Electric or boiler-fed shower with combi boiler?

[Walkacrossthesand]

I’m having a bit of a refurb done, with a downstairs shower room being installed. Current (ageing) standard boiler being replaced with a combi, pressures are good the plumber says.
Upstairs, I already have an electric shower. The electrician says the CU can cope with 2 electric showers.
The plumber/boiler man recommends a combi-fed shower with a ‘thermostatic mixer bar’ (if I’ve remembered the name right?)
My worry is that, if anyone runs a tap/flushes a loo/an appliance anywhere starts to fill, while someone is in this shower, the pressure and shower flow will drop, even if thermostatic bar maintains the temperature. Does anyone have any experience of this? Is @pigletjohnstill around?

Thermostatic is best all day and everyday if you can.

We have this set up, electric shower upstairs, combi fed shower downstairs. Everyone loves the combi-fed shower and the electric shower constantly breaks down and needs replacing (we've been in the house 8 years and are on electric shower no 4, when we redo the bathroom we'll swap to combi-fed). No pressure problems with the combi-fed shower, it's far better than the electric shower.

Thanks @Tortephant@FruAashild. So no sudden drops when something else is drawing water? And (separate question) does the bath fill reasonably quickly? I was around when combis were first introduced and they didn’t have a good reputation - I guess they’ve improved!

If you have low water pressure then it may not be for you. outside of that as long as your system is spec'd to cope it will be fine. your plumber can advise on the specifics or any tweaks you need to make.

I have a combi fed shower, in a flat so same level as the boiler. The pressure does drop if someone runs a tap. I also find the thermostatic valve breaks frequently, and depending on your shower, can't always be replaced. I have heard you can get digital showers now where you set the exact temperature you want. Tempting!

We have a combi with a thermostatic mixer shower, pressure is fabulous and can easily be adjusted too. I love it.

Appliance filling doesnt seem to affect it, toilet flushing slightly lowers the pressure for a minute if that, but nothing significant.

We don't run the kitchen tap at the same time or attempt any washing up while someone is showering though, as that affects the temperature.

We don't have a bath so can't comment on that but based on the shower pressure I'd say a bath from our combi would fill quicker than from a header tank.

Thanks for comments. My experience of electric showers is that I’ve never had one break - but I’m a swift showerer, in and out under 5 minutes, maybe they cope better with that than longer continuous use?

Electric showers tend to max out at 10kw or so whereas combi boilers are often 28kw or even 35kw. You will get far more water pressure.

You used to be able to get pressure balanced showers so the flow drops but temperature remains constant.

Our combi fed shower remains a consistent temperature but there can be a pressure drop if someone e.g. flushes the toilet. But the pressure when it drops is still way better than the electric shower it replaced, so it's not an issue at all.

Good points, @Flippoflak @Walkden- it’s all relative! Thanks all

We have a combi but two electric showers. Very useful if /when the boiler breaks, we still have hot water.

Also, we can use both showers at the same time. Very useful with a busy household. We did have to get our water pipe from our meter to the house upgraded for a fatter pipe though.

I have a gas combi with thermostatic bar. It's great, on demand water, cheaper than electric Pressure drops a little when there is a lot of water being used elsewhere but stays a constant temperature due toe the thermostatic valve. If you have higher throughput mains feed than us you may not have that issue.

If I was having a second shower I would probably get an electric one, especially with more electric (solar and battery) energy these days. Plus having two water heating mechanisms is great when one fails. We were without a shower for 2 weeks over teh winter when our boiler broke.

Most combi-boilers can't provide enough hot water to run 2 showers at full pressure, hence a lot of en-suites have electric showers, the alternative is to have an unvented system, then you need space for the tank and a specialist installer. Yes the bath fills slower with a thermostatic mixing valve. Part G of the building regulations applies, there is a maximum water temperature allowed to fill a bath. Without a mixing valve, you could turn hot and cold on fully, a mixer won't do that.

Water hardness is a key factor in electric shower longevity. In soft water areas they can work for decades. In hard water areas they have problems much sooner.

The shower works fine off the combi for us but we are only a 2-3 person household (DS at uni). I don't wash up when anyone is in the shower or have washer or dishwasher on. It's not that you absolutely can't, it's just better if you don't. This is a downstairs shower and the combi boiler is located upstairs. We live in a quirky cottage where mod cons have to fit where they fit 🤣

We are in a soft water area so no idea why we have such bad luck with them. I'm blaming my DH's love of scorching hot water and the teenagers excessively long showers.

As far as water pressure goes we have no problem with the downstairs combi-fed shower. The dishwasher and washing machine have no impact, the toilet flushing has a small impact but there's no fluctuation in temperature like you can get with an electric shower. If the boiler is sized for a large house/family it should be OK.

Pressure is not the same as flow. A combination boiler can usually deliver around 12 litres per minute, if the pipes can deliver enough flow.

Try filling a bucket at your kitchen cold tap (and your garden tap, if you have one), time it, calculate litres per minute. Older houses often have narrow pipes leading to poor flow. Showers and bathtaps can't deliver more water than the pipes provide to the entire house.

The best HW supply comes from an unvented cylinder. Any modern gas boiler can heat it, very quickly, and it usually has one or two immersion heaters that can be used on the infrequent occasions when the boiler us out of action (or if you have solar panels).

I agree with the above.

Unvented cylinder is the gold standard, can supply multiple appliances at once with high flow rates without varying the temperature of the water. Plus you have a cylinder with an immersion heater that can be used if the boiler goes off line.

A lot of people over the last couple of decades have swapped out their system boilers (with cylinder) for combis, only to find when they then move on to heat pumps they then need to find the room for all the equipment.