Combi or system boiler

[xyz123]

Hi, we currently have a conventional boiler with hot water tank.. Only have 1 bathroom supplied from a 2.5bar pump and kitchen sink.

We are having loft conversion done in few months so boiler needs to change. Loft will have a bathroom so total 2 bathrooms and kitchen tap. I know system boiler should work but it takes up space and I was wondering if a bigger combi boiler would OK.

I spoke with 2 plumbers and both gave different advise re combi or system boiler. How do I go about objectively deciding between the 2? I need to cater for 2 bathrooms being used at the same time as kitchen sink. Can I somehow measure incoming water pressure and flow at kitchen tap? If so, how do I know what is acceptable for combi boiler?

Thanks

[miroku1]

Baths in both ?

[xyz123]

Thanks for your reply. One shower and one bath.

[OutInTheShed]

A combi boiler should be an 'OK' solution if the water supply is adequate.
Does the incoming supply keep reasonable pressure when you flow a suitable rate of litres/min for two showers or a shower and a bath?
A medium sized combi should have enough power to heat that flow.

But with an eye to the future you might want a tanked system, to be heat pump or solar PV/immersion compatible?


Bad plumbing with too much pressure drop can ruin either system.
You may need to make sure the various users share reasonably, poorly laid out pipework can make that difficult.

[trevt]

We have 3 x bathrooms and went with system when we installed. Apart from the hot water storage tank it really doesn't take up much more room. Our tank is remote to the boiler in a laundry room.

When i researched before deciding the major manufacturers had reasonable tools that would guide you to the right type of system and recommended boiler. Whilst none recommended a combi - 3 bathrooms - they all did have big enough boilers that could cope. But, in comparison to the system boiler we installed, we would have seen a significant uplift in boiler output (~30K to 40+K BTU).

My point is you might need a bigger combi boiler and maybe use more gas as a result?

Flow rate on mains pressure, how hot you like showers and how likely you'll have both running at the same time. For me the system gives very hot showers, all three running at once and no detriment to performance.

Someone made the point think what you might do in the future, eg kids that grow into teenagers that take more showers

And finally, my trust in plumbers was severely dented when the last one i called to service the system found a problem, explained to me that a he'd never seen a system boiler as it was commercial and a combi was ideal for our needs. The boiler pressure vessel was faulty and he wanted to replace the whole system. A more realistic plumber installed a remote pressure vessel to "bypass" that in the boiler.

[Sheepshanks]

If you haven't got space for a pressurised hot water cyl then you haven't got a lot of choice.

We do have a cylinder, as, to my mind, the business of starting and stopping a combi every time someone wants to rinse ther hands (and they probably won't even get hot water in time anyway) seems a bit of a nonsense.

The water cyl won't run two rainfall showers and the kitchen hot water tap at the same time - well it will, but the pressure / flow would drop a lot, you just don't get scalded in the shower as the hot and cold are pressure balanced. You'd run out of hot water eventually too, although the cyl reheats pretty quickly - not far off as fast as you're using it. A combi, obviously, just keeps producing hot water.

A big advantage of a pressurised cyl over a combi is it fills the bath very quickly

[Daniel-3ov01]

A system or heat only boiler and unvented hot water cylinder will always be better than a combi boiler when it comes to drawing off hot water from simultaneous outlets, a standard combi boiler will be compromised on flow rate from multiple outlets even if you get a large 40kw one.
You could get a storage combi (basically a combi with a small built in hot water storage tank) but thay are expensive very big and can be a pain in the ass to fix in my experience.
Assuming your house has the water pressure and flow to support simultaneous water draw offs you could consider having a combi boiler running all the how water demands and an electric shower in the second bathroom, you could then have one shower or bath running off the boiler and the second (electric) shower could run at the same time with no detriment.
If you have the space for a hot water cylinder then an unvented cylinder is the best solution though.

[Nemophilist]

We have two bathrooms (two showers , one bath) and when we changed our boiler we opted for a combi but made sure it had the output to cope with the amount of rads and bathrooms we have.
Im happy with that choice. It copes well. Our water pressure is generally good if that might have made a difference.

[Patio]

Had a system boiler with tanks in loft and was persuaded by plumber to go combi when we had the rear extension

Biggest mistake I ever made

Have since put solar on the garage and can't take advantage of an immersion, and shower pressure not as good as before

Regret it every sunny day

[Trustmeimadoctor]

You can have a cylinder and immersion with a combi

Pressure is very unlikely to be any less but flow could be but it can only supply what your mains can supply really. A combi if speced right can easily supply many simultaneous showers

[Patio]

Trustmeimadoctor said:

You can have a cylinder and immersion with a combi

Pressure is very unlikely to be any less but flow could be but it can only supply what your mains can supply really. A combi if speced right can easily supply many simultaneous showers

Had a pump with the old system but don't think you can with a combi unless I'm mistaken?

My cylinder and water tank are still in loft, could they be reconnected with a combi?

[glennjamin]

I'd stick with the tank, I heat my water using diverter from solar panel pretty much free hot water heating from march to end of September.

[Patio]

glennjamin said:

I'd stick with the tank, I heat my water using diverter from solar panel pretty much free hot water heating from march to end of September.

Exactly where I was prior to going combi

System and tanks is the way to go

[Clockwork Cupcake]

I needed to replace a very old and inefficient traditional boiler (it was way past EOL too) and I was advised that combi boilers potentially operate at a higher water pressure than traditional boilers and that the joints on the pipes in my old house might not take it (cap'n) so I replaced my traditional boiler with a newer more efficient condensing traditional boiler, and kept my hot water tank and everything else. It was also the cheapest option too as it was the minimum change.

Edited by Clockwork Cupcake on Sunday 2nd March 02:01

[Crumpet]

Patio said:

glennjamin said:

I'd stick with the tank, I heat my water using diverter from solar panel pretty much free hot water heating from march to end of September.

Exactly where I was prior to going combi

System and tanks is the way to go

We’re on a combi in a large five bed and in the summer when the heating is off the gas usage for the hot water side of things is about £12. I assume that would be similar throughout the year even when the heating is on.

As nice as it would be to have free water heating from solar for six months of the year the numbers just don’t stack up for return on investment for a long these things.

The combi is fine for us as a family of four but two showers running at once are on the underwhelming side! But then they would be as it’s only a 28kw boiler and the max hot water flow is 12lpm. I’ll step it up to a 40kw boiler that’ll do 18 litres per minute next time.

[croyde]

So the age old question. How should you run a System boiler?

I'm in a 2 bed rental flat with an electric shower, so hot water is only needed to wash hands, shave and to rarely fill the bath.

Even just sticking it on for an hour a day seems a waste so I never turn it on and just boil a kettle for the dishes every evening.

Not sure why the landlord fitted it, brand new, before I started the tenancy as a Combi would have made much more sense, or am I wrong

[Clockwork Cupcake]

croyde said:

So the age old question. How should you run a System boiler?

I'm in a 2 bed rental flat with an electric shower, so hot water is only needed to wash hands, shave and to rarely fill the bath.

Even just sticking it on for an hour a day seems a waste so I never turn it on and just boil a kettle for the dishes every evening.

Not sure why the landlord fitted it, brand new, before I started the tenancy as a Combi would have made much more sense, or am I wrong

Certainly sounds like a combi would have suited that Use Case much better.

[OutInTheShed]

Clockwork Cupcake said:

croyde said:

So the age old question. How should you run a System boiler?

I'm in a 2 bed rental flat with an electric shower, so hot water is only needed to wash hands, shave and to rarely fill the bath.

Even just sticking it on for an hour a day seems a waste so I never turn it on and just boil a kettle for the dishes every evening.

Not sure why the landlord fitted it, brand new, before I started the tenancy as a Combi would have made much more sense, or am I wrong

Certainly sounds like a combi would have suited that Use Case much better.

If the tank is well insulated, there's nothing wrong with a system boiler in this case, I'd assume it also heats some radiators?
A tanked system with an immersion as back-up can be a good idea for landlords, as if the boiler stops working the tenant still has HW.

I wouldn't take out a tank to fit a combi, because in the long run electric is going to be the only option.

[C-J]

Crumpet said:

We’re on a combi in a large five bed and in the summer when the heating is off the gas usage for the hot water side of things is about £12. I assume that would be similar throughout the year even when the heating is on.

...

£12 - is that per day or week or month?

[Crumpet]

C-J said:

Crumpet said:

We’re on a combi in a large five bed and in the summer when the heating is off the gas usage for the hot water side of things is about £12. I assume that would be similar throughout the year even when the heating is on.

...

£12 - is that per day or week or month?

Sorry, that’s per month in the summer, based off the smart meter readings from June to October when the heating is off. 80kwh per month, although at 24/25 prices it might be a few more £. In theory that’s just under three hours of running water at 28kwh, which doesn’t actually seem enough.

Rest of the year for gas we use in the region of 25,000-30,000 kWh so water heating barely registers. Combi is now 13 years old so the stop/start nature of combis doesn’t seem to wear them out quicker, although we’ve just replaced the igniter this winter for the first time.

I’m a fan of combis. I’ve used a well set up system boiler and tank and it was great. But I also know other people who run out of water fast and still can’t run two showers at the same time - my parent’s system boiler and tank is terrible. Given how flaky and st a lot of plumbers are I’d be worried I ended up with a crap system.

An electric shower covers redundancy for the very few periods when the boiler isn’t working.