New boiler needed: combi vs standard?

[pototea]

Hello!

I am getting a loft conversion done and need a new boiler as the existing one is barely enough to cover the existing house.

I seem to be finding a lot of people who are against combi boilers (what my existing is) and I honestly don't know why.

My house will have three bathrooms once the loft is converted. It is unlikely that there will be more than two showers used simultaneously.

I understand that the difference in flow rates is 15 litres/min for a combi and 55 litres/min for a standard boiler with hot water tank.

My issue with a standard boiler is the following:

1) I do not want the hot water to run out half way through the shower as has happened in EVERY SINGLE house I have ever lived in.
2) Lack of space! I have a long narrow terraced so where would the hot water tank go once I get the loft converted? Loft space and hallway space will be very restricted.

Does anyone have any advice?

What boiler would you get if you have a three storey house with two main bathrooms?

 

[breezer]

a combi can not run two showers at the same time.

it relies on the incoming water pressure, which it heats instantly. so if there are 2 showers running the best you will get is half the pressure at each.

as you have as combi, turn the shower oin , now flush the loo, what happens to the shower?

 

[simond]

As we have done to death here before, the water horsepower into the property (flow and static pressure) measurements are important before fitting a mains pressure system.

The flow problem is easier to rectify than a low pressure scenario.

There is nothing wrong with a hi flow combi providing you appreciate its lmitations, which, by the sound of it, you do.

Try looking at Worcester Bosch Highflow 440, (wait for the rumoured 550) or the Vaillant 937, Viessmann 333.

The ACV Heatmaster 35TC is the big daddy, but these are £2500 ex VAT specialist item and a few other components needed so are in a different league.

Please let us know the static pressure and flow rate otherwise we are all pi**ing in the breeze.

And finally, you can't beat getting some experienced installers round for a quote. Sometimes this is where to start for the best.

 

[pototea]

Thanks for the quick replies!

I have already got two plumbers round for a quote.

Neither of them have specified what the incoming pressure is in their quote so I will call tomorrow to ask.

I assume it is them who need to provide this info?

 

[Agile]

As we have done to death here before, the water horsepower into the property (flow and static pressure) measurements are important before fitting a mains pressure system.

I recommend a boiler with at least 50 horsepower to run two showers if the mains flow rate is sufficient, say 20 li/min with a dynamic pressure of at least 0.5 Bar.

A possible variant on a combi would be to use a small heat only boiler for the CH, probably 12 kW or 15 kW as required and for the hot water a Rinai instant water gas heater. They do some very nice small but powerful models which are more expected to be used in commercial applications like restaurants etc.

New electric shower(s) might enable you to keep your existing combi boiler though.

Tony

 

[BigBurner]

a combi can not run two showers at the same time.

They can you know Maybe your combi doesn't. Some high flow jobs can do three bathrooms.

 

[BigBurner]

I understand that the difference in flow rates is 15 litres/min for a combi and 55 litres/min for a standard boiler with hot water tank.

55 litres/min? Very rare! Combis can do 22 litres/min, which is about the same as most cold tank/cylinders. Some floor standing jobs can do 45 litres/min if the cold supply can handle it. The HeatMaster was mentioned, as does the Gledhill Gulfstream.

Look at the Mikrofill Ethos 54kW. It will give 22 to 23 litres/min for ever, as with the Eco-Hometec.

As mentioned the separate Rinnai multi-point is a good option and can be fitted outside.

Keep your existing combi and buy another to do the extra bathroom. That is a cheap way of get what you want. Depending on models you cab join the outlets for the bath using check valves and an shock arrestor. You can also split up the CH system (zoning) and have upstairs off and downstairs on, saving energy.

Take no notice if ignorance towards combis. Good quality combis are brilliant. Many plumber don't like them as it means less work installing bulky cylinders.

 

[Nixt]

Take no notice if ignorance towards combis. Good quality combis are brilliant. Many plumber don't like them as it means less work installing bulky cylinders.

No that's the reason they do like them. What they don't like is having that call on a Sunday or over Xmas if it packs up and the customer has nothing. Of this many people have many years experience. But as you have no practical experience, Drivel, it won't apply, hence your ill-informed generalisation.

 

[simond]

Harsh but fair, Nixt.

 

[BigBurner]

Take no notice if ignorance towards combis. Good quality combis are brilliant. Many plumber don't like them as it means less work installing bulky cylinders.

No that's the reason they do like them. What they don't like is having that call on a Sunday or over Xmas if it packs up and the customer has nothing.

Don't fit poor quality boilers, which most are. You get what you pay for. Fit an Atmos and see if you get call backs. They are the best and not expensive.

 

[Nixt]

Don't fit poor quality. Get what you pay for.

As a retired design engineer you should've noticed that the days of bespoke components in combi's are long gone. The extra paid for top brands only guarantees a well advertised badge, scores of suits and hundreds of engineers on the road burning fuel unnecessarily. Any real quality/reliability benefit is harder to notice if they are installed to the best standard.

 

[BigBurner]

As a retired design engineer you should've noticed that the days of bespoke components in combi's are long gone.

I am not a design engineer. Look at an Atmos or Viessmann. Have you heard of them?

Those who cry about combis fit garbage like baxi/pott. Biasi, Ravenheat and other assorted tin.

Look at an Atmos 32kW combi that gives over 14 litres/min. Around £900 and comes with weather compensation and OpenTherm compatible. Then look at what it doesn't have, which makes it stand out. No 3-way valve, no plate heat exchanger. The pump can fail and the DHW still works.

Open the case and all is before you - a doddle to work on. So simple. That is if you ever have to fix one of course.

You get what you pay for!!!! And the Atmos is dirt cheap for about the best boiler in the world.

 

[Agile]

As a retired design engineer you should've noticed that the days of bespoke components in combi's are long gone.

As I gather, you were a design engineer working on the burner parts of commercial boiler, that is before you retired!

Tony

 

[Nixt]

Those who cry about combis fit garbage like baxi/pott. Biasi, Ravenheat and other assorted tin.

Look at an Atmos 32kW combi

There are plenty of complaints with all brands. Your favourite makes sell next to f/all by comparison so can't be properly assesed for failures. There have been many bi-thermic type combis sold and where some give increased reliability they can furr up easier. They're not really better or worse.

In any case the point I'm making is about your comments about installers reluctance with combis (which Water Systems has copied word for word on Screwfix forum).

Combi boilers were developed in the 60's and mass produced by the 80's. However it was only after they became the cheapest type of system did they become the most sold. Any installer will have by now had plenty of combi experience so it can hardly be seen as new technology. Indeed in these days of thermal technology, starting a gas burner every time you run the cold main to get HW is about as new an idea as using an internal combustion engine to transport yourself.

 

[simond]

The Atmos may be the best boiler in the World. Frankly their installed base is far too low to make that claim.

I don't see Atmos making any significant claims for reliability neither does their warranty.

I think it is disingenious for a person known not to fit any boilers and who obtains recent product knowledge from scouring internet spec sheets to pontificate in this way.

By all means lets hear the fruits of your research, Water Systems (or Big Burner as you now call yourself), I for one are not saying you are stupid, quite the opposite.

Those of us in the trade can tell you have no experience fitting this stuff, why pretend otherwise? It is sending out the wrong message to folk who think they are getting opinions from tradesmen.