Loft Conversion- What would be the best heating/hot water system to use?

29 Jan 2017
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United Kingdom
HI all,

I am currently in the process of having a loft conversion done, of which I am planning to upgrade/change the heating and hot water system as the house will be a 5bedroom with 3 toilets once complete.

I currently have a condensor boiler which is 24kw, which has the usual cold water and hot water vented setup. As part of this I'd like to upgrade to an unvented system but I have a few questions about this.

1) Direct or indirect? My builder recons direct unvented costs less to heat up water than indirect, I would've thought using electricty would be more expensive? He recommended running a direct unvented for the hot water, and changing to a combi for the heating. -Could someone shed some light on if they would recommend this please.

2) If I were to keep my existing boiler, could I just add the unvented and remove the cold water storage in the loft and the vented hot water cylinder? Or will I have to keep the cold header tank?

3) I have a feeling a 24kw boiler will be too small to heat around 18 radiators (once finished)- Should I change the boiler at this point too? If so what boiler/setup would you recommend

I will certainly be using a G3 installer for this, so nothing to worry about there. Just wanted to get some background info before consulting plumbers.

I have also checked, there is a 22mm mains water pipe coming into the house.

Many thanks!
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Either you , your builder or both have terminology mixed up here.

All modern boilers are condensing - they wouldn't meet efficiency requirements otherwise.

An indirect cylinder has a coil of pipe inside, water in that coil is heated by the boiler and therefore heats the hot water in the cylinder. It's the standard deal which most houses with a gas boiler and cylinder will have.
A direct cylinder is one which has an electric immersion heater inside. They are only used with all-electric installations and as you have already noted, cost far more to operate because electricity is 4x more expensive than gas.

A vented hot water cylinder is supplied via a cold water tank in the loft above, water pressure is entirely due to gravity and will be low. A traditional option which could still be fitted today but probably won't be.
An unvented hot water cylinder uses cold water direct from the mains, and therefore the hot water is supplied at much higher pressure. No cold storage tank required. Usually what new/better installations have but only if the incoming water pressure and flow is suitable.

Whether vented or unvented, they can be supplied by your existing boiler.

A combi boiler is a device which heats radiators and also attempts to heat hot water on demand. Their advantage is that no hot water cylinder is required.
They are only suitable for small properties with low hot water requirements, typically one bathroom. Totally unsuitable for a 5 bed, 3 bath house.
Other boilers are usually called heat only or system boilers, they just heat water in a closed circuit for radiators and a hot water cylinder.

Whether your existing boiler is too small depends on the heating requirements for the building. Unless your house is vast and has no insulation, 24kW should be ample, and may already be oversized. The number and size of radiators means very little.

Finally - your builder is clearly NOT the one to be suggesting hot water or heating systems. Find someone else before expensive mistakes are made.
Hi thanks for clearing that up! Will provide some more information below:

The property is a 1950s build, has had new radiators fitted 3 years ago and the 24kw boiler was fitted in 2011(Worcester Bosch Greenstar) There is around 300mm loft insulation and cavity wall insulation(beads blown into the walls). Double glazed windows too. Do you think the 24kw is sufficient to accommodate the new proposed size?

So if I were to keep my existing boiler and replace the indirect vented cylinder for an indirect unvented cylinder, would this work? I have two tanks in the loft, one large cold water storage and a smaller one which i belive is the header tank for the heating. I know the large cold water tank will be removed but will the header tank for the heating need to stay? To provide water for the central heating?

Looks like I won't be taking advise from my builder when it comes to water systems!
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So if I were to keep my existing boiler and replace the indirect vented cylinder for an indirect unvented cylinder, would this work?
Yes, provided your water supply is suitable. The pressure and flow must be measured first.
You can get some idea by taking a 10 litre bucket and filling it from a cold water tap as close to the incoming supply as possible - kitchen tap, outside tap or possibly a washing machine connector. With the tap open fully, you should be able to fill a 10 litre bucket in less than 30 seconds.

but will the header tank for the heating need to stay?
Not necessarily. If you have one, it means the heating system and boiler is open vented (unrelated to the hot water cylinder being vented). It should be possible to convert this to a sealed system, although that will require some additional equipment installing and in some cases can result in leaks as the water in the radiators and pipes will be at a higher pressure than it is now. However with new radiators only fitted a few years ago, that is very unlikely.

For the boiler, you need someone to do a heat loss calculation for the entire building. There are various online calculators which you can use yourself, just search for 'heat loss calculator'.
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Thanks for the reply

Will try to measure the flow and pressure tomorrow. Just out of curiosity what would happen if the flow/pressure is inadequate? Would the unvented tank simply not work? Would I need to upgrade mains or buy some sort of accumulator?

I wouldn't mind keeping the header tank as it's quite small so can tuck it away in a corner(if that will work with the unvented setup), and remove the cold storage tank... if not may consider upgrading to a system/closed circuit one and get it out the way now rather than later

Looks like I will need to get my plumber out sometime soon!
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