I am thinking of a loft conversion and wondering if I should upgrade to an Unvented system. However house heating layout is non standard. We have goundfloor underfloor heating (Nuheat) and first floor radiator Happy with both and would prefer adding radiators to loft conversion (2 x bedrooms with ensuite). Currently have recently fitted Baxi 100HE boiler and standard imersion tank running off gravity fed loft headers. Underfloor can be converted I know, but what is needed to convert the rest ?
Perhaps DD can answer this. Boiler is on ground floor in attached garage (loads of space around it). Hot water tank is in airing cupboard on first floor (space also available). Need to run 3 showers at same time (teenaged daughters as well as us). 2 showers will be in converted loft, almost above current airing cupboard, so access no problem, loft also heated by radiators. Will a heat store serve this layout well ? Will I need the expansion tank to be in loft ? Heatweb didn't quite cover this with their otherwise excellent site.
Firstly, you want sealed pressurised operation of the primary heating circuit, so that you can have radiators on the top floor. Secondly, you may want mains pressure operation of the domestic hot water (from the taps) so you can have an en-suite shower or such like.
In most cases, it is simple to convert a gravity primary circuit to sealed pressurised operation. The main thing to check is that the boiler is certified for such operation. If so, a simple and cheap kit of parts and removal of the header tank is all that is required. Occasionally you will get leaks on corroded radiator connections or pipework when the pressure increases. If so, it would soon have expired anyway and at least you've got a plumber on site to fix the leaks.
If you also need hot water on the loft floor, your choices are:
Point of use electric heaters. Ideal if you want the minimum of disruption to your system. They also reduce dead leg problems. However, if a shower is desired, you do have to put up with electric shower performance which is low.
Negative head pump. Simplest to install, as it doesn't affect the existing system. However, they can be noisy and inconvenient (some designs require you to pull a cord after turning on the tap).
Unvented cylinder. Provides mains pressure water. Efficient to use, as you can run the boiler at a lower temperature. However, they require annual safety inspections due to the large quantity of pressurised hot water.
Heat bank. Also provides mains pressure water. Less efficient, as you need to turn your boiler temp right up. However, has much higher fundamental safety, as the hot water storage itself is vented.
Combi boiler. If you are replacing the boiler anyway, you could install a combi boiler that provides instantaneous mains pressure hot water. If you also have additional bathrooms or you prefer soaking in the bath, an instantaneous combi may be too small, so an alternative is to keep your existing hot water cylinder for the main bathroom and use the combi to supply the new loft extension, where it will be ideal for showers.
On Tue, 30 May 2006 12:39:19 +0100, snipped-for-privacy@metronet.co.uk wrote (in article ):
You can get a heatbank (e.g. DPS Pandora) with an expansion vessel in the top. Other types have a small header tank.
In effect, you have three lots of water.
- The primary, which circulates though boiler, radiators and a coil in the heatbank can be sealed or open vented.
- Secondary, which is the bulk water in the heatbank. This is heated via the coil. It is pumped through a plate heat exchanger when hot water is used.
- Mains water through plate heat exchanger is heated by the secondary water and delivered to showers and taps.
A heatbank of this design can be positioned at any height and there is no need for any kind of roof tank.
Direct heatbanks where the bulk water of the cylinder becomes part of the heating circuit obviously have more restrictions on positioning because they are open vented.
The main things to watch is adequacy of the cold mains supply and the capacity of the store.
A decent shower requires 15 litres per minute of warm water at 40 degrees. Three showers will require 45 litres/min and the mains has to deliver it all. This may be a struggle and may require an upgrade to the service pipe from the road. Do check before committing yourself.
Ask the females how long they want the shower to run. Many like to wash their hair and take 15-20 mins to do so. Calculate back how much hot water that will be in the winter when the mains is at 5 degrees. I suspect that you would need at least a 250 - 300 litre store.
There are only two of us and a 750 liter sealed pressurised tank. We run out of how water quite regularly. It just takes one person to leave a hot tap dribbling, and the other to take a bath up to the overflow (regular occurrences) and there is bugger all left to wash the dogs paws in.
IMHO there is no such thing as too large a hot water tank, especially if it can bi sited inside the insulation of the house, where any heatloss is a net gain to the house..
A decent sized boiler with a priority system (all the boilers output goes to the cylinder) will virtually eliminate this. With a heat bank using a blending valve ob the Flow and Return only 75C water goes into the top of the cylinder and this is used directly to heat DHW.
Something is going wrong here. My 180 litre heatbank will do two baths even with the boiler turned off. The boiler could easily keep up with a running tap, too. 750 litres should do 5 baths even with no boiler.
Say, price up a 300 litre thermal store. It gives x baths/showers. It may be borderline or just too small, so 450 litres is required. Maybe too big for the cupboard/heavy for floor and more expensive too.
By arranging the pipework so that the hot bathtaps and shower lines are on a flow switch, the boiler can come in immediately on a priority setup (the boiler heats only the hot water). The flow switch will override the normal cylinder stat(s). For about a £30 flow switch a lot can be saved in thermal store/heat bank costs and most likely you will not run out of hot water too. This combines the energy of the boiler and the cylinder.
For normal operation on the other taps, the heat bank works off the normal cylinder stat(s)
In actuality, I find the standard thermostat comes on within seconds of a substantial draw anyway, so I'm doubtful of the benefit of logic to fire up the boiler immediately.
Also, even if you decided to implement it, a relay would be a cheaper way, as a flow switch is already in system. Wiring it up would introduce the potential for accidentally overheating the store, too.
15 to 20% of the hot waer may have gone by then. Also when having two stats to prevent boiler anti-cycling the flow switch on the bathtaps/showers is necessary.
That will bring in the boiler if there is any flow, as would a combi. Having a flow switch on the bathtap/shower lines only, ensures that the boiler fires up when with large DHW draw-offs.
You could always have a high limit stat set to 90-95C. In reality if there is a draw-off of DHW it will not overheat as enough cold water is being introduced. A 30kW boiler will give one good shower, so in reality the DHW temp will stay the same, at worst just above the cylinder temp setpoint.
500 liters(sic)? That's half a tonne of water at 40degC (minimum) for a 'good soak'.
What are the dimensions of your 'good soak' device?
500 litres would be 'water' - not extranl bath dimensions- of two metres (long) by one metre (wide)by half-a-metre (deep). That's some bath! Can you cite the make and model number? Are you one metre wide?
"Standard" baths appear to be 1.7m x .7m x.5m.
One presumes you're not using a water meter - which company provides water to your establishment (where you aren't concerned about a dripping tap wasting three quarters of a tonne of water into the drains overnight - (dribbling taps will cause the tank to empty)?
Why not just extended and move the tanks into area not used by conversion? my brother did for his loft conversion. Move CH header tank into space above loft stair well to be above the radiators and HW feed tank elsewhere. Fitted a negative head shower pump for the ensuite shower in the loft and all works fine without having to change any plumbing in the house.
However, it can be good use of space to use the overstairs area for the heatbank. This is what I have done. The airing cupboard downstairs was seriously biting into the space for the 3rd bedroom, so the loft was the best place for it. The 3rd bedroom is now quite usable.
The cold tanks were in the rear loft, which is my only remaining storage area, so needed the tanks removed.
We only have a compact cylinder (Albion CF45) and with only a 12KW boiler it will give baths every 20 minutes, so with normal time spent in bath, continuous bathing is possible. I can't imagine needing 750L unless it's the football team's communal bath.
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