The new house, old combi boiler, second shower conundrum!

[Seb101]

I've just moved in to a new house (built 2003) and it's fitted with a Worcester CDi 35kw combi unit that's about 12 years old (installed with the build). It's got a few problems, it doesn't seem to be able to regulate the DHW temperature properly, so it just goes through the cycle of heating water up to scalding hot, then over-temping, flame cut out, 10 second back-off then re-ignition - which repeats ad-infitum. The result at the tap is water that starts ok, goes scalding hot, then lukewarm and repeat. This is fine for sink/bath use as it still supplies instant hot water for washing hands etc, and if you're running a sink/bath you don't notice the fluctuations in temperature.

Where it is a real problem is the shower - for obvious reasons.

There is one additional factor, I need to add a second shower over a bath to the house, for a lodger. I was originally set on just getting a new combi unit and regular shower, but I keep reading more and more posts online that even high-kW combi units will struggle to adequately supply two showers simultaneously.

I want the most cost-effective short-term solution (money is tight after buying the place - budget is like £2000 tops), I'll be ready to pay for a more long-term solution in a couple of years. With this in mind, I don't want to spend large amounts now, only to change it in a couple of years.

I see my options as:
1. Fit 2 electric showers and live with the boiler as-is for sink/baths. Estimate cost ~£500
Pros - cheapish!
Cons - not a huge fan of electric showers, will require messing up the tiling in the existing shower. Still have old under-performing boiler.

2. Fix the current boiler, install 1 electric shower for the lodger. Estimate cost anything from £300 to £800
Pros - potentially cheapest, depending on repair.
Cons - repair cost hard to estimate, research suggests anything from simple thermistor replacement to long drawn-out sequential parts replacement to find the issue (thermistor, diverter, HEX, gas valve, main circuit board). Then still living with old boiler out of guarantee.

3. Install new higher DHW output (42KW Worcester?) Combi boiler, run two regular DHW fed showers. Estimate cost £1600+
Pros - new efficient boiler, DHW fixed
Cons - not confident that any residential Combi can deliver enough DHW to run 2 first floor showers simultaneously?

What would your expert experienced opinions be on this? - bearing in mind I am willing to save up and invest £5k in a few years time to put in a tank-based system if that is what is really needed.

Disclaimer - I am a competent electronics engineer and domestic plumbing DIYer. I have access to a gas-safe registered heating engineer to do boiler work at 'mates rates'.

 

[Agile]

Your boiler should modulate and provide a constant hot water temperature. I suppose you have not bothered to get it serviced and repaired?

You can fit just ONE electric shower!

You can also fit a small cylinder for one shower and heat that electrically for the lodger. With more work and cost to heat it from the current boiler but frankly its not worth the bother.

Can your mains supply really power two showers together?

Another no cost solution is to only use one shower at a time.

Tony

 

[Seb101]

Thanks for the reply Agile If you read my post you will see I have just moved into this house, so I am currently deciding whether to service and repair the boiler, or replace it. It's not a case of 'not bothering' - the previous owner was guilty of that.

Using one shower at a time is not going to be realistic, it would be OK if you have a family, but I can't really tell a paying lodger they can't shower while I'm showering.

I would have to check the mains capacity - good point.

 

[picasso]

If you find the main water flow and pressure to be adequate you could use your existing combi to heat an unvented cylinder, make the dhw side of your combi redundant, then when you have the money you could replace the boiler and havent lost the 2k your spending now.

 

[endecotp]

I can't really tell a paying lodger they can't shower while I'm showering.

Well you could tell them that, and reduce the rent by 2p to compensate them.

Or you could choose a lodger with a job that means they'll get up half an hour before or after you do. Or who doesn't wash

Or you could tell yourself that you can't shower while your lodger is showering.

I suppose what I'm saying is - if I were a lodger, I'd be happy to accept that the plumbing doesn't cope well with two simultaneous showers. It's not the end of the world.

 

[Seb101]

If you find the main water flow and pressure to be adequate you could use your existing combi to heat an unvented cylinder, make the dhw side of your combi redundant, then when you have the money you could replace the boiler and havent lost the 2k your spending now.

Mains water flow is very good where I am. I like this idea. Obviously I will get qualified professional in the do the work, but I like to understand my systems... So essentially the existing Combi will heat the DHW in the unvented cyl from the CH supply through an internal HEX? This cylinder would then supply the showers in the two bathrooms (which are next to each other thankfully - so pipework easier). Then the Combi continues to supply DHW the other taps in the kitchen/bathrooms? Does this setup mean that you have to configure the CH timer to come on specifically to heat the water during the summer? Or are there electronics that allow the tank to request CH?

Also - couple of questions..
What stops the water in the unvented tank from overpressure when the CH is on constant and where does the pressure relief go?
My CH is though narrow gauge pipe - is this a problem for cylinders?

Thanks!

 

[endecotp]

You'll have a motorised valve or valve that directs the water to either the cylinder or the radiators (or possibly both), just like a non-combi system, with appropriate control electronics.

Hot water for the other taps could come from the cylinder or from the existing combi DHW output, with obvious pros and cons.