Combi boiler - bathroom and shower - which model

[agnesf]

Can people with newish combi boilers and 2 showers tell me what make/ model of boiler they have and whether it good or rubbish.

We have 4 bed house, 4 person family (sporty kids getting to the end of primary so likely to be wanting more showers). Currently have one bath & seperate shower but will be installing seperate shower in near future.

We have been recommended a boiler with tank by one plumber and a combi by another. I prefer the idea of a combi but have read that they are not so good if all want to shower at once (unlikely but who knows as kids get older).

I'd prefer a well known make as the one recommeded to us is a bit obscure and am worried about reliability/ parts.

We just got rid of a boiler and tank, even though they were new (in a refurbed house), and installed a combi instead. Loads better. The one we got was a Worcestor Bosch. We've had one before and always found them reliable, plus there was a good warranty available.

A combi can deliver a limited amount of hot water at any one time. It will be limited by (1) the amount of water that your incoming water main can deliver to the house, restricted also by your internal pipework, and (2) the amount of heat the boiler can deliver to warm up this cold water. The more water that it tries to heat, the cooler it will be. This is why a combi is fine in a dwelling where only one person will ever be running taps at any one time, but very annoying if two or more people want to use the water.

You can measure (1) by running the kitchen tap into a bucket and timing how long it takes to fill. That will be the most you can get out of all the taps and showers in your house, hot and cold added together, if you get a combi. For example it might be 12 litres per minute, which is quite a fair shower. But it will be 6 litres each if two people are using it, or 4 litres each for three. You can usually increase the flow by digging up the water main and laying a new, larger, plastic pipe all the way to the road, and paying the water company to connect it. You may also need to fit larger pipes inside the house if the old ones are small.

(2) will depend on your boiler. A typical modern boiler can heat in the region of 12 to 15 litres of water per minute. It may need a new larger gas pipe all the way to the gas meter. Modern boilers tend to be in the region of 30 kW power, which is often double the power of most old boilers which heated radiators and a cylinder. A typical bath is 100 to 120 litres so will take maybe ten minutes to fill which is quite slow. The faster you fill it the cooler it will be. A bath filled from a cylinder and a water storage tank will be much faster if it has been run in 22mm pipe with no restrictions and a British tap with good wide waterways (flashy Italian taps are smaller inside and restrict the flow)

If you have very good incoming water flow and pressure, you can get a Megaflow or similar which is a sort of high-pressure hot water cylinder that runs straight off the incoming water main. This allows you to pre-heat and store hot water which means that (subject to the incoming flow) you can fill a bath quickly.

Hi PJ - thanks for helpful reply.

Have tested tap in kitchen and it did 10 lites in 25 secs so I think this is ok. As for the gas connection - well we don't have one at the moment as are on oil so presume we would get a big enough pipe as all the pipes are going to be installed at the same time as the boiler.

I like the idea of combi as we currrently seem to water lots of money heating up our tank and then not using the water as all our appliancces are cold fill.

Are they reliable & cheaper to install? Have got the impression that although cheaper to install they have a reputation for being less reliable but this may be based on older models.

A combi boiler is more complicated than a conventional, so has more inside it to go wrong. Some of the parts which are now internal (such as the CH circulating pump) are present, but elsewhere, in a conventional boiler so easier and cheaper to fix without needing a gas-qualified heating engineer or a special part. Additionally, some of the early combis were cheaply made and unreliable (some tended to leak or block, and some had aluminium heat exchangers which corroded within a few years). A Combi has an additional heat exchanger for the hot water.

When having a new boiler fitted to an old installation, it is vital to have it properly cleaned, usually by powerflushing, with a sediment filter fitted to catch the remaining dirt, and an anti-corrosion chemical added. Heat exchangers, especially in combis, are easily blocked by sediment, and this will not be covered by the manufacturers guarantee.

Installers like combis because they can be installed fast, so maximising profits. Some buyers like them because they are cheap. The combination of fast and cheap does not usually go with a good-quality installation.

The better brands like Vaillant and Worcester-Bosch are considered quite good. There are still some cheap ones, especially in the DIY sheds, that are bought by people who want a cheap product.

When I had a new boiler in my own home, I got a Viessmann, because it has a Stainless Steel heat exchanger which will not corrode, and a long parts and labour guarantee. I chose a Conventional boiler because it has hardly any internal working parts to go wrong, and because I like baths and cylinders. A Megaflow would have been a more modern alternative.

Look at the manufacturers' websites for an approved installer who has been on their (free) product training course. When you phone, ask "do you repair xxx brand boilers?" because you want someone who is capable of working on it, not just bunging it on the wall and running off with your money.

You can usually rely on BG to give the most costly quote.

p.s.

you mentioned heat loss from the cylinder.

Provided your cylinder and pipes are well insulated (which they should be) the heat loss from a cylinder is between half a kWh and two kWh (depending on whose calculation you believe) per 24 hours. This is between 2p and 8p worth of gas a day. It does not matter in winter because any heat lost contributes to the heating of the house.

I went through this last year, trying to get the compatibility between new showers and our existing combi boiler. It's worked out well.

The boiler has a flow rate of 18 litres per minute and the shower has a limiter of 15 litres per minute. We have good water pressure from the mains. The showers are thermostatic which I would consider essential with a combi. So even if other taps are run when the shower is on there is no change in temp and barely any drop in pressure.

We've only run both showers at once one time and whilst the drop in pressure was noticeable it was no where near as bad as I imagined it would be. Our boiler is a Worcester Bosch.