Living with a combi boiler

Rosa_Damascena

ComicGeek said:

Not sure I've ever heard of an Ideal being called the Ferrari of the boiler world, but each to their  own.

More importantly, are they running hot, cold and gas copper pipework across the top of the underfloor heating pipework though??? What on Earth are they doing there????!

I don't understand what you mean? Are you referring to the 3 pipes on the bottom LHS of the first shot? Surely they have to cross at least one point?!

The boiler is 40Kw, that's the Ferrari reference and I was told that only Ideal do that size?

coffeehound

Time for a crisis meeting with your plumber, Rosa.

Rosa_Damascena

Jeepers_Creepers said:

Gulp - a Salamander Home Boost pump.

I suspect you may have misunderstood some of the things your plumber has told you, Rosa.

First things first - that is definitely a combi boiler, as it has 2 x 15mm pipes (cold supply and hot out) and 2 x 22mm pipes (CH flow and return) as well as gas and stuff. Combis, as you know, are only as good as the mains supply - the incoming mains flow and pressure is what drives the hot and cold supply to your taps, yes?

If your mains F&P are good, then you should be happy with the general performance. A 30kW combi, for example, will heat hot water at the rate of around 12 litres per minute, and - for a shower - this will be blended with around 6 lpm of cold in order to give you a shower flow in the region of ~18lpm. That's a lovely shower!

BUT, if you try and run another shower at the same time, then clearly you are only going to get half that, possibly less. In practice, the shower first in line will likely take the lion's share, so - really - you won't usually be able to have two folk showering at the same time. Three? Forget it.

That's a 30kW combi. You can get larger ones, such as 32, 36, and even 38kW (even more, but let's not go there). At 38kW jobbie (16lpm hot, with 8lpm cold = 24lpm = 2 x 12lpm showers) could well provide two showers at the same time, tho' each flow would be a compromise (12lpm is fine, but hardly gushing...)

Next 'BUT'. Combis are only as good as the mains supply, remember? So to have a 38kW jobbie delivering its max of 24lpm for two showers would depend on the cold mains supply being at least 24lpm, ideally more. This would need to be backed with a solid mains water pressure of at least 3 bar, I'd suggest, or else you could find flow rates dropping off very rapidly as soon as you open a second tap or shower, or are trying to supply this flow to a higher floor.

So, what mains flow and pressure did your plumber measure?

A warning sign for me and others is the potential requirement for that mains booster pump. These are usually only required when your incoming cold mains flow is below 12lpm (obviously). This is important; 12lpm - ie. the most that pump can (is permitted to) supply is just about adequate for a household. I fib - it isn't. It's pants. There is no way on this planet that 12lpm will provide 2 simultaneous showers of adequate value, and even one will be just 'ok' (ie, better than an electric shower, but that's not saying much).

Phew. That's one issue. The other is, what actual DHW system(s) are you going to be left with? It is usually - almost exclusively - a choice between a combi (instant DHW supply and no tanks) and a 'stored' type like the one you currently have (the hot cylinder). Your current one is 'vented, gravity-fed', but you can also get 'unvented' which is mains pressurised.

The 'stored' hot water types obviously need something to heat them up, yes? This is usually a conventional 'heat only' boiler - that's the 24kW jobbie you already have. Are you really keeping this other boiler as well?! That just doesn't make any sense - 2 boilers? Blimey. Ok, assuming you are not keeping it, what will now heat up your hot cylinder? Well, an immersion heater will, but that's daft as leccy is expensive. Can the new combi heat it? Well, er, 'yes', but only if the CH output from it (ie the flow to the rads) is diverted via a valve in order to also heat the hot cylinder when required, just like your current 24kW does. Frankly, that's a crazy system, tho', as you are asking this combi to also be a conventional boiler, so it can heat a tank of hot water as well as providing instant water to other places. Nuts!

It surely cannot be this they are thinking of?! So it begs the question - what IS their plan?!

1) Are they removing the existing 24kW boiler? Ie - is this going to be a 100% 'combi', instant DHW system.

2) What is the cold mains flow and pressure? We need figures!

3) What size - kW rating - is that Ideal combi?

4) Why do they need a 12lpm mains booster pump? WHY?

Now this is a helpful post but its taking time to digest. I get that its combi / DHW system and since the combi is in the rest has to come out.

Have I made the mistake of thinking this is simpler than it should be? The example of 38kW will give me 2x12L/min showers (mine is 40 so possibly slightly higher) - so that's upstairs sorted. Adding the pump to the downstairs shower in the as yet unbuilt bathroom gives me the 12L/min there I need. Right or wrong?

I do understand that for any of this theory to work out the water pressure needs to be a minimum of 3 bar though. I'll find out what it is next week now.

ComicGeek

Rosa_Damascena said:

ComicGeek said:

Not sure I've ever heard of an Ideal being called the Ferrari of the boiler world, but each to their  own.

More importantly, are they running hot, cold and gas copper pipework across the top of the underfloor heating pipework though??? What on Earth are they doing there????!

I don't understand what you mean? Are you referring to the 3 pipes on the bottom LHS of the first shot? Surely they have to cross at least one point?!

The boiler is 40Kw, that's the Ferrari reference and I was told that only Ideal do that size?

Yes, exactly that location on the 1st photo of the boiler. You will have screed poured on top of the UFH pipes, that will probably be about 65mm minimum to prevent the screed cracking. However, you have now got copper pipework running over that (the copper also can't be exposed in the screed as it will corrode, so needs wrapping) so the amount of screed is now considerably more to cover those pipes. Not only does this affect levels etc and mean more screed and cost, you now have more screed to heat up before the UFH heat comes into the room.

The normal way is to cut slots in the insulation, properly sleeve the pipework within these, then run the UFH pipework above with the minimum screed depth.

In hundreds of installations I have never seen pipework run above the UFH pipework.

Slight porkies on the boiler size, there are 4 other manufacturers that I can think of that do a 40kW combi, and probably others as well.

Rosa_Damascena

coffeehound said:

Time for a crisis meeting with your plumber, Rosa.

I'm not panicking, I have confidence in my team.

@ComicGeek - the screed will be at least 65mm, the floor level is high across the house so we are looking up to the point of the first breezeblock visible, ie after the darker lower section you see in the salamander shot. The piping isn't yet complete as there are a number of new radiators to be fitted (my request and everyone thinks its a bizarre one) so there is more pipework to come and insulation will come when the pipework is completed, with the plan for screed to be done on Friday. So whilst I get your point about more screed taking longer to heat up there was no way round this really but I don't see that pipes over the UFH tubes is the cause for alarm your reaction suggested. If it is they can be disconnected and forced over the copper pipes - although I think that will mean the level of the screed will come even higher and interfere with the fit of my kitchen, and that really would be a problem.

Tell me what is wrong with this plan.

Eta: the rest of the copper piping can be sleeved and buried in the Celotex I am sure. 

coffeehound

Rosa_Damascena said:

coffeehound said:

Time for a crisis meeting with your plumber, Rosa.

I'm not panicking, I have confidence in my team.

The fact that you aren't sure of the situation with the hot tank sounds alarm bells that you aren't being kept informed or included in the decision making.  It may be that you trust them enough to take the decisions.  The generally accepted rule though is that for multiple showers, a well-sized HW storage tank gives the best result.  I don't know if his proposed use of the 12 L/m pump will work as you describe; normally they are installed at the mains incomer to boost the whole house.  Perhaps it will all work fine and I hope it does, but the communications don't sound good enough

Rosa_Damascena

coffeehound said:

Rosa_Damascena said:

coffeehound said:

Time for a crisis meeting with your plumber, Rosa.

I'm not panicking, I have confidence in my team.

The fact that you aren't sure of the situation with the hot tank sounds alarm bells that you aren't being kept informed or included in the decision making.  It may be that you trust them enough to take the decisions.  The generally accepted rule though is that for multiple showers, a well-sized HW storage tank gives the best result.  I don't know if his proposed use of the 12 L/m pump will work as you describe; normally they are installed at the mains incomer to boost the whole house.  Perhaps it will all work fine and I hope it does, but the communications don't sound good enough

That may be down to me. My team are under instructions to use their judgement and whilst they keep me informed every step of the way, my job is very demanding and they are not to interrupt woman-at-work. Bottom line: if the money doesn't come in this project cannot happen.

The discussion we had about this was about 5 minutes long. What sold it to me was not the promise of how good this new boiler would be, more the lure of a good sized storage cupboard where the tank will be removed and to a lesser degree the space gained in the loft if the tank goes.

So where does the question about the HW tank and immersion heater arise, you ask? Someone else is rather insistent that I keep them just in case, and although I understand the risk I would rather not. Instead I can shower at someone else's home, that someone else coincidentally being the lucky recipient of a 2 year old 24kW boiler still under guarantee.

ComicGeek

Rosa_Damascena said:

coffeehound said:

Time for a crisis meeting with your plumber, Rosa.

I'm not panicking, I have confidence in my team.

@ComicGeek - the screed will be at least 65mm, the floor level is high across the house so we are looking up to the point of the first breezeblock visible, ie after the darker lower section you see in the salamander shot. The piping isn't yet complete as there are a number of new radiators to be fitted (my request and everyone thinks its a bizarre one) so there is more pipework to come and insulation will come when the pipework is completed, with the plan for screed to be done on Friday. So whilst I get your point about more screed taking longer to heat up there was no way round this really but I don't see that pipes over the UFH tubes is the cause for alarm your reaction suggested. If it is they can be disconnected and forced over the copper pipes - although I think that will mean the level of the screed will come even higher and interfere with the fit of my kitchen, and that really would be a problem.

Tell me what is wrong with this plan.

Eta: the rest of the copper piping can be sleeved and buried in the Celotex I am sure. 

The way round it is that you install thicker insulation to bring the floor level up without increasing the screed depth.

Fitting the copper pipework (with a plastic push fit bend) above the UFH pipework is a very basic error. Those are your key gas and water pipes, they just don't go there. IMO it shows a complete lack of understanding and planning.

Jeepers_Creepers

Rosa_Damascena said:

Now this is a helpful post but its taking time to digest. I get that its combi / DHW system and since the combi is in the rest has to come out.

Have I made the mistake of thinking this is simpler than it should be? The example of 38kW will give me 2x12L/min showers (mine is 40 so possibly slightly higher) - so that's upstairs sorted. Adding the pump to the downstairs shower in the as yet unbuilt bathroom gives me the 12L/min there I need. Right or wrong?

That combi is 40kW? That's a biggie all right!

The 40kW version will heat up to 16.4lpm according to a surf. That's very good. It should be clear, tho', that just to supply hot water at that rate, your cold mains supply would need to be 16.4 lpm or above? In order to supply even one gushing shower - which would require a fair dose (roughly half again) of cold water blended in - your cold mains supply would have to be at least 24lpm. With a decent pressure pushing that 24lpm along.

So, IF your mains flow is over 24lpm, then fill yer boots - and have an amazing shower! If you turn on two showers at the same time, then enjoy 2 pretty decent showers - not gushing, but certainly a lot better than a leccy shower. Try and run three showers at the same time? In theory, this is possible, and each shower should be around as good as a powerful leccy shower - a bit mediocre.

In practice, I suspect there would be problems trying to run three showers, especially if your incoming mains is at the minimum for this to work - ie around 25lpm. For instance, if one shower user opens up their shower's flow control to 'max', and they happen to be closer to the boiler, then they are going to steal the lion's share :-) The other two will suffer to some degree. Another possible issue is that, as each shower's thermostat adjusts the shower temp - 'opens and closes' in order to maintain the correct shower temp - then this might set up an imbalance in the whole system where the boiler keeps trying to modulate its output to cope with the ever-changing demand. Might.

BUT, in any event, for anyone to entertain fitting a combi of this size, they'd need to be confident of the mains supply - flow and pressure. (If the pressure is weak - say 2 bar and below - then the effect of another tap or shower or loo being used will have a more dramatic effect.)

Ok, let's assume that your incoming flow is above 25lpm (which it should ideally be for this size of boiler, and the wish to run 2 or three showers at the same time). That 12lpm mains booster pump serves no purpose. It has to be on the 'cold' mains pipe, and if it's to help the combi's hot flow, it would have to be fitted on the cold mains pipe before the combi. The cold mains pipe before the combi is (must be...) at a higher flow rate than this anyway, so it'll do nothing but get in the way.

And, yes, since you are going 'combi', the old boiler and tanks really need to go. To try and still employ them in some convoluted manner is not a sensible thing to do. Either your combi will do the task - in which case 'great!' - or it won't. In which case it should have been chosen.

Phew!

Yes, please find out what the cold mains flow and pressure are.

And ask what the 12lpm booster pump is there for.

If it turns out that your cold mains supply is borderline or inadequate, and if upgrading your incoming supply pipe cannot fix it, there is an ultimate solution, and that is a mains booster accumulator. This is roughly the size of your hot cylinder (!), but can be fitted in your garage or outhouse or even a purpose-made box next to your house. What this does is to pump the mains water into a pressurised tank up to 3 bar or above, and then this will supply your house with as much flow and pressure as you could shake a stick at. These are made by a few folk such as Challis, Salamander, Grundfos, etc., and will sort out any lack-of-flow issue. Budget around £1k or so.

Rosa_Damascena

Jeepers_Creepers said:

If it turns out that your cold mains supply is borderline or inadequate, and if upgrading your incoming supply pipe cannot fix it, there is an ultimate solution, and that is a mains booster accumulator. This is roughly the size of your hot cylinder (!), but can be fitted in your garage or outhouse or even a purpose-made box next to your house. What this does is to pump the mains water into a pressurised tank up to 3 bar or above, and then this will supply your house with as much flow and pressure as you could shake a stick at. These are made by a few folk such as Challis, Salamander, Grundfos, etc., and will sort out any lack-of-flow issue. Budget around £1k or so.

My garage? You are looking at it, complete with Salamander

But if the water pressure and flow rate (>3bar and 24L/min respectively) are sufficient I'm happy that I'll get a 12L/min shower in each if all 3 are running concurrently.