I have had installed a Vokera Mynute HE system boiler with a Pullin Evolution unvented water tank.
The plumber advises me to keep the hot water on 24 hours as this will work out the most efficient rather than having to heat the tank from colder temperatures. This doesn't sound right to me but there is a hint of logic to it.
However our hot water use is pretty predictable, showers in the morning, dinner dishes at night and possibly bath, shower in evening.
Should I be using the time clock to have the water heated for these times rather than 24 hour?
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What does the instruction manual say to do?
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How well insualted is your hot water tank?
If the boiler does not fire during the unused night or day periods, then the idea has merit.
Its always one of these things that you can argue about, but is entirely dependent on how good the tank insulation is and how hot you want the water.
Mine is on 24 hours.
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If water use is predictable and therefore water use reproducible, you can check for yourself can't you? Note the gas meter reading and do a test on 24x7 and another when it's on timer.
If the difference for always being on is negligible then I think I'd go with that option.
I'm about to have a combi boiler for the first time starting next Thursday.... and according to United Utilities a water meter (vendor says there isn't one!?!?).
Last edited by: rtj70 on Sun 23 May 10 at 14:49
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they may have a policy of installing them for new customers.
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Zero, if you're referring to us then the existing owners say there isn't one (the HIP and United Utilities says yes).. we'll find out on Thursday. From what I can tell, we might actually be better off with a water meter anyway.
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they probably dont even know they have one.
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>>From what I can tell, we might actually be better off with a water meter anyway.
We were. It cut our bills in half.
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wait till autumn before you get one
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>> wait till autumn before you get one
I know someone.....who waits until the single bloke next door has gone to work before connecting his neighbour's garden tap to a hose to fill his kids paddling pool.......
:-)))
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Argh! Just go and look for it, it's not difficult!
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Rob, good idea re meter readings, just might do that!
Good luck with the combi, how new is it? Ours was 15 years old and showing signs of age. Biggest struggle for it was keeping a constant running temperature, ie a shower was fine but filling a bath it really struggled with.
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Well its a brand new water tank. Toquote its blurb "it has a stainless steel inner vessel, and an outer casing is a combination of resilient thermoplastic mouldings and plastic coated corrosion proofed steel sheet. All products are insulated with CFC free polyurethane foam to give good heat loss performance"
So to answer your question, I don't know!! :)
Interestingly after they installed it, we had to use it manually until the electrician came and did his bit and wired up the timeclock. When he did this, he inadvertently wired up one wire wrongly which meant that the water heater wasn't working. But it was not until the next night that we realised when the water was all cold. By that time we had dinner dishes done, night shower, morning showers, evening dishes all from the tank so I reckon it must be quite well insulated!
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Out of interest, why a stainless steel tank? Always thought copper was better, particularly in preventing bacterial growth etc in the stored water.
Last edited by: Everest Pete on Sun 23 May 10 at 17:35
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The amount of heat energy needed to warm a tank of water by x-degrees is fixed by the amount of water heated and the temperature rise x. If you know the tank size and the temperature rise, it is easily calculated.
I cubic centimeter of water heated by one degree C requires 1 calorie of energy. To get from that to watt-seconds (joules) multiply the calories by 4.2. You can then get from watt-seconds to watt-hours and then to kilowatt hours.
The longer the tank stays hot, the more heat you lose.
So, as a pure physics question, it must consume more power/gas/electricity to keep a tank hot continuously than to heat it up only when you want to use the hot water.
Of course, in practice, the amount of heat loss depends on how well the tank is lagged and, according to that, what you will save by not having hot water all the time.
P.S. I once had a galvanized steel cold tank in the loft and my hot copper cylinder failed 3times due to steel-copper electrolysis. I changed it and then lost one more tank going porous, and none since. The existing one has so far easily outlasted the previous three.
Last edited by: busbee on Sun 23 May 10 at 18:28
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Bzb
Whilst I don't disagree with the basic physics, you need to factor in other variables. Does the boiler efficiency change depending on the temperature of the water it is heating for example?
You also need to include, for example, the transfer characteristics of the heating coil in the tank, and the thermal loss from the pipes feeding the hot tank.
I am not saying that it necessarily changes the conclusion you have reached in practical examples, but I am sure that it would be possible to design a system that had the necessary characteristics to obey the plumbers statement.
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...unvented hot water tank...
Er, isn't that the essential ingredient of a steam engine?
I wouldn't want to sleep next to the blinkin' thing.
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In the UK people are paranoid about unvented hot water tanks. IIRC they need (expensive) annual certification inspections by a suitably qualified plumber. In France they are the norm - fed directly from the cold water main. A cheap and nasty blow off valve is fitted to vent overpressure water into an overflow. Sold in every French DIY store from about 200€. Often fitted in cheap flat conversions in the SE of England!
In France they are regularly fitted into habitable rooms without a containment enclosure - but you do not seem to hear about many shrapnel deaths!
Last edited by: pmh on Sun 23 May 10 at 21:18
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>> I wouldn't want to sleep next to the blinkin' thing
I've been covering boilers and hot water storage at college over the last couple of weeks - let's see if I can remember it all...
BS EN 60335-2-73 ensures that boiler explosions cannot happen in newly installed systems, as pmh says they are fitted with a blow off valve as a last resort. The hot water storage cylinder and the pipework are designed in such a way that the conditions for explosion can never arise - explosions are caused by pressurised superheated water (>100°C) being subject to a sudden drop in pressure such that a state change occurs and the water expands 1,600x in volume as it turns to steam. Wholesome hot water should be kept at no more than 82°C by thermostatic control, and all of the components in a high pressure unvented system are designed to fail safe.
In answer to the OP's question, rtj70 has the right idea about doing a comparison test over 2x 7-day periods and balancing energy consumption with system performance. The installer is unlikely to be on a kickback from your energy supplier, therefore the advice to run it 24 hours a day is likely to be aimed at getting the best performance from your hot water supply.
And to Everest Pete, stainless steel is used rather than copper because domestic water (especially soft water) is slightly acidic and can corrode copper cylinders and pipework. Bacterial growth only occurs in systems with a cold water storage cistern in the loft - it can be prevented by keeping all light out out of the cistern. This is achieved by fitting a Byelaw 30 kit consisting of an opaque, well fitting cover over the cold water cistern with mesh screens on the ventilation and overflow pipes. The OP's high pressure unvented system does not need a loft cistern.
Hth,
Dave
Last edited by: Dave_TD {P} on Sun 23 May 10 at 23:30
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"Does the boiler efficiency change depending on the temperature of the water it is heating for example?"
Heat transfer is less rapid as the water gets hotter, which complicates things.
DTD
Do you know which is best in a condensing boiler, the use of stainless steel or aluminum for the heat exchanger? Do they both react with the copper in the system or do they both develop an inert layer that stops this?
Last edited by: busbee on Mon 24 May 10 at 10:50
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Boilers aren't covered in-depth on my Level 2 course, that's something they save for Level 3. However, in condensing boilers the main corrosion risk comes from the condensate which has a pH value of 3 or 4, hence S/S or aluminium alloy is used for greater resistance than cast iron. They haven't been around long enough for any long-term effects to be noted, unfortunately.
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After reading this I'm glad I have an instantaneous system for heating and hot water - no tanks
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Only a couple of months since we had a new condensing boiler fitted to the existing system. It has always been my way with the old boiler, as I shall do with this one, to use the timer for HW. Our needs are from 7-9am and 5-11pm so I don't want the boiler to be topping up the hot water for the other 16hrs.
For two reasons...
Condensing boilers only work to their claimed high efficiency when working harder... ie cool to hot....
and experience with ours shows it tends to fire up for short periods at a time when maintaining hot water (when none is being drawn). This must add wear to the boiler fan and other parts.... bit like stop-start driving??
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I had a scrappage voucher but decided not to use it as I was reliably informed that new condensing boilers would be lucky to last 10 years but the old Potterton might go on that long anyway.
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>>I had a scrappage voucher but decided not to use it as I was reliably informed that new condensing boilers would be lucky to last 10 years but the old Potterton might go on that long anyway<<
I had a scrappage voucher but decided not to use it as I was reliably informed that new condensing boilers would be lucky to last 10 years but the old Glow Worm might go on that long anyway.
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Ours was replaced on scrappage and the £400 is a very welcome discount.... particularly as I used an honest installer who didn't find reasons to uplift the cost to match the scrappage!
Our 25yr old boiler was leaking, pilot would only keep going if set like a blowtorch and we needed the boiler moving 2m for some house mods.... so for once it all came together in the most economic way.
Our house is an old one with solid walls and in a very exposed location... our heating bills are high so the savings should be noticeable.
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Yet another boiler query - Bigtee
