[Tom Tallon]

Following from RB's thread a few months ago on changing CH boilers, I thought I'd document my experience of trying to replace my boiler.

Problem

Ferroli Optimax boiler of 10 years standing has given a problem each year since I got it. Its a gravity fed regular boiler with the old fashioned copper cylinder now know as a vented system. To make two showers work concurrently I have had to install a Salamander pump which works upstairs on hot water supply. The pump is extremely noisy. My requirement is to get a reliable boiler and replace the noisy pump while having two shower outlets which work together.

Household

Just my wife and me but often use the showers at the same time in morning. We have 12 radiators but have cold fill washing machine and dishwasher. We have a shower over bath and a shower in a separate shower room. 4 bedroomed detached house.

Solution 1

Install new unvented system with system boiler and pressure cylinder of say 150 litres standing hot water capacity. cost circa £4500 because of the need for two expensive bits of hardware - boiler say £1200 and the cylinder say £800. Quite a big job to make changes to pipework and possibly relocate the cylinder.

Solution 2

Install top of the range combi. In my case, Worcester Bosch 38CDI. It has 40 kw hot water output and 30 kw central heating well enough for 12 radiators.
Boiler can provide 16.4 litres of hot water per minute.

Outcome of deliberations

Went for Combi option at £2740 agreed price. Problem on hot water will be that the two shower outlets will split the mains input which is 20 litres per minute. The hot water will also be split with 8.2 per minute litres available to each shower within the 10 litres available in total from mains pressure. It is calculated that a shower uses roughly 2/3rds hot to 1/3rd cold water. Thus I should have 10 litres available to each shower of which 6.66 litres may be hot water (within the split of 8.2 litres available to each shower from the boiler peak output).

The next step was to check what a 10 litre per minute shower would feel like! So I found a low pressure shower head with great reviews on the Internet. Its called a Methven Maku from New Zealand and is available in Homebase at £24 for the handset.

Its provided with a 9 litre per minute water inhibitor and I tried it on my pumped shower with the inhibitor. It worked a treat so I surmise a Combi could do the same for each shower within the calculation above.

Only one slight reservation was that the Maku works by criss crossing the shower sprays into each other which gives an invigorating feel but can feel a bit cooler as it mixes more air in the jets.

Will it work with Combi? Will tell you in mid July but hope so.

[1 AKY]

Subscribed. Please keep us posted.

Have a similar project pending at my parents' home.

[73henny]

I went through this about 6 years ago when we moved into our current house. I went with the unvented option and it's been great. There's not just the two showers to consider as someone could turn on a hot tap to wash their hands for example. The shower flow from a combi is also affected by how cold the water is coming into the house so you may find the shower is 'better' in the summer. It'll probably be fine having said all that.

[Tom Tallon]

Originally Posted by 73henny

I went through this about 6 years ago when we moved into our current house. I went with the unvented option and it's been great. There's not just the two showers to consider as someone could turn on a hot tap to wash their hands for example. The shower flow from a combi is also affected by how cold the water is coming into the house so you may find the shower is 'better' in the summer. It'll probably be fine having said all that.

Thanks - yes as I said I knew there were other issues for the combi which I missed out. As you say where the heat rise is from water at 10 degrees C in winter rather than 20 degrees C in summer, the higher mixture of hot to cold may be needed. You'll note that on my calculations I still have a bit more hot water capacity to use in winter over summer ie 6.66 litres per minute used but 8.2 available at peak to give a bit of a reserve if needed.

[430GH]

simple solution = shower together

[Tom Tallon]

Originally Posted by 430GH

simple solution = shower together

Thanks mate. That was suggested by quite a lot of people I spoke to! also the plumber said why not shower one after another!

[XK140]

The other obvious solution is to convert one of the showers to electric.

[Tom Tallon]

Originally Posted by XK140

The other obvious solution is to convert one of the showers to electric.

That was an option I also omitted - but..... my wife hates electric showers - sorry i should have said that it was completely taboo, that one!

[teaston]

I am interested in why you chose a combi if you wanted the most reliable boiler? I know they have improved in recent years, but aren't the heat exchangers still quite a weak point on them?

Also how big is your house? I was under the impression combi's are meant for smaller houses?

[JLR1969]

Originally Posted by teaston

I am interested in why you chose a combi if you wanted the most reliable boiler? I know they have improved in recent years, but aren't the heat exchangers still quite a weak point on them?

Also how big is your house? I was under the impression combi's are meant for smaller houses?

Combis come in different sizes so house size is less of an issue, suppose it depends on how many people live in the house

[six]

Last year we went from vented old school to a full on unvented system. On the old system we had a Stuart Turner 3 BAR pump which was noisy as hell. Frankly unacceptable, but an amazing water flow rate and pressure when combined with Grohe 22mm feed taps and shower.

The unvented solution now with a big cylinder and mains pressure hot water is very good but the pressure and flow is nowhere near as good as the pumped solution. Two showers at the same time is only barely acceptable and if another tap is turned on, hot or cold it's crap.

I wouldn't do it again, I'd box a pump in to silence it. There are several extremely effective ways to do this including using a concrete slab sat on thick rubber carpet underlay to isolate vibrations then enclose in a sound proofed box.

Some other tips...

- Don't forget although your washing machine is cold fill if it's on when your in the shower it will affect the pressure for hot and cold supply as will hot or cold taps or toilet flush

- New condensing boilers need a condensate drain and a vent pipe. This can be a pain in the arse depending on where your boiler is located in the house

- The hot water cylinder will require a large bore vent pipe to a safe ground level location outside. The diameter of this pipe grows the further the distance travelled. If your using an existing airing cupboard like us this can be a challenge to install within regs as airing cupboards are generally in the centre core of the house, not on an outside wall

[73henny]

Originally Posted by six

The unvented solution now with a big cylinder and mains pressure hot water is very good but the pressure and flow is nowhere near as good as the pumped solution. Two showers at the same time is only barely acceptable and if another tap is turned on, hot or cold it's crap.

This must be due to the incoming water pressure to your house. Is your house on a higher area than the surrounding streets? In fact I had to have a pressure reducing valve fitted as the incoming pressure was too high for the unvented tank. Water authorities usually have a minimum pressure that they supply to properties. I'm surprised that you find two showers barely acceptable. I can hardly tell when the other shower in our house is being used.

[six]

Yeh I think the incoming pressure is on the low side sadly. On high ground and we find summer is worse than winter. Also early in the morning is better than during the day / evening... air wonder if you can have a pump on the incoming supply which would boost pressure across everything...

[teaston]

Originally Posted by six

Yeh I think the incoming pressure is on the low side sadly. On high ground and we find summer is worse than winter. Also early in the morning is better than during the day / evening... air wonder if you can have a pump on the incoming supply which would boost pressure across everything...

I suppose theoretically you could, but it would be illegal, and very inconsiderate to your neighbours whose water pressure you would reduce whilst your pump is sucking the water away from their houses!

[six]

Originally Posted by teaston

I suppose theoretically you could, but it would be illegal, and very inconsiderate to your neighbours whose water pressure you would reduce whilst your pump is sucking the water away from their houses!

Good point, although they have been irritating me recently

[EvilDrPorkChop]

Originally Posted by Tom Tallon

Following from RB's thread a few months ago on changing CH boilers, I thought I'd document my experience of trying to replace my boiler.

Problem

Ferroli Optimax boiler of 10 years standing has given a problem each year since I got it. Its a gravity fed regular boiler with the old fashioned copper cylinder now know as a vented system. To make two showers work concurrently I have had to install a Salamander pump which works upstairs on hot water supply. The pump is extremely noisy. My requirement is to get a reliable boiler and replace the noisy pump while having two shower outlets which work together.

Household

Just my wife and me but often use the showers at the same time in morning. We have 12 radiators but have cold fill washing machine and dishwasher. We have a shower over bath and a shower in a separate shower room. 4 bedroomed detached house.

Solution 1

Install new unvented system with system boiler and pressure cylinder of say 150 litres standing hot water capacity. cost circa £4500 because of the need for two expensive bits of hardware - boiler say £1200 and the cylinder say £800. Quite a big job to make changes to pipework and possibly relocate the cylinder.

Solution 2

Install top of the range combi. In my case, Worcester Bosch 38CDI. It has 40 kw hot water output and 30 kw central heating well enough for 12 radiators.
Boiler can provide 16.4 litres of hot water per minute.

Outcome of deliberations

Went for Combi option at £2740 agreed price. Problem on hot water will be that the two shower outlets will split the mains input which is 20 litres per minute. The hot water will also be split with 8.2 per minute litres available to each shower within the 10 litres available in total from mains pressure. It is calculated that a shower uses roughly 2/3rds hot to 1/3rd cold water. Thus I should have 10 litres available to each shower of which 6.66 litres may be hot water (within the split of 8.2 litres available to each shower from the boiler peak output).

The next step was to check what a 10 litre per minute shower would feel like! So I found a low pressure shower head with great reviews on the Internet. Its called a Methven Maku from New Zealand and is available in Homebase at £24 for the handset.

Its provided with a 9 litre per minute water inhibitor and I tried it on my pumped shower with the inhibitor. It worked a treat so I surmise a Combi could do the same for each shower within the calculation above.

Only one slight reservation was that the Maku works by criss crossing the shower sprays into each other which gives an invigorating feel but can feel a bit cooler as it mixes more air in the jets.

Will it work with Combi? Will tell you in mid July but hope so.

I'm sorry but this is totally the wrong solution for you. In my experience in the industry i've never seen a combi being able to power two showers at once maybe with the exception of a combi with a hot water store (So either a Worcester High Flow or Vaillant EcoTec Plus 938). Put it this way, my parents had a Worcester 42kw Combi and that wouldn't do two showers at a reasonable flow rate.

Also where are you testing you 20l flow rate from the mains from? Also what is your standing pressure, has your plumber tested this? This will be crucial as at the minute your pump is giving you the flow rate and drawing the water from the tanks in the loft - so pressure in your current situation won't affect you but will when you go onto a combi (Hence your shower heads working fine at the minute).

For a 4 bedroom house you really require a unvented solution, especially if you are wanting two showers at once. To achieve this you need a good sized incoming main into the property (25mm). From that you need a 22mm water feed to the cylinder, and then 22mm feeds from the cylinder to your showers with a equalising pressure feed. This will give you the best chance of the maximum flow rate through the house to each shower.

You also have to take into account the running costs. When your using your hot water you'll be using 40kw of gas, say over 20 minutes (10 mins for 2 showers) A unvented cylinder will only use 8kw to heat up over 30 mins, then use a minimal amount of gas to keep topping it up once it's heated. Plus onto of the fact that when your running your hot water your combi boiler won't be doing your heating, on a traditional system both can be heating at the same time so you aren't having any downtime on your heating.

Don't get me wrong, a good combi is fine, especially in certain situations. But one don't expect it to work miracles they have their limits, and two for you I think IMO it is totally the wrong solution.

[TimMcC]

For what it's worth, we moved into a place with 3 showers back in January. All 3 showers are decently powerful (as good as pumped shower in last home). Not tried running all 3 at the same time, I expect it'd be rubbish and cold. But we can run two nice warm showers at the same time, albeit with slightly less good than normal but both still decent.

We have an Ideal Logic+ 30, and a 4 bedroom house.

[EvilDrPorkChop]

Originally Posted by TimMcC

For what it's worth, we moved into a place with 3 showers back in January. All 3 showers are decently powerful (as good as pumped shower in last home). Not tried running all 3 at the same time, I expect it'd be rubbish and cold. But we can run two nice warm showers at the same time, albeit with slightly less good than normal but both still decent.

We have an Ideal Logic+ 30, and a 4 bedroom house.

From what it sounds like you must have good pressure which allow you to achieve this. This is the issue, you need good pressure along with a good flow rate which can't always be achieved.

The point I was trying to make was if you are starting from fresh I'd be putting the best possible system/solution in for what I required.

[HighlandPete]

Originally Posted by EvilDrPorkChop

From what it sounds like you must have good pressure which allow you to achieve this. This is the issue, you need good pressure along with a good flow rate which can't always be achieved.

Mains pressure does seem to be a key issue here, as you have clearly stated.

The same can apply to running electric showers, I can run two showers together, with excellent flow rates and output. Mains pressure is ample, some services require pressure regulation in my home, to stop 'thumping', as mains pressure is so high.

[Tom Tallon]

This has been useful to me.

Firstly to EvilDrPorkchop. Yes I agree entirely that from a blank sheet of paper, a 4 bedroomed house with 2 or more bathrooms would equate to a system boiler with an unvented cylinder and at least 200 litres of standing hot water. Indeed I have examined new houses and they all conform to this pattern.

However Tim McC says he is running two decent showers from an Ideal Logic 30 combi. Thats my plan. I will also upgrade his boiler from 30kw DHW to 40 Kw and the flow rate to 16.4 litres per minute heat rise from his 12.4 litres pm.

I wish to have this facility to run two showers simultaneously for say 5 minutes per day. His evidence says I can do so with a good mains pressure which I have,

As Highland Pete says its all abouts mains pressure. If you have less than the minimum 1 bar pressure at your property an unvented sytem with 500 litres of standing hot water isn't going to help a great deal as you're not going to push it out fast enough to make a flea wet. You'll need some form of pumped assistance eg electric shower or shower pump.

However I'm very grateful for the fors and against views. I'll report back at end July to see if my plans have gone asunder!

[Tom Tallon]

My update after the WB boiler has been fitted.

Well it works. The very good mains flow which is probably 22 litres per minute through the combi has been tested through both showers at the same time. One has a flow restrictor of 9 litres per minute and the other no restrictor. Both shower heads are Methven Maku aerated shower heads, one shower is a manual Hanagrohe mixer and the other a Hansgrohe thermostatic shower.

Both produce strong wide flows of water - not quite power showers but not much short of this. So plenty of power and vey good temperature in both.

Right so its July and the heat rise from the combi is from warmish water to which to add the 35 degrees per minute temperature rise on each 16.4 litres hot water per minute.

The question will be what happens in December when the mains water temperature dips. Obviously the pressure will be OK but will it be at reduced heat or will the boiler have a surplus to be used to cope. Theoretically the restricted shower still has a reserve of 2.4 litres hot water not being used in the mix.

To summarise. this powerful combi boiler will service a 4 bedroom two bathroom house for two people at least with decent mains pressure and 40 kw output for domestic hot water.

[MarkG_M5Comp]

Sounds like you have a good result.

I have bad experiences with combi boilers - has one fitted to my old house and had 2 major issues:

1) When we came across pressure issues (growing family with multiple bathrooms in use together) we could do nothing about it; you can't pump incoming mains water or hot water from a combi. I'd stick with my storage tanks now for that reason alone.

2) When something goes wrong - we had expansion tank leaks, PCB failure on 2 occasions - you lose EVERYTHING. No heating or hot water. At least with a water tank you can turn on the water heater for showers etc. Costs a fortune but possible. I had too many cold shower to care for a return to that!