Woohoo - good water pressure, combi's on!

Waterline came out unprompted after an email enquiring what our pressure is. Flow rate is 13 litres a minute, pressure is 5 bars at 11:00am. He did warn they will be dropping pressure to avoid leaks from burst pipes in the future, but our position means it shouldn't be too bad. Woohoo.
So all we need to decide now is what gas combi model would be good for us (moving from oil). We want 2 really good showers. We looked at the condensing Vaillant ecoMAX 835/2 E 11.9 litres a minute at 42 C. Will look at other makes too. Few questions:
1. Is it OK to put in an understair cupboard, with a vent to outside?
2. Will we need a flue? It would be 3 storeys to the apex of the roof, we like to avoid a big ugly flue if we can.
A preparatory pipe is all ready laid under a new drive to bring the gas in at this point, and an outside drain is only a few feet along.
3. Can we drain the condensate through the wall and along a pipe into this drain, or will that cause a proble with freezing?
4. On the Vaillant site there are VANTAGE unvented hot water cylinders. Would we need one of these too for any reason?
5. If somebody turns the a hot/cold tap are we likely to get frozen/scalded in the shower? (Or even is there a wee flutter of temperature change?)
6. Is 5 bars at 11:00am likely to drop dramatic at the peak 7:00 to 9:00am?
TIA Suzanne

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Suzanne, this is not really a good flow rate at all. The static pressure of 5 bar is fine but I think that you will find 13 litres/min disappointing for running two showers - that's just about enough for one.
My supply does 20 litres/min at the kitchen cold tap and is inadequate for mains pressure hot water IMO. Each of my showers does between 20 and 25 litres/min depending on head setting, using a roof storage tank to provide the peak water requirement.

This is a well regarded product, but is not going to deliver the goods for what you want. The specification that is useful for comparison between makes is the flow rate at a 35 degree dT (temperature rise). This boiler delivers 14.3 litres/min (if you had the flow available).
If you had approx. twice the flow rate that you do from the mains (say 26 l/min, then the boiler would do a reasonable job from spring to autumn when the water temperature of the mains can be 20 degrees. However, in the winter, it can drop to 8 or even less. In these circumstances the boiler would be pretty much flat out delivering water at 40 degrees or so (shower temperature). Ergo, the maximum water rate at this shower temperature would be 14 litres/min, limited by the boiler.
I think that if you want to go the combi route and have two decent showers, then you will need to consider upgrading the mains water pipe. You will also need a larger boiler.
Of course this does depend on your definition of "really good shower".
Mine is 15-20 litres/min minimum with the head in drench mode (which still has a fair pressure), and about 15 litres/min in jet mode which borders on the uncomfortable at close range.
To give you a comparison, piddly electric showers do about 4-6 litres/min.

If you look on Vaillant's web site, you will find the installation instructions for this boiler - and that answers these questions.

The flue can go through the wall, but there are rules on location relative to other things. It shouldn't need to go to roof level, but you may also want to give consideration to the plume of water vapour that will exit the flue and direct it accordingly
You don't need any other ventilation for this model, according to the manufacturers.

You will, but as long as the rules are met, there is a lot of flexibility.

You can although it's better to run it internally if you can. Outside, it is better to increase from the 19mm overflow pipe used to connect to the boiler, up to 32mm waste, to prevent freezing.

This is a pressurised hot water storage cylinder. It would get you over the performance limitation of the boiler in that you would have X litres of water stored at 60 degrees. This could be used for one or even both showers and the combi part of the boiler used to provide supply to either the other shower or to the rest of the house. It won't get you over the limited flow rate, though.

If you use a good quality thermostatic shower valve and the rest of the system is installed properly it should be fine.

It could do, and in the evening. Mine varies between 3 and 6 bar.

To email, substitute .nospam with .gl

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