Boiler advice

Jeepers_Creepers

Hi 330d.

I don't think that JB is actually claiming that their 30kW combi is successfully supplying TWO showers at the same time; almost certainly it is not.

Your aunt's example might not be completely relevant either, since we don't know if she has a good mains supply - it could be so pants that any second tap will make the flow collapse completely.

Let me be clear: YES, if someone turns on a hot OR cold tap, flushes a loo, sets the W/M or basically farts in the wind, the person in the shower powered by a combi WILL notice. It should just be a drop in water flow - it shouldn't normally freeze or scald you - but you will notice it. How MUCH you'll notice this will depend on the flow and pressure of mains coming in to your house. I think a hot tap being turned on will affect it more than a cold, as it's not only stealing some water pressure & flow, but specifically some of the boiler's limited hot water.

My mains supply is pretty good - I suspect it's around 3bar and 20lpm. When the shower is being used, the mains flow is split between the hot and the cold - if a nice shower temp is 39oC, then that will be made up of (roughly) hot at 60oC+ and cold at 5-10oC, so - I dunno - roughly twice as much hot as cold? If someone runs a hot or cold tap, that will quite dramatically affect one or the other, but the thermostat will then balance things to keep the temp stable, so will basically adjust BOTH supplies = a very noticeable drop in shower flow.

It's still usable, and all we do is chug through until full flow is restored.

If you had TWO showers running at the same time, then that would be the situation throughout the shower - barely half the flow each, possibly even less.  If someone then turned on another tap, then the showers could actually turn off completely as the thermostat won't allow the shower to become scalding.

How to overcome this? Well, you can't have two GOOD showers running at the same time from a 30kW combi, and not even perfect ones from a 38kW jobbie, because 30 or 38kW split between two won't ever be as good as serving just one (although should still be better than any electric shower!). BUT, if you had a pressurised accumulator fitted, then - in theory - you at least won't be affected in the shower by someone flushing a loo (cold), running a W/M (cold) or running a cold water tap. It'll also help maintain a hot flow, but bear in mind you'll now have the hot split between three outlets - to obvious detriment.

Soooo, I reckon (only 'reckon' - I'm not a plumber) that a 38kW combi supplied by an accumulator WILL give you two 'adequate' (better than electric) showers at the same time, and a bludy brilliant one if only one shower is run. You can even go bigger with combis - there ARE models that'll supply two good showers, but I wonder if you really want to go there?

Soooooooo, if you absolutely need TWO VERY GOOD SHOWERS AT THE SAME TIME, then go 'unvented'. If you'd be happy with two ok showers for the rare times they are run together, and superb showers individually, then a 38kW combi + accumulator (might or might not be needed) should do the job.

I investigated mains accumulators for my bro - 6 floors up in a flat in London with a trickle coming out the cold tap. I was quite taken by the Challis make as it pre-charged a tank with an EV inside it, and then released this pressurised water in a 'natural' way. I called up Challis to discuss and spoke to Mr Challis himself - that was nice. However, the guy my bro got to do the job would only do Grundfos. (This cracking guy fitted the Grunfos and a new WB combi for £4.5k - in London!)

My bro's system is amazing; he even had to have a PRV fitted on the Gundfos' outlet to reduce the pressure... One negative for the Grundfos is that it seemingly stores the mains water and pumps it out on demand - and he can hear it as it's on his flat's roof. The same would apply if you fitted one in your loft - it would seriously annoy you.

Do you have room in your garage? Or build a small insulated 'shed' for it.

Challis: https://www.challisshowers.com/about-the-company

Two basic types - one is pressurised by the mains. This is for supplies which have good pressure (3bar+) but poor flow. The other is for power pressure mains and has a pump added to pressurise the accumulator (even if you ran the tank dry, it would still pump mains at 12lpm forever!). The output flow from either will be whatever you want - 40+lpm and the rest :-)

330d

Jeepers_Creepers said:

Hi 330d.

I don't think that JB is actually claiming that their 30kW combi is successfully supplying TWO showers at the same time; almost certainly it is not.

Your aunt's example might not be completely relevant either, since we don't know if she has a good mains supply - it could be so pants that any second tap will make the flow collapse completely.

Let me be clear: YES, if someone turns on a hot OR cold tap, flushes a loo, sets the W/M or basically farts in the wind, the person in the shower powered by a combi WILL notice. It should just be a drop in water flow - it shouldn't normally freeze or scald you - but you will notice it. How MUCH you'll notice this will depend on the flow and pressure of mains coming in to your house. I think a hot tap being turned on will affect it more than a cold, as it's not only stealing some water pressure & flow, but specifically some of the boiler's limited hot water.

My mains supply is pretty good - I suspect it's around 3bar and 20lpm. When the shower is being used, the mains flow is split between the hot and the cold - if a nice shower temp is 39oC, then that will be made up of (roughly) hot at 60oC+ and cold at 5-10oC, so - I dunno - roughly twice as much hot as cold? If someone runs a hot or cold tap, that will quite dramatically affect one or the other, but the thermostat will then balance things to keep the temp stable, so will basically adjust BOTH supplies = a very noticeable drop in shower flow.

It's still usable, and all we do is chug through until full flow is restored.

If you had TWO showers running at the same time, then that would be the situation throughout the shower - barely half the flow each, possibly even less.  If someone then turned on another tap, then the showers could actually turn off completely as the thermostat won't allow the shower to become scalding.

How to overcome this? Well, you can't have two GOOD showers running at the same time from a 30kW combi, and not even perfect ones from a 38kW jobbie, because 30 or 38kW split between two won't ever be as good as serving just one (although should still be better than any electric shower!). BUT, if you had a pressurised accumulator fitted, then - in theory - you at least won't be affected in the shower by someone flushing a loo (cold), running a W/M (cold) or running a cold water tap. It'll also help maintain a hot flow, but bear in mind you'll now have the hot split between three outlets - to obvious detriment.

Soooo, I reckon (only 'reckon' - I'm not a plumber) that a 38kW combi supplied by an accumulator WILL give you two 'adequate' (better than electric) showers at the same time, and a bludy brilliant one if only one shower is run. You can even go bigger with combis - there ARE models that'll supply two good showers, but I wonder if you really want to go there?

Soooooooo, if you absolutely need TWO VERY GOOD SHOWERS AT THE SAME TIME, then go 'unvented'. If you'd be happy with two ok showers for the rare times they are run together, and superb showers individually, then a 38kW combi + accumulator (might or might not be needed) should do the job.

I investigated mains accumulators for my bro - 6 floors up in a flat in London with a trickle coming out the cold tap. I was quite taken by the Challis make as it pre-charged a tank with an EV inside it, and then released this pressurised water in a 'natural' way. I called up Challis to discuss and spoke to Mr Challis himself - that was nice. However, the guy my bro got to do the job would only do Grundfos. (This cracking guy fitted the Grunfos and a new WB combi for £4.5k - in London!)

My bro's system is amazing; he even had to have a PRV fitted on the Gundfos' outlet to reduce the pressure... One negative for the Grundfos is that it seemingly stores the mains water and pumps it out on demand - and he can hear it as it's on his flat's roof. The same would apply if you fitted one in your loft - it would seriously annoy you.

Do you have room in your garage? Or build a small insulated 'shed' for it.

Challis: https://www.challisshowers.com/about-the-company

Two basic types - one is pressurised by the mains. This is for supplies which have good pressure (3bar+) but poor flow. The other is for power pressure mains and has a pump added to pressurise the accumulator (even if you ran the tank dry, it would still pump mains at 12lpm forever!). The output flow from either will be whatever you want - 40+lpm and the rest :-)

Thanks for the info.

I think a combi boiler is too high of a risk to take. For example, I can spend £1500 on a new combi boiler, accumulator pump ( which seem to be about £1000), remove the current hot water cylinder and relocate the boiler in the loft. It would be a tragedy if after all this expense I am getting poor water flow and/or pressure. Yes it would be very energy efficient but not great performance.

On the other hand I keep my current unvented system, just replace the boiler to a larger capacity and move it to the loft. This way I am not risking any poor pressure situations. Plus this option might be cheaper than above as I am not buying an expensive accumulator. 

Jeepers_Creepers

330d said:

Thanks for the info.

I think a combi boiler is too high of a risk to take. For example, I can spend £1500 on a new combi boiler, accumulator pump ( which seem to be about £1000), remove the current hot water cylinder and relocate the boiler in the loft. It would be a tragedy if after all this expense I am getting poor water flow and/or pressure. Yes it would be very energy efficient but not great performance.

On the other hand I keep my current unvented system, just replace the boiler to a larger capacity and move it to the loft. This way I am not risking any poor pressure situations. Plus this option might be cheaper than above as I am not buying an expensive accumulator. 

1) You DON'T currently have an unvented system - you have a vented cylinder that relies on gravity to provide its flow. It depends for its performance on the height of the CWS tank above the outlets. Clearly this isn't quite good enough at the moment because a shower booster pump has been added. I really don't know if moving your EXISTING type of tank up in to the loft, along with raising the CWS a bit more, will work. You definitely WOULD still need a shower booster pump (or two), but it might not work even with that. You'd need pro advice for this option.

2) You COULD move everything in to the loft but swap your hot cylinder for 'unvented', which means it's now powered by mains pressure. The only uncertainty about this setup is how good your mains supply - P&F - is. However, if you find it's not as good as you hoped, the addition of an accumulator WILL fix it - no question. That is your 'ultimate' solution, but also potentially the most expensive, especially if an accumulator is required. At the moment we simply don't know if it is. If you want my best guess based on what you've said, I think the chances are high that it'll be 'fine'. By fine I mean that one shower running will be awesome, and two showers running will still work but with noticeably reduced flow. I would take this option over the one above (vented + shower pumps) any day; I hate supplies being boosted by pumps on demand.

(3) You could go 'combi' in the loft either as it stands and see how it goes, or with an accumulator - in which case it'll work as well as a combi can work. Which is to say it'll be fine for almost everything, but will 'struggle' with two large demands at the same time.

To sum up:

1) The vented cylinder + system boiler in the loft + shower booster pumps will quite possibly not work at all (but seek plumber advice) since the cylinder and CWS will be at close to the same height. (The use of 'home county' flanges will be essential, at the very least). Personally, I hate booster pumps, so would avoid this - but that's my personal thing.

2) The new unvented cylinder + system boiler in the loft might work just fine, but it'll depend directly on yer mains.

3) The new unvented cylinder + system boiler + accumulator will give you everything you desire, but an empty bank balance.

4) A large combi + accumulator will give you pretty much everything you want, but with some compromise with two showers running at the same time. I'd guess that the compromise would basically be that each shower runs at half-flow. This option will be around 2/3rds of the cost of (3)

330d

Silly question. But I have a separate pressure vessel. I thought you only have this for unvented systems?

Jeepers_Creepers

Not a silly Q.

There are two parts to your whole CH/DHW setup, and the water in them don't ever mix.

(1) One is the boiler + radiators + heating coil in the hot cylinder.

This is the 'system' water and it never mixes with anything else - it is continually circulated. You add 'inhibitor' to it to keep it from corroding your rads from the inside out.

This water can be (a) in a VENTED system (open), or unvented (sealed & pressurised). A vented type has a SMALL water tank in the loft - around 5 gallons - and does NOT need either a pressure gauge or an expansion vessel. And (b) the UNVENTED - sealed - type has no tank, is not open to the air at any point, DOES need an expansion vessel (usually built IN to the boiler casing) and DOES need a pressure gauge - so you can tell what's going on.

So, your 'BOILER' can be either vented or unvented.

(2) And the other part of your system is the DHW - the hot water that comes out your taps. If you heat this in a cylinder, that cylinder can - again - be vented or unvented. (a) Vented is what you currently have - this has a LARGE CWS tank in the loft which supplies it with fresh cold water. This does not need an EV or a gauge. The second (b) type of cylinder is unvented. This does NOT have a supplying tank in t'loft as it's effectively sealed and is supplied directly by the mains and under mains pressure. This, therefore, DOES need an EV and a gauge to keep tabs on what's happening.

So, your 'HOT CYLINDER' can be either vented or unvented too.

(And a combi is something else again but is not relevant here as it does not really store water. However, since combis are sealed (unvented), it means they, too, must have EVs and gauges to keep tabs.

Phew.

Ok, your hot cylinder is vented. Your boiler was presumably bought as a vented type - it doesn't have a gauge or EV - but at some point a plumber converted it to 'unvented' by removing the small header tank in the loft, sealing it off and adding an EV and a gauge.

You have an unvented boiler and a vented hot cylinder.

fezster

A "good shower" is highly subjective. Some people consider a 10KW electric shower with a flow rate of 11 litres/min to be "good".

In my experience, a "power shower" experience is 15 - 20 litres/min. This means that you need roughly 10 litres of hot and 10 litres of cold going into a thermostatic shower (obviously it wont be exactly half and half depending on a number of factors).  For a cylinder of water at 60 degrees, it will be close to 50% each in summer.

A 30KW combi boiler will give you around 14 litres/min at a 30 degree rise. Even in the height of summer, when the incoming water is 20 degrees, you will be using proportionally more hot water than cold to have a comfortable 40 degree shower. So your 14 litres/min barely stretches to 2 showers operating at 15 litres/min. And in winter, when the incoming water is 10 degrees or lower, you are just about able to run a single 15 l/min shower. In other words - 30KW does not cut it to run two "good showers" IMO.

Jeepers_Creepers

I agree, Fezster.

I know my own 30kW combi - which gives a great single shower - would be seriously compromised if two showers were drawn from it. With the plumbing being in 15mm pipe, there would probably be too much of a pressure drop as well as the boiler simply not having the guts.

If you could maintain whatever flow is required, tho', then that would leave just the lack of heating ability. I suspect a 30kW jobbie split between two showers would still each give better showers than electric types, and a 38kW would - I think - actually be very satisfactory even running two. But it would need that enhanced mains flow supply.

A 38kW combi like the WB 38 CDi can deliver more than 16lpm of 'hot'. With all of that being used by a very greedy shower (or two normal showers), cold would have to be added at the rate of roughly 5-8lpm, so easily up to 25lpm mains flow required. That just ain't going to happen in the majority of homes. Even if the two showers are running reasonable well at a lower flow rate, it'll be drawing the max from the mains, so one more tap/toilet opened and the flow will collapse. It would be a system running on the edge.

Now imagine being able to supply whatever flow you want - ie with an accumulator. 40lpm @ 3bar? No problem. The 38CDi would now be able to supply its max output without issues, the best part of 12lpm combined H&C to each of two showers, and with plenty of flow in reserve should a twit flush a loo.

That is MILES better than any electric shower. "For a Mira Sport 10.8kW on full power setting with incoming water supply at 10ºC, to achieve a showering temperature of 42ºC (a temperature rise of 32ºC), the flow rate will be 5 l/min."

I honestly think a 38kW combi supplied by a Challis would be impressive, and leave very little to be desired.