poorly designed combi system - need technical, objective advice.

Hi all, first of all thanks to anyone who takes the time to read about this problem, and more thanks to anyone who replies

Trying to give advice to a customer regarding an 'inadequate' central heating design. He has, a Victorian three floor terraced house with toilet on the ground floor,an ensuite with an electric shower as well as the main bathroom containing a bath and a large steam cubicle with shower jets etc. (It's specifications state that it requires 10lpm cold and 10lpm hot water at 1.5bar) on first floor, and a further en-suite ont he secon floor containing a bath and an electric shower.

The newly installed Biasi 28 combi, (which states at best 12 lpm (summertime) but more likely around 8lpm.) is required to supply the four basins, two baths, kitchen sink and steam cubicle,

There is a 100% certainty that the three bathrooms will be frequently used simultaneously

My suggestion... the bath and steamroom in the main bathroom should be supplied from the cylinder, all other points around the house from the combi. My assumptions, bathrooms will rarely be used at the same time as the toilet and kitchen on ground floor. The bath and shower in the main bathroom will rarely be used at the same time.

There is space on the second floor which could accomodate a cylinder and header tank . The tank would sit about 1.6 to 1.8m above the steam room shower head, but only 0.3m above the bath/basin in the en-suite on the same second floor

My questions.

  1. Based on the required flow rate for the steamroom shower, what size cylinder should I recommend?
  2. Will the 1.6m head of water create sufficient pressure for the multi headed monster steamroom shower?
  3. Should I take the cold water for the steamroom shower from the header tank to ensure like pressures between hot and cold - or would a pressure equalising valve be sufficent to balance cold mains water pressure, and the cylinder fed hot water, and how efficient are they at coping with fluctuation on e.g the cold, when someone flushes a toilet and the runs a basin concurrently?
  4. Based on the above, what size header tank should I recommend?
Reply to
sparkydude
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I reckon a shower is at least 50 litres, and if the HW is at +60C and the cold ( winter ) at +5C, then a +40C shower needs to be about 2/3 hot water, i.e. 2/3x50=34litres. Call it 100 litres for two consecutive four minute showers, minimum.

Your shower specs may call for 10l/min hot water and 10l/min cold water, but that is more of a desireable characteristic of the supply piping, not necessarily what it will use in practise. Half and half hot and cold water will give you a cool shower, so I don't think you will exceed 15l/min total supply to the shower head in practise.

The specs call for 1.5 bar, and simple maths says that your header tank will deliver 0.16bar. Doesn't look good.

I'd fight shy of having cold water at mains pressure and hot at 0.16bar. I'm sure you'll have to do something. However, as I said, will your shower cope with hot water at 0.16 bar?

Sorry I can't suggest a decent solution, someone else out there will know more.

Andy.

Reply to
Andy

If mains flow rate is high enough and you are going to install a cylinder anyway then it seems like a thermal store or heatbank would be appropriate.

Await the large flood of people recommending heatbanks and the odd person recommending one combi for each bathroom. :-)

Mr F.

Reply to
Mr Fizzion

For an installation like this, I'd be looking at 180 litre plus.

Not a hope in hell. Not even close. You would need at least 15m to meet the minimum spec and it would appreciate twice this.

Personally, I would measure the flow rate available at the house. If sufficient, I would install a mains pressure hot water storage based system, either a heat bank or unvented cylinder (probably the former). The combi hot can be used solely for the kitchen. You could run one of the showers off it too, to provide backup if the storage system is down for maintenance.

If the flow rate available from the mains is less than 40lpm or thereabouts, I would use a gravity cylinder with pumped output, with a 50 gallon header tank. I would feed the cold either from the header tank, or direct from mains (if the flow is adequete, say around 30lpm) with pressure balancing valve. Other alternatives are to replace the water mains, or to fit a very large accumulator on the mains supply, if the static pressure is good enough, but the flow inadequete.

Christian.

Reply to
Christian McArdle

Thanks to all for replies so far. The method of adding a second combi, I had discounted as a suggestion because I am not convinced that it would be a 'definite' solution, there is also a lack of wall space and a wish to keep his costs down, however this does now seems to be the cheapest option. The heatstore/unvented cylinder option would require a degree of knowledge/expertise/confidence that the plumber does not posess, I believe the cylinder option I had initially been most keen on still has merit when combined with a twin pump for both hot and cold, from a 180+litre cylinder and a 50 gallon tank respectively and a 'traditional' plumber. Thanks everyone for your assistance - and if you feel I'm risking my reputation by suggesting the above. Please feel free to tell me so.

Reply to
sparkydude

Rightly so. Remember the drencher shower requires 10 lpm HOT water, not 40C tepid. If it wants it at 60C, that is equivalent to a combi claiming 16 litres per minute. That leaves nothing left for additional showers or hot taps. So you would need to find a 45kW combi to run it, or find a way of reliably combining the outputs of two smaller combis.

Quite frankly, if you can't install a heat bank, then you shouldn't claim to be a plumber. They require you to plumbing in the cold mains input, the hot mains output and the indirect coil in just the same way as any other cylinder. Any plumber who couldn't do this would not be allowed past my door.

Christian.

Reply to
Christian McArdle

Where can you get heatbanks from? preferably online at a reasonable price. All I've seen are comparable in price to a combi.

Reply to
<me9

thereabouts,

Best go for a heat bank and have the combi do the kitchen and one shower. The mains supply should be tested and replaced as suggested. Also take a

22mm cold feed pipe from stopcock to the heat bank. At the stopcock run another pipe just to the cold outlets. The cold supply for the showers take off just before the heat bank.

Another suggestion is supplement the combi boiler with a 75kW MAN Micromat combi boiler which will deliver about 30 litres/min, plus the 10-12 litres/min of the BIASI. The MAN is not cheap though, but v good. You will need the next size up gas meter fitted for the MAN.

Reply to
Doctor Drivel

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