Flow rate of Vaillant Eco Tec v Vaillant Eco Tec storage combi

12 Sep 2020
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United Kingdom
We're looking at replacing our ageing combi boiler and are weighing up whether it's worth getting a storage combi versus a conventional combi. (We've considered a system boiler and cylinder, but we don't think we can make it work from a space perspective; in any event, a combi should be sufficient for our needs.)

My main concern is hot water flow rates, but looking at the Vaillant website, it's not clear how much better the flow rate is on the storage combi.

The ecoTec plus 38kw combi boiler (which as far as I can tell is their most powerful conventional combi they do) is stated to have a max flow rate of 15.9l/m (see https://www.vaillant.co.uk/homeowners/products/ecotec-plus-combi-832.html#specification)

The ecoTec plus combi store 938 is also stated to have a flow rate of 15.9l/m (see https://www.vaillant.co.uk/homeowners/products/ecotec-plus-combination-store-1152.html#specification). However, the marketing blurb talks about a max flow rate of 20l/ minute.

Does this mean on the ecoTec combi that you get a flow rate of 20l/minute for a very short period of time (until the 20l storage tank is exhausted) after which it reverts to 15.9l/m? Or would you actually get that flow rate for longer because you would be instantly starting to top up the storage tank?

From a space perspective, I would only want to go with the (more hefty) storage combi if there was a material difference in hot water flow rate.

Any insight on this very much appreciated!
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For what it's worth, I e-mailed Vaillant about this directly, and this was the response:

"You are correct. The ecoTEC plus 938 is able to give a blended flow rate of 20 L/min for 10 minutes after that the actoSTOR will be depleted so the boiler flow rate will drop to 15 L/min, all Vaillant combi boilers come fitted with a restrictor to restrict the flow to give a hot water temperature rise of 42 degrees over the incomes mains water temperature, the actoSTOR water is held at 65 degrees when cold water is added to make it useable you will then achieve the 20 L/min for 10 minutes which is sufficient for two showers running together, to achieve this an external blending valve must be fitted.

When a small demand is needed from the boiler it would still fire up the boiler and use water from the storage tank and then recharge what has been used in the tank, if you fit a Vaillant digital programmer you will be able to time the charging of the actoSTOR this will allow the store to be changed when there is high demand and during the rest of the day the boiler will work as a standard combi.

I would add that whether 20l/minute is sufficient for two showers at once depends on your showers, but thought this was a helpful explanation nonetheless.

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