Conventional boiler.... comeback Combi all is forgiven!

[TellMeWhy]

You are referring to the q60. A boiler over a metre wide! !!

Have you seen the size of a 400 litre unvented cylinder? Never mind the cost.

60kw!!!!.

Blimey.

Are you suggesting such a boiler for the OP?

Yes, as it fits the bill, is cheaper overall, less complex that a system boiler and all the unvented cylinder controls and valves. A domestic gas meter will cope.

 

[TellMeWhy]

Yes seen and fitted lots of them, dont confuse any high flow combi with a well installed well sized unvented cylinder, theres no comparison.

Yes and they belt out the hot water. I doubt you have fitted the large 25 litres ATAG, or seen one. They are a bit rare.

A large combi will pour out 250 litres of hot water in 10 minutes. 500 litres in 20 minutes. Drain a 400 litre cylinder of hot water and then see how long it takes to reheat and you have no hot water in the meantime.

Any combi fitted in a property with more than one bathroom would just be wrong.

You need to know how to use figures, and stop putting a wet finger in the air. And stop taking notice of plumbers at the trade counter.

 

[TellMeWhy]

And what's it's minimum flow rate when running the heating ?
...
...
That's a criticism I have of all the boilers I've looked at the manuals for. Regardless of how low they can range, they still require a minimum flow rate based on max output (or thereabouts). Since combis are almost invariably oversized for the heating (or they are sh*te at making hot water), this means you are automatically installing something that's not suitable for the heating side.

The top ATAG is about 51 kilowatt. It modulates down to about 8. The only way to get a condensing boiler working efficiently most of the time is isolate it from the heating system operating at the most efficient hydraulic environment and then drip feed heat to the heat emitters. That means a well designed thermal store system.

As you wrote, modulating very low is a general problem with all boilers not just large ATAGs.

 

[ALEC1]

Or fit weather comp which makes the house the thermal store, and by the way the way it's done in Germany and Holland

Thermal stores have no place in a modern gas heating system...

 

[TellMeWhy]

Back on topic, clearly that cylinder is undersized. A upgrade sounds like its required. To make life better for now, try running that cylinder from first thing in the morning to last thing at night, set it up to come on an hour before you get up, example could be 6:30am till 10:30pm. That way when any water to drawn off the boiler is there to get that cylinder back up to heat.

In the long term, get the boiler upgraded, a modern high recovery unvented cylinder installed, you'll be impressed by the results, plus you can loose that shower pump as unvented offers near mains pressure hot water

A well installed stainless steel cylinder will last for years, the average run time of a boiler is 15 years, but if the system it's attached to is a good on it'll give you many years of fault free service, just make sure you opt for a good brand that's a simple as possible, that way there's less to go wrong.

Trouble free? Where? With expensive zone valves, expansion vessels and cylinder pressure controls? And all this complexity will not go wrong over 15 years? And that is just on the cylinder alone. What parallel universe are you in?

The system boiler to heat this cylinder is not much different to a combi. A combi has a 3-way valve and plate heat exchanger over and above. A system boiler and an unvented cylinder is far more complex, and expensive and offers no more than a large combi in performance. Like a combi an unvented cylinder is depended on the mains water pressure and volume, so no gain there. Anyone with the slightest engineering know-how can see this.

 

[TellMeWhy]

Or fit weather comp which makes the house the thermal store, and by the way the way it's done in Germany and Holland

Thermal stores have no place in a modern gas heating system...

Doing a search I see SimonH2 is the expert on thermal stores. I will let him come in and tackle you. BTW, I am putting a thermal store in an out-building heated via a boiler ion the main house. The perfect answer for the situation.

You appear to think weather compensation on a boiler will stop boiler cycling? How can it magically do this? Most top boilers have the weather compensation built in. This not a black Teutonic art.

 

[ALEC1]

Because weather comp seeks to match heat loss to heat load, by varying the flow temp.

Obviously it's dependent on emitter type, but an UFH install I have has three-4 hour burn time in winter, coming on 4 timed a day

The oldest weather comped boiler I have seen is a 14 year Viessmann, one sensor developed a fault, the DHW one as it sits in the cylinder

 

[SimonH2]

The top ATAG is about 51 kilowatt. It modulates down to about 8. The only way to get a condensing boiler working efficiently most of the time is isolate it from the heating system operating at the most efficient hydraulic environment and then drip feed heat to the heat emitters. That means a well designed thermal store system.

As you wrote, modulating very low is a general problem with all boilers not just large ATAGs.

So the answer to fixing the problems by having a high output boiler capable of heating a decent DHW supply, is to add a cylinder and all the controls that going combi was supposed to do away with !
It's much, much less of a problem with a system boiler and stored heat - for the simple reason that such a setup can be correctly (or better) sized for the demands. You'd need an abnormally high heating demand with abnormally low DHW demand for any combi to come even close to being optimum for either - and in a typical domestic environment it will be badly (often very badly) matched to one or both of it's duties.

When a mainstream manufacturer, with a mainstream boiler, comes out and supplies a boiler where the CH can be fully TRV, with a modulating pump, and the boiler will cope with an arbitrarily low flow rate - then, and IMO only then, will that stop being a problem. I gather some combis do now have modulating pumps - so at least there's some progress in the right direction.

Thermal stores have no place in a modern gas heating system...

I thought that blinkered attitude would rear it's head sooner or later.
Anyone who states that "<something> has no place in ..." like this has just demonstrated that they are not competent to be advising customers on what they should be installing. You've just admitted that you would not consider <something> regardless of situation, and your prejudice would probably prevent you spotting situations where it was appropriate - so your advise is worthless. Unless, like the financial services industry are required, you state upfront that you can only recommend certain products and that there may be other products more suited to the customer - so the customer knows they are getting incomplete advice.

 

[Corgigrouch]

But Dr Drivel, You don't have an ATAG boiler or a thermal store......Do you?

 

[TellMeWhy]

The top ATAG is about 51 kilowatt. It modulates down to about 8. The only way to get a condensing boiler working efficiently most of the time is isolate it from the heating system operating at the most efficient hydraulic environment and then drip feed heat to the heat emitters. That means a well designed thermal store system.

As you wrote, modulating very low is a general problem with all boilers not just large ATAGs.

So the answer to fixing the problems by having a high output boiler capable of heating a decent DHW supply, is to add a cylinder and all the controls that going combi was supposed to do away with !
It's much, much less of a problem with a system boiler and stored heat - for the simple reason that such a setup can be correctly (or better) sized for the demands.

Read what I wrote.

You'd need an abnormally high heating demand with abnormally low DHW demand for any combi to come even close to being optimum for either - and in a typical domestic environment it will be badly (often very badly) matched to one or both of it's duties.

Thais just plain untrue. Experience a large ATAG and you will think differently. I know of one in a large London flat that supplies a body jet shower. He can stay in it all day. A cylinder to cope would have been a small commercial job. The combi was the perfect solution.

When a mainstream manufacturer, with a mainstream boiler, comes out and supplies a boiler where the CH can be fully TRV, with a modulating pump, and the boiler will cope with an arbitrarily low flow rate - then, and IMO only then, will that stop being a problem. I gather some combis do now have modulating pumps - so at least there's some progress in the right direction.

The old large water content cast iron boilers fitted that bill, as do some current oil boilers. To get the water capacity with a modern boiler you have to add a thermal store as an extension of the boiler with the boiler only heating the stored water - as in the old cast iron jobs.

 

[SimonH2]

Trouble free? Where? With expensive zone valves, expansion vessels and cylinder pressure controls? And all this complexity will not go wrong over 15 years? And that is just on the cylinder alone. What parallel universe are you in?

The system boiler to heat this cylinder is not much different to a combi. A combi has a 3-way valve and plate heat exchanger over and above. A system boiler and an unvented cylinder is far more complex, and expensive and offers no more than a large combi in performance. Like a combi an unvented cylinder is depended on the mains water pressure and volume, so no gain there. Anyone with the slightest engineering know-how can see this.

Zone valves are simple and cheap - understandable by almost anyone that should need to work on a system, and available off the shelf at your local plumbers merchant. If they do fail, they also normally have a manual override so the end user can deal with common issues themselves while waiting for the pro to come and fix it properly.

I agree that the valving on an unvented is non-trivial (another plus point for an OV thermal store IMO), but the much maligned open vented DHW cylinder is quite adequate for many people if they separated what they need from what they think they need. That's not a suggestion that everyone should make do with low pressure DHW, but most people could if they just chose fixtures more carefully.

But where I do take issue is the suggestion that a combi is no different than all this rolled up into one box - it isn't. Once you roll it up into one box, you have a cramped and often hard to work in system, where leaks fall on teh electrics, using manufacturer specific parts that aren't off the shelf at your local plumbers merchant. What's more, the whole box is often ridiculously over-complicatyed for what it does, meaning that your choice of who you get to fix it becomes quite critical (too many plumbers* really don't understand the internals).
Doffing my landlords hat, the more that's integrated into the one box, the less I'm able to do to deal with tenants' problems. In the flat, the boiler has a separate combustion chamber cover - so arguably I'm able to take the outer casing off and work on (say) the electrics (eg time clock problem) as long as I don't do anything related to the gas system or it's safety systems. I had to do that a couple of years ago to diagnose the very simple fault (failed microswitch on the flow sensor/diverter valve) that the supposedly professional heating engineers (note the plural) had failed to diagnose during about 5 visits over the course of a week. That's not the case in the house where the outer casing is also the front of the combustion chamber.

And there's one thing the combi cannot do, no matter how perfect it's design - have an immersion heater that acts as a useful backup for when it breaks down. That's something just about every other option supports, or even has as standard.


* Problem here. To the vast majority of the public, all people who deal with pipes are plumbers - whether they be jobbing pipe fitters, boiler technicians, heating engineers, etc, etc. So the average Mr/Mrs Public doesn't really know that they need a particular sort of "plumber" to sort their boilers - and lets face it, we've heard plenty of tales on here alone about "plumber couldn't fi the boiler" which will mostly be because the wrong sort of plumber was called.

Doing a search I see SimonH2 is the expert on thermal stores.

Oi, no I'm not. I've taken the time to understand how they work (unlike, it seems, a lot of people here), but I'm most definitely not an expert.

 

[Dan Robinson]

I can't believe anyone would be dumb enough to fit a q60 in a 4 bedroom house.

Welcome back Drivel. We missed ripping your ridiculous ideas to spreads.

 

[TellMeWhy]

But where I do take issue is the suggestion that a combi is no different than all this rolled up into one box - it isn't. Once you roll it up into one box, you have a cramped and often hard to work in system, where leaks fall on teh electrics,

This is like the wet finger in the air plumbers. Do not compare quality combis with cheap and nasty B&Q models. Quality combis are generally easy to work on as they have superior design. Do the figures and costings and large combis walk all over a system boiler (not much less complex than a combi) and a complex control setup and unvented pressure controls. Replacing some unvented pressure controls can be very expensive.

I think the next added function on combis must an integral immersion to give low flow hot water when the gas controls have failed. This appears not a problem overall to the public as combis outsell all other types of boilers. Few are bothered about a few days of no hot water as kettles are do for a few days.

 

[ALEC1]

Simon, a thermal store, which has to run at and kept a very hot temperature, should not be confused with a buffer vessel that allows all sorts of heating and hot water permutations

Are you confusing the two?

 

[Tom1981]

Back on topic, clearly that cylinder is undersized. A upgrade sounds like its required. To make life better for now, try running that cylinder from first thing in the morning to last thing at night, set it up to come on an hour before you get up, example could be 6:30am till 10:30pm. That way when any water to drawn off the boiler is there to get that cylinder back up to heat.

In the long term, get the boiler upgraded, a modern high recovery unvented cylinder installed, you'll be impressed by the results, plus you can loose that shower pump as unvented offers near mains pressure hot water

A well installed stainless steel cylinder will last for years, the average run time of a boiler is 15 years, but if the system it's attached to is a good on it'll give you many years of fault free service, just make sure you opt for a good brand that's a simple as possible, that way there's less to go wrong.

Trouble free? Where? With expensive zone valves, expansion vessels and cylinder pressure controls? And all this complexity will not go wrong over 15 years? And that is just on the cylinder alone. What parallel universe are you in?

The system boiler to heat this cylinder is not much different to a combi. A combi has a 3-way valve and plate heat exchanger over and above. A system boiler and an unvented cylinder is far more complex, and expensive and offers no more than a large combi in performance. Like a combi an unvented cylinder is depended on the mains water pressure and volume, so no gain there. Anyone with the slightest engineering know-how can see this.

I live in this universe. I see plenty of boilers and systems that have never failed. I can look on my terminal and see just annual service after annual service, the customer can tell me they've never had a single issue. A well installed system can last many years. I've serviced an old Ideal Élan that was never serviced, never attended to in 15 years, if I un maintained boiler and system can do it, when can't a attended to appliance?

A Combi has a greater amount of weak points than a system, mainly due to the size of the kit squeezed into a ever decreasing case. I know exactly what a Combi and a system boiler is, what a unvented cylinder is. But I do know that some homes can not take a system, that's where the Combi comes into it. I have a oil fired Combi with a thermal store. If I could afford it I'd pull it out and replace it with a system, or better still a air source heat pump.

It's all down to personal choice, from what I've found to be a better way of doing things, you clearly like a Combi.