler and/or Megaflow

[Agile]

In that case it will be seriously over powered and consequently less efficient.

I am sorry you dont want to answer my question about the boiler cycling although its your perogative. That would have identified if the boiler is really under powered or just that the system is not designed or functioning correctly.

Oh well! Its difficult and frustrating being classified as an ignorant "plumber" who left school at 15 and is very thick and so plumbing was the only career available and so could never offer any useful or correct help!

Tony Glazier

 

[kevindgas]

OK chaps. Thanks for all your comments and ideas. It has been very useful. We are going for a heat only boiler with roughly double the power of the Minimiser. The combi idea was a complete mistake on my part. Sorry about that.

there is no mistake in asking the right questions to ensure you get the right equipment.

as tony has asked and not had a reply

a rough check of allowing 5,000 btu per rad and 15'000 btu for hot water will give you an approx heat requirement in btu divide by 3412 to get KW's if your boiler is more than 15% above this figure it is more than likely oversized

 

[Agile]

Kevins quick check is a little high and in particular the addition for water requirement should only be 6300 BTU according to the energy efficiency training we should all have attended.

However, Micky G is aware of an estate in Herts where they have just that system and he is not aware of any under powered issues there!

That confirms my expectation that your existing system has a fault rather than just that the boiler is underpowered. A 15 kW boiler will adequately heat a 4 bed detached house !

It always surprises me that some home owners prefer to follow their uninformed guesses rather than take professional advice.

Pearls before swine ?

Tony

 

[ChrisR]

A 15 kW boiler will adequately heat a 4 bed detached house !

Contentious! There will be some where it will, and many where it won't.

the addition for water requirement should only be 6300 BTU according to the energy efficiency training we should all have attended

That's 1.8kW. That would be grossly inadequate for someone using say a Megaflo with a 25kW coil, who uses a lot of hot water.

A 16.8kW boiler in many 4 bed houses with half a dozen people, could be disastrous!

Each installation has to be assessed separately.

And does anyone have figures to show by how much a 24kW boiler modulated down to provide 16kW is LESS efficient than a 16kW boiler working flat out?

 

[kevindgas]

Kevins quick check is a little high and in particular the addition for water requirement should only be 6300 BTU according to the energy efficiency training we should all have attended.
Tony

allowing my figures is actually not that far out as the water requirement not only allows for the heating of the water but also covers the water content of the pipework and associated heat losses
personally i have found this to be a very good guideline but it is no substitute for using a mears calculator for 100% accuracy
as for 6300 BTU i wouldn't ever go that low 10.000 minimum.

as i have had a few house over the years i can say from bitter experience a 16KW/55,000 BTU struggles in the winter and personally i invariably use a 16 KW in a 3 bed house but a 19KW in a 4 bed house

thats not to say that is correct everywhere don't forget I live by the sea not in central London where ground / air temperature is a little higher and so a margin is allowed for this

 

[BigBurner]

Big Burners suggestion would work in theory,

It works brilliantly in practice. I recently did one using an Alpha combi. The existing cylinder was a direct electric immersion unvented. A new full heating system was being fitted from being all electric. This saved a fortune on a new cylinder, no 3-way valve or complex wiring just a bronze pump on the cold feed to the combi. The recovery rate is exceptional, as all the boilers output is being pumped into the cylinder. No inefficient coils. Most makers have no problems having a combi rigged up this way - it must be a model that accepts a hot inlet through the cold inlet and is activated by a flow switch, not a pressure valve - most are these days.

This had the (flow) hot outlet connected to the cylinder by a Surrey flange into the cylinder's DHW draw-off tapping. The advantage was that when the cylinder stat switched in the combi as water was being drawn off, the combi's output was merged with the cylinders at the draw-off tapping. This meant it never ran out of DHW. It would drop to what the combi could supply when the cylinder was exhausted of heat.

It will work equally as well on a vented cylinder.

but manufacturers of both cylinder and boiler would not allow it,

You are guessing.

so any warranty would be dust. I would also question whether it complies with building regs, I doubt it.

Totally conforms. Just because they don't do it around your way doesn't mean it will not work. All it is doing is passing water through the DHW section and heating it as a combi does in the normal way.

In many cases it is highly cost effective and delivers far more DHW than just heating a cylinder via a coil and system boiler. It heats the cylinder top down not bottom up. Response is fast.

 

[Agile]

Each installation has to be assessed separately.

Sorry, I should have typed 6800 BTU as equivalent to the 2 kW which we are instructed to add to the heat load to calculate the boiler size required.

The logic is that the hot water is heated at different times to the heating.

Of course, there can be rare situations where greater boiler power might be needed such as when its tenanted to groups who fill it with more than the expected number of people. A three bedroom Norfolk house caught fire and a neighbour counted 18 chinese running out and not staying around!

To Kevin, obviously you will not go wrong by over stating the heat requirement, but housebuilders and heating designers are expected to use the official guidelines. They are only designed to provide the target figures with -1° outside and high winds will increase heat loss.

In this particular case we are told a little about the house and can deduce that its got good insulation and cavity walls and I expect that 16 kW will be quite adequate. Thats why I am so strongly suspecting there is a fault or design deficiency in the heating.

Tony

 

[gas4you]

It works brilliantly in practice. I recently did one using an Alpha combi.

You have never touched a boiler in your life

 

[JohnD]

I recently did one

To clarify, does that mean you installed one, or had one installed in your house, or what?

 

[kevindgas]

It works brilliantly in practice. I recently did one using an Alpha combi. The existing cylinder was a direct electric immersion unvented.

Post the pictures of this install !!!!!!!

 

[BigBurner]

Post the pictures of this install !!!!!!!

What don't you understand about the description?

 

[JohnD]

I recently did one

To clarify, does that mean you installed one, or had one installed in your house, or what?

I really would be interested in your reply.

 

[kevindgas]

What don't you understand about the description?

no pictures then!!!!! whats up can't you find a book with it in

 

[ChrisR]

no substitute for using a mears calculator for 100% accuracy

Roflol
TRy doing it longhand taking everything into account, you'll find the Mears iis wildly inaccurate.

 

[Agile]

No pictures then!!!!! Whats up can't you find a book with it in

BB knows that if he copies pictures from his brochures then we will be able to identify them.

He is scuppered on this one!

Watch him wriggling!