Boiler system suggestions to run 2 showers

abankerbutnotafatcat

Hello, we are doing major development/extension work on our house and will be ripping out all existing pipe work and starting from scratch.

The finished house will be about 180sqm with about half solid brick and non thermo-efficient windows and doors. The other half will be new-build; cavity walls and double-glazed. Fortunately, it is the north-facing part that will be new build. We are in the SE.

Judging by the kitchen tap (sorry if this isn't a good way to judge this) our mains water pressure is quite good.

The primary heating system will be wet underfloor with some radiators as considered necessary.

We will have four showers and would like a boiler(s) that will enable two of them to be run at the same time.

Any suggestions? I was wondering if two combis could work?

MX5huggy

You need a system boiler and a suitably sized unvented cylinder (megaflow being a common brand).

DorsetLad

First, a disclaimer: I am not a boiler expert but I do read a lot of DIY forums!

I agree with MX5huggy's reply: system boiler + unvented cylinder is the way to go. Having two combis can cause a number of problems, e.g.: Combis use a lot of gas and together they could require more gas flow than can be supplied by your domestic gas pipe. If they are plumbed in parallel (if this is what you are thinking of ) then you would need twice as much water flow through a tap or shower just to switch them on. You would not have these problems with a system boiler + unvented cylinder.

It's very important to know how good your mains water supply is (maximum flow rate and water pressure when the flow is maximum). As part of your house is fairly old, is the rising mains lead or iron pipe, or has it been replaced by modern blue plastic (MDPE) pipe? Only the latter is likely to give you adequate water flow for several showers plus hot water tap, so replacing the mains pipe from the road to the house (if necessary) should be one of the first things to do.

Clive_Woody

We recently had our bathrooms replaced and went for a unvented system to replace the old traditional hotwater cylinder and coldwater tank in the loft. We went for a pretty large cylinder for the new set up and the volume and pressure is fine to run two showers simultaneously with no noticeable change in pressure. We're really pleased.

Make sure you find someone qualified for installation of unvented systems.

Alex1983

As above definitely system boiler and unvented hot water. Also make it a sealed pressurised system.

DaftyDuck

Don't rule out the idea of having some electric showers. A combi like a Vaillant 937 will run two showers simultaneously.... just. Using an electric shower when the boiler would struggle with demand can be a simpler, cheaper overall option.

Mind you, from the sound of your op, system boiler is probably the way to go. Just worth realising that some combis are pretty powerful.

abankerbutnotafatcat

Thank you all. You have all been very helpful. We will ensure we have a qualified heating engineer experienced with the system we choose.

One reply says that an unvented system replaces the hot and cold water tanks. Does this mean that it doesn't have tanks? Apologies, I am only familiar with 'normal' gravity-fed boilers and combis.

We like the lack of tanks and energy efficiency of combis hence my query. We are considering have the system on two loops, each controlled by a Nest type device, which will enable us to only heat occupied areas, and we want to only produce hot water when required. There are only 2 of us living in the house all the time but we want the hot water system to cope when there were many more occupants.

Our original house is 150 years old so must have had updated pipes at some stage. It had a large extension including a bathroom built 15 years ago so I understood any water pipe upgrade would have been required then but haven't seen documentary proof of this. It's on an ordinary surbuban road. We can incorporate any necessary upgrades as part of the development. Is there a way I can tell about mains pipes/water pressure?

abankerbutnotafatcat

DaftyDuck wrote: »

Don't rule out the idea of having some electric showers. A combi like a Vaillant 937 will run two showers simultaneously.... just. Using an electric shower when the boiler would struggle with demand can be a simpler, cheaper overall option.

Mind you, from the sound of your op, system boiler is probably the way to go. Just worth realising that some combis are pretty powerful.

Thank you. Although I can see the advantages, especially as a back up if boiler fails, our impression of electric showers is that there is poor water pressure which is particularly an issue for those washing long hair like me. Is this the case?

Alex1983

With unvented you will have a boiler, 1 cyclinder and no tanks in the loft.

You can have the system zoned, each part of the house could be controlled independently and without the need for everything being on and all controlled by a multi zoned controller like hive, don't think the nest does multi zone but I may wrong with that.

Jonesya

abankerbutnotafatcat wrote: »

Thank you. Although I can see the advantages, especially as a back up if boiler fails, our impression of electric showers is that there is poor water pressure which is particularly an issue for those washing long hair like me. Is this the case?

You can get 10kW showers with good flow but you may have problems if you want more than one electric shower, due to limitations of your electricity supply.

Typical domestic electricity supply is 60, 80, or 100A. A modern shower needs 40-50A, so you could only have one, unless you upgrade your supply.

If it were me, I'd have a boiler system with a big hot water tank to feed 3 of the showers (and an immersion in case of boiler failure) along with 1 electric shower.

That way you should always have a shower available, even if there's a problem with the boiler or the hot water system.

fezster

An unvented cylinder (I have megaflo, but it is overpriced and cheaper alternatives available) is mains driven and so does away with the cold water storage tank in the loft. It replaces your hot water cylinder, but can be sited pretty much anywhere with access to drains and plumbing to the boiler.

You can measure your incoming flow rate using a bucket to time how many litres per minute you have. Use an outside tap closest to your incoming water main. A G3 certified plumber is required to fit an unvented cylinder and they will measure your flow rate and pressure. Typically, you need 2.5 bar and 20 l/min as a minimum. If you have big rain type showers which consume 15 l/min, you'll need at least 30 l/min to run two of them simultaneously. If your water main is already blue mdpe pipe of at least 25mm, you may not be able to upgrade it further. However, even I'm this circumstance you can use an accumulator to boost your incoming water supply.

Best to discuss this all with a competent heating engineer as the time and cost you'll save by doing it right once will be more than offset by his/her additional fee compared to someone who doesn't know the ins and outs.