Living with a combi boiler

Rosa_Damascena

ComicGeek said:

Hopefully it's not directly pumping from the main with that amount of flow rate, that would be completely against the water regs. For that flow rate there would have to be a cold water storage tank to provide hydraulic separation from the main.

I struggle to understand the logic of throwing out a 2 year old boiler though. 24kW is a lot of heat, so unless it's a huge property would likely be absolutely fine. Better to spend a couple of hundred pounds confirming heat loss calculations then spend £2k? fitting a new boiler.

The boiler is not being thrown out, I am happily gifting it (and that will be my plan B should the new combi ever fail, if the car doesn't start then I really will be in trouble). As for details I don't have a clue, but Jeeps gave me the reassurance I needed when I was thinking about it last night.

Rosa_Damascena

Jeepers_Creepers said:

I await, with tingling anticipation, Rosa's description of this 'pump'

I guess it must be a pumped accumulator as you suggest, but - even then - can a combi really supply 3 showers?! I guess three not-full-on showers?

Well I have just investigated and am posting some pretty pictures. The pump is yet to be fitted. 

coffeehound

Rosa_Damascena said:

What I want to know is whether I should be taking out the tank and immersion heater. I'm very wary of lacking a plan B for HW should the combi fail for whatever reason, but also tempted by the thought of having a completely empty loft to play with and a handy storage cupboard on the FF. How do people manage, other than having a reliable gas engineer to hand?

So is your hot water currently in two separate systems, vfm?

Rosa_Damascena

Until yesterday I had a standard boiler for CH and HW, and an immersion heater which I used only when I was having the boiler and radiators replaced a couple of years ago. AFAIK they run off the same plumbing...?

coffeehound

Did the plumber say he was going to plumb things so that the hot water storage cylinder would still be in use?  If not, the ship has already sailed, and the HW tank is probably already out of use!

Rosa_Damascena

coffeehound said:

Did the plumber say he was going to plumb things so that the hot water storage cylinder would still be in use?  If not, the ship has already sailed, and the HW tank is probably already out of use!

Both the tank and HW cylinder are still in situ. Nothing has been messed with upstairs and the opportunity to retain is still there AFAIK but Jeeps has indicated that this would be a complex plumbing job. Not that it is beyond my team, I just think there is more that could go wrong for relatively little gain.

ComicGeek

That pump will only boost up to 12 litres per minutes, so you're going to be very disappointed trying to run more than 1 shower at a time.  

That's 12 litres per minutes cold water, of which some/most will be passing through the combi boiler to be heated up - most showers are going to be at least 8 litres per minute flow rate, so nowhere close to being able to run even 2 showers at a time with that. If you've got ok water pressure, then it probably won't ever kick in because it will always be at least 12 litres per minute anyway.

It would need to be either an accumulator system (as long as mains pressure is higher enough) or a separate cold water storage vessel and pump set to run multiple showers.

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????!

NSG666

Is the combi going to feed hot water to the whole house or will the cylinder still be feeding hot water to some of it? If the former then that pump won't give 3 decent showers all at once, it is designed to be used where the incoming water flow rate is below 12 litres per minute which should have been checked beforehand. Did your plumber discuss what the flow rate was when advising you to fit this pump?

Jeepers_Creepers

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 with 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?

NSG666

We've gone off track a bit from the original question to trying to double guess what the installer has done and why they might have done so. The short answer to the question 'can I remove my hot water cylinder and cold water storage tank from my loft' is - 'not if it is still being used to supply some parts of the house'.