There has been quite a lot written on this subject but I am still not clear how I get around it. I have an Ideal Isar 30 Combi Boiler which should produce 35C temp rise at a flow rate of 12l/m. The incoming water pressure is such that the hot water flow rate is only 9.5 l/m. I would like to be able to run two showers simultaneously. At this flow rate/water pressure the results will be pretty miserable.
I am thinking of:
a. A larger combi boiler. b. Can one use a pump on the cold water supply?
Are you certain that the limiting factor is the cold water supply? In other words, is that with the tap full on or are you having to turn the tap down to get the desired temperature?
If it's the supply, then you could check the flow rate at the kitchen cold tap. If that is greater than about 20lpm then the issue is not so much the supply to the house, but the plumbing inside it. If you have pipework of 15mm size, upgrading it to 22mm should help.
If the problem is that the water supply to the house is poor, then increasing the combi boiler size will not help. There are three alternatives if that is the case. FIrst get the static pressure tested by the water supplier. If that is reasonably high (say 3bar plus) then an improvement coule probably be made by upgrading the size of the service pipe from the road. You would have to pay for that. Another option would be to install a water accumulator which effectively stores water under mains pressure, refilling slowly but able to deliver quickly for a period of time.
One is not allowed to use a pump on the cold mains supply because the suppliers are concerned that a negative pressure could be created in the main under certain circumstances and contaminants sucked in at places where there are leaks.
The third alternative is to fit a loft tank. This will provide a store of water that is not dependent on mains behaviour.. It is also likely to be the least expensive option. You can supply a combi from a loft tank. Of course, the issue will then be that the pressure available for showers will have been reduced and a shower pump may be needed.
Once the water flow issue is resolved, you still have the issue of heating it. 14lpm flow rate in the winter will be the maximum that is available as the incoming water temperature drops to 5 degrees or so. This is marginally OK for one shower, but completely inadequate for two. Going for a larger combi would help a bit, but realistically if you want to run both showers simultaneously you would need some form of storage of hot water. This could be done by going for a combi with internal storage (although capacities are limited), or alternatively some kind of hot water cylinder. This would also avoid the need to replace the boiler, since the existing combi functionality could be left in place and could feed the hot taps and perhaps one shower, while the cylinder could be heated from the CH side of the boiler using a motorised valve, as though it was a system boiler. Overall, this would probably be less expensive than replacing the boiler as long as you have the space.
On 9 Apr 2006 02:15:17 -0700 someone who may be "john" wrote this:-
Despite the claims of a noisy but small minority, combination boilers are designed for small households, where it is unlikely that several fittings will be in use at once.
If you really want to run two showers at once then the best way of doing it is to install a storage system. This means that you don't need an excessively large boiler for an occasional use. Given the low flow rate of mains water the way to do this is probably to install a suitably sized conventional hot water cylinder, heated by a separate zone, which feeds two pumped showers (unless you have enough head to avoid pumping). Depending on where the combination boiler is located and pipe runs it may be better to re-arrange the plumbing so this cylinder feeds other nearby fittings as well.
If the second shower is only required occasionally why not consider an electric shower? At least there would still be a sort of working shower any time the combi was out of service.
If you really want frequent simultaneous showers look to a hot water storage solution where you can design out all the problems, but first you have to accurately categorise the cold water supply.
Despite the claims of a noisy minority, combination boilers are loosely thrown together for tiny households, usually of less than one person who never have a bath deeper than 1.73 inches and are quite content with water coming out of the hot tap being at any temperature between 5 deg c and 80 deg c - usually the former and alternating every 4.219 seconds depending on the alignment of the planets and the flapping of a butterflies wings 87 miles away. Utterly useless would be overestimating their capabilities.
Storage IS the future although combi's may suit the stinking French for centuries to come.
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