No Cold Water Header Tank - Is this a problem?

We just moved house and next week we are getting the existing baxi bermuda back boiler removed and replaced with a combi (with new rads etc). The bathroom airing cupboard currently contains the HW cylinder and header tank and I had assumed that the CW header tank would ben in the loft. Having had a mooch up there yesterday there is nothing other than a few dead flies and load of lagging. It would appear that all our CW feeds currently come off the mains CW.
What are the implications of not having a CW header tank? Is it even an issue?
Thanks Chris.
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On Tue, 02 Dec 2003 18:37:38 GMT, Chris

The main operational points with having a roof tank are
- you are not limited to the delivery rate of the mains supply, so can run, for a period of time, multiple showers and baths. This is also true of mains derived systems as long as the mains supply is adequate but should be checked.
- that the water using appliances can be independent of one another as regards flow. This can be achieved almost as well with mains connections if the plumbing is organised appropriately and the possible use of flow restrictors.
- that you have a store of water if the mains fails
Hopefully, your installer will have checked that the mains flow rate (not the pressure so much) is adequate. If it is less than about 20 litres per minute at the kitchen cold tap you could have problems if you are looking for good shower and bath filling performance. You can check this yourself by timing how long it takes to fill a measured container like a bucket.
Secondly, hopefully your installer is fitting an adequately sized combi boiler. Your existing system heats the water in the cylinder to 60 degrees or thereabouts and you get to use almost all of it as quickly as you like. You heat the water over time to reach that point.
With a combi, the effect is different. You are applying a fixed amount of heat to cold mains water which varies in temperature during the year and in flow rate. Combi boilers are specified in terms of a flow rate for a 35 degree rise in temperature and the commonly sized boilers will deliver 11-15 litres per minute according to rating. In the summer this is OK, but in the winter, the mains water is down to 5-8 degrees. You can work out from this that under these conditions, the output temperature will be at around 40 degrees which is shower temperature, not hot water. So in effect, the rated output of the boiler is what you will get from the shower in the winter and/or a lower flow rate if you want hotter water.
All of this can be OK if your requirements are modest or you have a large boiler. Note that for combi boilers the heat output to the CH normally modulate downwards, so that even if you choose a large boiler from the HW standpoint, it does not mean that it is necessarily oversized for the house.
Take a look at the spec. of the proposed boiler and make sure that you are happy with what it will do. You can simulate the flow rate by experimenting with turning a tap to a given level and measuring the rate as before.
The other factor, is that depending on the plumbing layout, you may experience more or less interaction between appliances. On a newly planned and implemented system designed for mains pressure operation this can be done quite easily, but more may be involved when modifying an old system in terms of new pipe runs.
.andy
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Chris wrote:

Not really, no. I have seen the setup you describe used a lot - in fact you can get HW tanks witha header on top of them. This has often been a simple upgrade for older properties with gravity HW systems anyway - rip out old tank, header tank, wire all cold taps to mains, and install small HW tank and immersion in wardrobe.
You want mains cold anuyway with a combi, since HW is now mains pressure. And you can rip out the HW tank as well.

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I presume that the header tank in the airing cupboard is for the existing hot water system? It's quite normal to feed all the cold taps directly off the mains rather than from a header tank. When your existing boiler is replaced by a combi, you'll presumably no longer have a stored hot water system - so the hot cylinder and header in the airing cupboard will disappear. [How will you air the washing?]
I presume that there must be an additional small header tank somewhere for the primary heating circuit? Perhaps it's in the attic, buried in dead flies and lagging? [As far as I am aware, Baxi Bermudas were designed for vented rather than sealed systems]. Your replacement system will have a sealed - pressurised - primary circuit with filling loop, pressure vessel and pressure gauge - so this additional header will also disappear.
FWIW, I've never been totally convinced about combis - except for small flats or single occupancy buildings - and would prefer always to have stored hot water. It's worth asking some pertinent questions to satisfy yourself that a combi will really do everything you expect of it before going ahead with this scheme.
Incidentally, although it's not too common, you *can* have stored hot water with a combi - if you use the hot water side just for the kitchen tap and maybe a shower - and zone the heating side so that it also heats a tank of hot water via an indirect coil in the cylinder as per your existing system. That would fill a bath *much* faster.
Roger
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In the same cupboard with vastly more space with a small bathroom radiator inside.

There are some 2 bathroom conbi's about. use one of those and you will be very convinced. You cisl also couple two cpmbi's toigether and see how they sing. Combi's are cheap!

I woudl go for a 15-16 litres/min job.

That is true, and in many cases the ideal solution as it saves spending 250 on a power shower pump alone. Use a combi cylinder cold tank and hot water cylinder combined and stick it in the loft and many problems solved.
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Yes, but in addition to the rate of heat input, there is an additional constraint around the rate at which the mains can supply cold water.
Roger
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On Tue, 02 Dec 2003 18:37:38 GMT, Chris

Existing header tank is for hot water tank and the proposed combi is a Baxi 105e. The mains CW flow rate certainly seems adequate, i'll try and quantify it tomorrow.
From the replies so far it would seem that the current arrangement is not an issue.
Thanks for the replies so far.
Chris.
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On Tue, 02 Dec 2003 20:54:59 GMT, Chris

This boiler has a DHW flow rate of only 12.7 litres/min for 35 degree rise. I would suggest looking at that carefully before committing, and see if you are happy with it at 40 degrees, less at higher temperatures.
It is also a very poor choice from the energy efficiency perspective. At 78.5% SEDBUK rating it is only just scraping into the bottom end of the requirements of the Building Regulations which require 78%. The Baxi Bermuda is at 72% if it's an old one, so you are gaining very little energy saving.
It is likely from next year or the following that the requirement will move to 86%, which will effectively take non condensing boilers off the market.
You would be much better off with a condensing combi boiler and perhaps a more powerful one than the 105e. Efficiencies are in the 90-91% range. I replaced an older 65% efficient wall mount boiler with a condensing model and the reduction in gas consumption is pretty much as you would calculate from the above numbers. A condensing model will cost around 150-200 more for equivalent power level, a little more for a more powerful model. You can calculate the payback period, but it is generally well within the lifetime of the boiler.

.andy
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wrote:

Try the :
"" Greenstar 40HE Plus Inc Clock & Flue Output - 11.4 - 39.1 kW . 38,900 to 133,409 Btu's
Domestic Hot Water Flow Rate - 16 L/min Built in fault diagnostic display.
SEDBUK Band - A 97% Efficiency Condensing In DHW Mode
Price Includes Flue, Clock and Delivery.
Price 1,351.25 Including VAT ""
http://www.discountedheating.co.uk/shop/acatalog/Greenstar_40HE_PLUS.html
16 litres/min flowrate will give excellent performance and low bills.
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On Tue, 02 Dec 2003 20:54:59 GMT, Chris

make sure you fill your kettle each night before you go to bed. Mains water interruptions are few and far between, but not unknown. I do not like to wake up with no means of making a drink - been there, done that. With a cold water tank, at least you usually have a tap (usually the bath) which can give you water that is drinkable after boiling.
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