BOILER SYSTEM

[Azazel]

Tony,

You say some of the allegations regarding combi boilers i put forward, don't stand up to investigation. I'm curious, which one(s)?

Do you actually believe a combi can cope with the hot water demands of a family with teenagers? (if you had an ample mains flow rate)

And I know I seem to picking an argument with you, but i'm not, i'm just interested in what someones views are who fits combis.

Cheers

 

[Agile]

I agree with you Belboz, a combi would be a waste of money in your situation.

A new heat only boiler and a new cylinder would probably come out at about the same price as a 35 kW combi. New cylinder required with new boiler.

A few arguments against combi boilers;

More bits and pieces in one place (the boiler) thus more to go wrong. And when a combi breaks down, you could lose both hot water and heating. At least with conventional systems you usually/should have an immersion heater to heat hot water in the cylinder, if/when conventional boiler breaks down.

Modern combis are very reliable so rarely break down if installed on a clean system.

Combi boilers have to raise temperature of incoming mains water around 30-40 degrees in a short time, this uses an awful lot of gas.

Sorry, but the same amount of heat is required however you heat the same volume of water!

Imagine you go to the toilet, then use the basin to wash your hands, and you've got a combi fitted. So you turn on the tap, combi fires up. Water is heated as it flows through boiler, so your wasting cold water as you wait for hot to come through.

You also waste water waiting for hot from the cylinder! Some combis have a DHW preheat or HW storage.

Also you are running your boiler at full rate, lots of gas been burnt, then you finally get some hot water. Wash your hands, and turn off the tap. Now the pipe and boiler cool down, and the next person to wash their hands has to go through the same process.

Your cylinder has to be heated in advance and then spends the whole day losing heat! The EST think a combi is MORE efficient for a hot water supply.

If a system with a cylinder (vented/unvented) is designed and fitted correctly it will give more than ample hot water performance.

I can totally agree with that!

Why do Albion advertise the fact that thier cylinders can be used to improve exsisting combi cylinders? (http://www.albionwaterheaters.com/Superduty.html)

A storage cylinder can be used with any combi and is a very good design to use. Particularly for an old couple who can use the combi most of the time and just heat the cylinder when the kids come to stay and use the second bathroom fed by cylinder.


Oh yes! Albion want to keep selling cylinders when everyone else is using a combi and their sales are falling every year.

Because combi boilers are useless for multi output hot water systems.

A 35 kW combi can feed a reasonable shower and a kitchen tap at the same time as long as the flow rates are correctly adjusted. For a one bathroom house thats usually of little consequence. But I agree that a cylinder will supply a greater total flow rate than a combi.

As a side note, the Albion Superduty Cylinders are brilliant! Worth every penny.

The only time I would recommend fitting a combi, is for a single occupancy dwelling like a flat, or a small bungalow.

Yes they are great on the central heating side, but then so is a system boiler.

I would always go for recommending a system boiler, and high efficiency cylinder. Usually a Superduty, or a Centre Cyl, which has a 'pancake coil'.

I know I went on a bit, but thats how strongly I feel about combi boilers.

Hope that helps.

I dont want to argue either but as you asked!

I dont usually fit boilers, but its often me who is advising people to keep a storage system when they all want a combi like everyone else and all the installers are just wanting to fit combis.

I have no axe to grind! All I want is for everyone to have the system which is most suitable for their circumstances and their property regardless of cost!

In the last 10 years I have probably fitted more heat only boilers than combis and thats in London where 95% of boilers are combis!

I consider that I keep a totally open mind! I can fix any combi or any heat only boiler so I dont have any fear of "complicated" combis breaking down which used to be a reason for many installers to stay clear of combis 10 years ago. As far as I am aware the reliability of each different system is probably about the same when controls and MVs are aded on a stored system.

Time for a curry now!

Tony

 

[Azazel]

points noted, though i'll not necessary agree!

cheers Tony.

 

[BigBurner]

Why do Albion advertise the fact that thier cylinders can be used to improve exsisting combi cylinders? (http://www.albionwaterheaters.com/Superduty.html)

Because combi boilers are useless for multi output hot water systems.

This posters experience of combis is at 100% odds to mine, and anyone else who knows this business. High flow combi boilers can do two bathrooms, never mind two outlets.

If the cold water mains pressure and flow is good enough, combis are the first to look at in economy of installation and running costs. No standing heat losses.

I think this man may work for Albion.

 

[BigBurner]

(a) I have no gas connection anywhere near a potential combi site

(b) With teenagers, I am not sure the amount of hot water usage would be satsified with a combi

A standard G6 domestic gas meter will do the largest of combis. With teenagers a combis is the one to go as you never run out hot water. A quality high flow combi is what you need, providing the cold water supply is good enough. Then there is the point of no power shower pumps.

 

[BigBurner]

I would generally agree that a cylinder system is often better if there are several people living there.

However in your case you will need a new cylinder and a new boiler both in a new position.

It may well be that a high output combi boiler will be a more economic solution but that depends on the mains flow rate as to whether its fesible.

Take that anti combi posting with a pinch of salt! Some of those allegations dont stand up to investigation!

Tony

This is a lot of common sense. Stored water combis give great high pressure flowrates.

 

[simond]

We're installing an ACV Heatmaster 35TC at the moment, with an accumulator front end to boost the flowrate.

41l/m hot water, who needs an unvented???

 

[namsag]

Only Disadvantage of a combi with teenagers in house is , yes ok you have constant hot water but to have constant hotwater for say 3 twenty minute showers in a row is the heating will be off for an hour.
Not really a prob unless weather is horrendous outside. But its one of those things people forget

 

[D_Hailsham]

Currently, there is a back boiler fitted in the livingroom downstairs.

Keep it until it breaks down. It may be more environmentally friendly to change it to a condensing boiler, but it's your money and the savings will never cover the cost.

I am looking to move the cylinder to the ground floor so I can enlarge the bathroom. The new position of the cylinder would be directly below the current position in the bathroom. The idea being that the existing pipework can be extended in a straight line.

Do it; though it might be worth replacing the cylinder.

I BELIEVE I have a fully pumped open vent system. Both hooked pipes are in the relevant position in the loft and the system has two 2-port valves and a pump.

If the pump is before the two valves and one valve feeds CH and the other feeds HW, then you have a fully pumped system

It would be worthwhile installing better controls, e.g Digital thermostat or an optimizing programmer such as the Honeywell CM900 series. TRVs on rads, except the room with the thermostat, are also a good idea.

 

[BigBurner]

We're installing an ACV Heatmaster 35TC at the moment, with an accumulator front end to boost the flowrate.

41l/m hot water, who needs an unvented???

Excellent indeed. I believe these are around £2.500 to buy. That is cheap for what it is and what flow rates it provides. The total system cost must be quite competitive. They are very efficient and condense on DHW. I have seen a few working and was impressed. They are a tank-in-tank cylinder with a burner running though it making it a big floor mounted combi. They are the only product on the market like this.

Let us know how it performs

 

[BigBurner]

Only Disadvantage of a combi with teenagers in house is , yes ok you have constant hot water but to have constant hotwater for say 3 twenty minute showers in a row is the heating will be off for an hour.
Not really a prob unless weather is horrendous outside. But its one of those things people forget

The great thing about combis is that they are generally oversized for CH. In between showers the boiler can re-heat the rads back up very quick. Not such a downside.

 

[BigBurner]

Currently, there is a back boiler fitted in the livingroom downstairs.

Keep it until it breaks down. It may be more environmentally friendly to change it to a condensing boiler, but it's your money and the savings will never cover the cost.

He is doing some major works. He may as well get the boiler to a top quality condenser while he is at it. The clawback may be only 3 years as gas prices rise and rise.

It would be worthwhile installing better controls, e.g Digital thermostat or an optimizing programmer such as the Honeywell CM900 series. TRVs on rads, except the room with the thermostat, are also a good idea.

I would avoid those programmers as they are difficult for ordinary people to understand. I know of one house with a CM67. They put on electric fan heaters in the off periods as they can't figure out how to override the off period. An expensive waste of time and their heating bills are high because of it. Controls have to be simple to use.

 

[D_Hailsham]

He is doing some major works. He may as well get the boiler to a top quality condenser while he is at it.

Moving a cylinder form one room to the room directly below is not, in my opinion, major works; though there are obviously many contributors who would like to turn it into a major job. It's the "I have the solution;what's the problem" approach.

The clawback may be only 3 years as gas prices rise and rise.

Three years to recoup £2,500? A typical back boiler has a Sedbuk rating of about 75% and condensing boilers of about 90% so, assuming a current gas bill of £1000 pa, this would be reduced to £833; a saving of £167. But you are loosing about £125 a year interest on your money (say 5% in an ISA), so you true saving is about £42 pa. Ignoring gas price inflation this will take nearly 60 years to recover. If you have to borrow the money, it's even worse. And gas prices would have to rise exponentially to give a 3 year payback!!!

I know of one house with a CM67. They put on electric fan heaters in the off periods as they can't figure out how to override the off period. An expensive waste of time and their heating bills are high because of it. Controls have to be simple to use.

I agree that the CM67 is not the easiest to use; the CM900 is much better. However, it is obvious the installer did not spend any time explaining to his customer how to use the programmer. If they are always having to use fan heaters, the time periods need to be changed.

 

[BigBurner]

A few arguments against combi boilers;

More bits and pieces in one place (the boiler) thus more to go wrong.

The components are in any system, in a combi they are all in one box, in a cylinder system they are spread over the house. This is a moot point.

And when a combi breaks down, you could lose both hot water and heating. At least with conventional systems you usually/should have an immersion heater to heat hot water in the cylinder, if/when conventional boiler breaks down.

The Gledhill and ACV Heatmaster floor mounted combis have full electric backup for CH and DHW. A small electric instant heater can be in a combi hot water draw-off for DHW backup.

Combi boilers have to raise temperature of incoming mains water around 30-40 degrees in a short time, this uses an awful lot of gas.

Which is not a point.

Imagine you go to the toilet, then use the basin to wash your hands, and you've got a combi fitted. So you turn on the tap, combi fires up. Water is heated as it flows through boiler, so your wasting cold water as you wait for hot to come through.

Dead-leg pipe is the same for a cylinder system. Insulate the DHW draw-off pipe from the combi or cylinder.

Also you are running your boiler at full rate, lots of gas been burnt, then you finally get some hot water. Wash your hands, and turn off the tap. Now the pipe and boiler cool down, and the next person to wash their hands has to go through the same process.

But the DHW draw-off pipe will have hot water in it and the plate heat exchanger will be arm as well. Another moot point.

Standing heat losses of a 150 litre cylinder with 120mm of insulation is 6% over a year. Insulation thickness is less than 120mm so about 8% over the year. Quite significant.

If a system with a cylinder (vented/unvented) is designed and fitted correctly it will give more than ample hot water performance.

So will a well sized combi.

Why do Albion advertise the fact that thier cylinders can be used to improve exsisting combi cylinders? (http://www.albionwaterheaters.com/Superduty.html)

That setup is NOT improving the flow of a combi. It is using the high pressure section of the combi for the shower only and having the cylinder do the rest of the house like a normal cylinder/tank setup.

There are small cylinders that do improve combi performance. These tend to be small thermal stores. Alpha do one.

If a combi is sized properly there is no need for a booster. They are are an after thought for poorly specified combi system

Because combi boilers are useless for multi output hot water systems.

Many combis do two bathrooms.

As a side note, the Albion Superduty Cylinders are brilliant! Worth every penny.

They are good in the right setup, but not exactly cheap.

A direct cylinder, plate heat exchanger and bronze pump heating a cylinder does it in about 2/3 of the time and when the cylinder is empty the boiler will revert to an instant water setup with the boiler providing water heating directly. This is called thermal layer heating. It heats the cylinder top down. A thermal layer setup is cheaper than many fast recovery coil cylinders and far superior. Many stored water combis use this type of cylinder reheating.

I have used a combis water section to heat an unvented cylinder directly. A Surrey flange on the top DHW outlet tapping and the DHW outlet pumped via a bronze pump into the top of the cylinder. The cooler water returned from a tapping at the bottom of the cylinder to the cold water inlet.When the cylinder was cold the combi provided DHW at the rate in which the burner can provide. The system never runs out of DHW reverting to lower combi flow rate if the cylinder went cold.

The only time I would recommend fitting a combi, is for a single occupancy dwelling like a flat, or a small bungalow.

If the cold mains is good in flow and pressure, A combi should be the first choice in a normal domestic house.

Yes they are great on the central heating side, but then so is a system boiler.

A combi is a system boiler with a water section added. They share most components.

I would always go for recommending a system boiler, and high efficiency cylinder. Usually a Superduty, or a Centre Cyl, which has a 'pancake coil'.

I would go for a combi is the water pressure and flow is good enough, and if not good enough a combi and accumulator.

I know I went on a bit, but thats how strongly I feel about combi boilers.

Hope that helps.

It didn't help at all. It just showed your lack of knowledge of heating systems.

I just had to have some fun with this one.

 

[simond]

The Gledhill and ACV Heatmaster floor mounted combis have full electric backup for CH and DHW.

ACV Heatmaster has electric backup? Not on any we've seen........