Unvented cylinder vs combi boiler

[Misty9]

My plumber, who is installing a new bathroom for me, has suggested that I have a few choices to get the best water pressure for a shower:

• leave hot water cylinder as is, but install pump for the shower. Problem may be small cylinder so would empty quickly, so possibly get a bigger one
• replace with unvented cylinder. Good water pressure and no pump needed.
• replace boiler completely with a combi. Most expensive but end up with good water pressure. Could then resite boiler from loo to loft, and lose cylinder and tank.

Current cylinder is dark green and quite small. Only me and two dc under 10, but they will get older and want showers too eventually.

Any opinion as to the best option?! Budget isn't a major issue.

I should have said, it's a thermostatic mixer shower I want (over bath)

Hopeful bump... @PigletJohn?

I would go for combi every time. Why would you not want limitless hot water on demand?

"Why would you not want limitless hot water on demand?"

Until someone else runs a tap or flushes a loo. Then your water pressure plummets.
I'd go unvented.

If your water pressure and flow rate are good (have you tested it? It's not difficult it like checking the air pressure in your tyres) then a combi is all you need (assuming you are on gas), it will keep up, save you space and they are so efficient these days. You won't look back. You only need unvented tank if you have multiple bathrooms where lots of hot water might be demanded all at once.
If you do notice the toilet getting flushed while you shower you can put in an accumulator to infill that pressure drop when someone draws water while you are in the shower. They act like a buffer. They take up less space than a water tank anyway so you would still save space.

@Clymene

I would go for combi every time. Why would you not want limitless hot water on demand?

Because combi boilers don't always give the best pressure I've heard.

@ReviewingTheSituation

"Why would you not want limitless hot water on demand?"

Until someone else runs a tap or flushes a loo. Then your water pressure plummets.
I'd go unvented.

Tap? Yes, flushing doesn't affect mine. How many times will the OP take shower and someone running a tap? Few times I guess.

I was put off from combi boiler for years because of your reasons. But once installed, it was no issue.

I'll say combi boiler.

Unvented cylinder would still register the toilet flush in a pressure drop unless you have it pressurised (which the accumulator can do for your combi)

@ChickensMightFly

If your water pressure and flow rate are good (have you tested it? It's not difficult it like checking the air pressure in your tyres) then a combi is all you need (assuming you are on gas), it will keep up, save you space and they are so efficient these days. You won't look back. You only need unvented tank if you have multiple bathrooms where lots of hot water might be demanded all at once. If you do notice the toilet getting flushed while you shower you can put in an accumulator to infill that pressure drop when someone draws water while you are in the shower. They act like a buffer. They take up less space than a water tank anyway so you would still save space.

Thank you. The water from the kitchen cold tap is the mains pressure isn't it? How do I test the flow?

It's a 3 bed semi with one bathroom and two basins, plus the kitchen. Just me and then 2 dc half the time. So probably not a huge demand on water.

Hmm. I'm leaning towards getting a combi - mainly because then I can lose the tanks and site it in the loft thereby losing the ugly pipework in my tiny separate loo!

I'd keep it vented & fit a pump for the shower. Reasons:

• you can perfectly match the pump to whatever shower you get, thus giving ideal pressure etc. Want to pressure wash your scalp every morning? That's an option.
• vented system boilers are generally cheaper/simpler/more reliable
• immersion backup for hot water
• cold water tank backup for cold water
• existing plumbing not all exposed to mains pressure - less leaks.

@FurierTransform

I'd keep it vented & fit a pump for the shower. Reasons:

• you can perfectly match the pump to whatever shower you get, thus giving ideal pressure etc. Want to pressure wash your scalp every morning? That's an option.
• vented system boilers are generally cheaper/simpler/more reliable
• immersion backup for hot water
• cold water tank backup for cold water
• existing plumbing not all exposed to mains pressure - less leaks.

Interesting... The pumps are pretty noisy though aren't they? I've had one years back in a rented high-rise flat. Also, it would be in dd's room.

I have had combi boilers for 30 years in 3 different properties. Never had an issue with pressure.

And with several people living somewhere, running out of water halfway through your morning shower is much worse than the water going cold/pressure dropping while someone is brushing their teeth!

My only experience with a combi was awful. Took ages to run a bath (no shower). Have had vented and unvented since and unvented was my favourite but pumps do seem to have got a lot quieter if you go vented.

An unvented cylinder can give you unsurpassed hot water.

How many people live in your home, and many bathrooms and shoerrooms are there?

Fill a bucket at the cold tap at your kitchen sink (and your utilty room and garden tap if you have them). Time it to fill. Calculate litres per minute. How many lpm does it deliver? How old is your house (and the water supply pipe, if younger)?

For your own interest, measure lpm at the bathroom hot tap as well. This is relevant because a combi may deliver less, especially in winter if you like a fully hot bath. Bear in mind that a bath takes around 100 litres of water.

What are the dimensions of your existing hot-water cylinder, and what colour is it? (this is not a joke.)

We got a combi 2 years ago
Boiler and pump are outdoors
Love it. No issues with pressure or noise from pump

@PigletJohn

An unvented cylinder can give you unsurpassed hot water.

How many people live in your home, and many bathrooms and shoerrooms are there?

Fill a bucket at the cold tap at your kitchen sink (and your utilty room and garden tap if you have them). Time it to fill. Calculate litres per minute. How many lpm does it deliver? How old is your house (and the water supply pipe, if younger)?

For your own interest, measure lpm at the bathroom hot tap as well. This is relevant because a combi may deliver less, especially in winter if you like a fully hot bath. Bear in mind that a bath takes around 100 litres of water.

What are the dimensions of your existing hot-water cylinder, and what colour is it? (this is not a joke.)

Me and two dc under 10 half the time. One bathroom with separate loo. Cylinder is dark green and quite small. I'd estimate 1m high? It's in the bottom half of the airing cupboard. I'm not too fussed about baths but I love a powerful hot shower. Currently it has an electric one. I haven't tested the pressure.

cylinders usually come in inch sizes. It might be 36" or possibly 42" high. It may be standing on wooden bearers with a gap underneath.

Is it 24" diameter, or 18" (the foam insulation is probably about an inch thick)

there might be a label with dimensions or capacity.

BTW you can get a shower with integral pump, so the noise will be in the bathroom, probably screwed to a wall which, if brick, will not carry the sound like a wooden floor. Aqualisa make some good ones but there are cheaper brands. I think you only need a pump for your shower. It will be much better than your electric shower.

You can test the flow like piglet John said or your plumber should have a little gadget, is like an asthma flow meter you just hold under the tap, dead easy

Combi boiler can keep the pressure up if you buy the right one.

I have poor water pressure in my house, and just installed a new combi last year, with the shower running off it. It is wonderful. It is a fantastic shower.
It does lose pressure if someone turns the kitchen tap on, but I think that is because of my existing poor pressure.

@PigletJohn

cylinders usually come in inch sizes. It might be 36" or possibly 42" high. It may be standing on wooden bearers with a gap underneath.

Is it 24" diameter, or 18" (the foam insulation is probably about an inch thick)

there might be a label with dimensions or capacity.

BTW you can get a shower with integral pump, so the noise will be in the bathroom, probably screwed to a wall which, if brick, will not carry the sound like a wooden floor. Aqualisa make some good ones but there are cheaper brands. I think you only need a pump for your shower. It will be much better than your electric shower.

This is the cylinder label. The cold water tap in the kitchen is very fast flowing but I don't have anything to measure it in as I'm not living here yet. Integral pump sounds interesting...

the label says it is 900x390mm

Which means it is 36 inches tall and 15 inches diameter, expressed in those trendy new metric numbers.

And it says capacity is 99 litres

So it is just, and only just, enough for a hot bath. When heated with the immersion heater, there will be less HW because the immersion element does not reach to the bottom of the cylinder.

Green is around 20 years old. Newer ones are light blue.

I strongly recommend upgrading to a bigger one.

When you know the water flow we will know if you can make use of an unvented cylinder or a combi.

Bear in mind that a good shower will deliver at least 10 litres per minute.

The cylinder might reheat in about 20-30 minutes from the gas boiler.

Here are some examples of Power Showers (incorporating a pump).

@PigletJohn

the label says it is 900x390mm

Which means it is 36 inches tall and 15 inches diameter, expressed in those trendy new metric numbers.

And it says capacity is 99 litres

So it is just, and only just, enough for a hot bath. When heated with the immersion heater, there will be less HW because the immersion element does not reach to the bottom of the cylinder.

Green is around 20 years old. Newer ones are light blue.

I strongly recommend upgrading to a bigger one.

When you know the water flow we will know if you can make use of an unvented cylinder or a combi.

Bear in mind that a good shower will deliver at least 10 litres per minute.

The cylinder might reheat in about 20-30 minutes from the gas boiler.

Here are some examples of Power Showers (incorporating a pump).

Thank you so much. Okay, so it's old then. The boiler is a glow worm, not sure how old. I'm going to take a bucket over for painting tomorrow so I'll measure the flow hopefully.