Heating dilemna - unvented/combi - or do nothing?

[Smurfin]

Hi all

I've got a bit of a dilemna and after speaking to two plumbers, I can't seem to get a straight answer.

In short, we moved into our house 9 months ago. The house is a 5 bed detached (circa 2100 sq ft), 1 main bathroom with bath/shower, ensuite with shower and a downstairs w/c. Currently it's a gravity fed system; we have a perfectly functional Ideal boiler in the kitchen (can't remember the model, but it's 75% efficient), a header tank in the loft and a 165ltr cylinder in the airing cupboard.

There's a couple of issues. Firstly, as our family grows I'm thinking of a bigger 250lt tank. There's 4 of us, and although our daughters are young we plan to be here for a good few years.

That's more of an aside really, because water pressure for both showers/main bath upstairs is pretty poor. One is on a pump which has now failed and it takes 20 minutes to run a bath now! The ensuite shower (without a pump) is useable, but water pressure isn't great.

What I really want to avoid is installing a new pump, and another one for the ensuite. I can't stand pump noise, and it's made me think about making some changes.

One plumber recommended unvented, but he didn't even measure the water pressure coming into the house. Hmm..

Another plumber recommended either a 42kw Worcester Bosch combi or unvented system, or even just installing a central pump in the airing cupboard and sticking with the gravity system. He measured 3 bar pressure on the cold water tap in the kitchen, and the cold tap in the downstairs w/c, which is right next to the stopcock, must be hitting somewhere around 4 bar + in my opinion. But hot water is really poor....

The right course to take has me baffled. We're laying new flooring upstairs in the next few weeks so any big changes with the plumbing need to be done sooner rather than later. I've almost discounted the combi idea as I just think it will be too frustrating going forward when there's several taps running at once. The question is, do I go unvented, or stick with gravity and fit a decent central pump?

If I don't change the boiler, an unvented install with 250ltr cylinder is going to cost £1530 plus VAT. The wording in my quote is as follows:

Drain down existing heating and hot water system and strip out hot water cylinder. Install new 250l unvented hot water cylinder with expiation vassal and pressure relief valve running to the outside of the house. Re-connect pipe work to new cylinder and wire up to existing connections. Install new expansion vassal on the heating system converting from gravity to pressurized system. Connect mains water supply to filling loop on heating. Add water treatment inhibitor into heating system. Fill heating system and heat test.

The other thing this plumber mentioned was that the hot water cost will obviously rise as I'm not planning to change the boiler but making the cylinder alot bigger.

And thoughts or advice?

Thanks
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[Dan Robinson]

A combi in your situation is a daft idea.

I would probably look at a set of accumulators in the loft, and an Unvented cylinder in the airing cupboard (safety discharge pipe permitting).

As for the boiler. Ideal made lots of boilers. The last decent on they made was called the Classic. If you have one, then it will hopefully last a good while. But if you are doing some major works, then an upgrade would be cheaper to do cumulatively now than as a separate job later.

Your pressure is, at this stage largely irrelevant. If you are moving to an unvented cylinder, the size of the supplying water main is as important, if not more so than the pressure.

It will of course cost more to heat a bigger cylinder, but overall running costs should not rise too much as the insulation means that you don't lose too much energy unless you draw it off from a tap.

You will need at least a 22mm probably 28mm rising mains water
pipe to supply the cylinder direct without the accumulator.

Your only other option is pumps.

The pressure is rubbish because the water is "falling" to the taps from the loft tanks.

 

[dcawkwell]

The first and most important thing to do is measure the flow rate of mains water coming into the house.
You need about 20 litre/minute minimum to go unvented.

A modern unvented cylinder will reheat in approx 30 minutes
with an internal coil of approx 21kW.

I would recommend installing a glow worm ultra power.
A cross between an unvented and a combi. It will work as an unvented
cylinder but should you start to run out of hot water it falls back to
delivering hot water at combi rates.

Boiler and cylinder in one neat box. Peak delivery rate is 36litre/minute of
hot water.

 

[Smurfin]

The first and most important thing to do is measure the flow rate of mains water coming into the house.
You need about 20 litre/minute minimum to go unvented.

Ok, should I be measuring from the tap closest to the stopcock, or the kitchen sink tap?

 

[Smurfin]

A combi in your situation is a daft idea.

That's what I thought!

I would probably look at a set of accumulators in the loft, and an Unvented cylinder in the airing cupboard (safety discharge pipe permitting).

Ok, dumb question time, what's an accumulator, and what does it do?

As for the boiler. Ideal made lots of boilers. The last decent on they made was called the Classic. If you have one, then it will hopefully last a good while. But if you are doing some major works, then an upgrade would be cheaper to do cumulatively now than as a separate job later.

It is an Ideal Classic! The thing is, if it's going to run for years, is there really any point in changing it?

Thanks all for the quick responses

 

[dcawkwell]

The first and most important thing to do is measure the flow rate of mains water coming into the house.
You need about 20 litre/minute minimum to go unvented.

Ok, should I be measuring from the tap closest to the stopcock, or the kitchen sink tap?

Kitchen mixer taps are usually poor flow.
If you have an outside tap try that.
If you have a bath tap (that isn't feed from the header tank) measure
there as well.
You might want to repeat the exercise when another tap is running
in the house to see how the flow is affected. Going to unvented
means all the flow is coming directly from the mains.
As Dan says an accumulator tank can help here storing a quantity of
mains cold at pressure to even out the high demand times(flushing toilet, washer etc).

 

[Dan Robinson]

An Accumulator works like a big ballon squirting water out at a steady pressure for as long as it holds its charge.

It gives you the affect of a pump, but without the noise and need for power (power cuts won't be an issue).

As for your boiler, it is impossible to say - we service many, and they are in pristine order. However, I went to one in Dunstable that had totally disintegrated internally and was pouring water over the kitchen.

Fundamentally they are good boilers, but a modern one can be as good, and 20% more efficient. Installation costs don't rise much just because the house is bigger, but fuel costs do. So your payback time (using percentiles) is greatly reduced over say a 1 bedroom flat.

Some people will b1tch and moan that modern boilers are less reliable than your Classic, but this certainly isn't the case with all of them.

Really though it is a decision only you can make. But if you are having a new kitchen for example, an d the cost is something you can absorb, then it would be less hassle, and costly (relatively) to have it all done at the same time.

You can go either way and not be too disadvantaged.

 

[Dan Robinson]

Its is effectively a big balloon. Mains water fills it under pressure, and is held there until you turn a tap on. The water is then squirted out of the vessel to the taps at mains pressure - quicker than if it was having to be squeezed through a 15mm rising main (which is what most houses have going up into the loft).

It doesn't increase pressure, rather maintains pressure and flow - silently.

They need careful sizing and setting, but are very effective.

 

[Dan Robinson]

Untitled

• Dan Robinson
• 12 May 2012
• 3

Here's three we fitted earlier .

Big bath fills very rapidly - house is 4 storeys with a pressure of 1.5 bar in the loft and a 15mm riser. Customer very happy indeed.

 

[Smurfin]

The first and most important thing to do is measure the flow rate of mains water coming into the house.
You need about 20 litre/minute minimum to go unvented.

Right, I've just checked out on the outside tap, and approximately I think I'm getting 20-22 litres per minute. Am I safe going down the unvented route then? (i.e. not a waste of money!)

Thanks

 

[Dan Robinson]

You should be OK, but will need to check the size of the pipe rising up to the cylinder location. If it is 15mm you will need it upgrading. If it is 22mm or bigger then you should be fine.

 

[Smurfin]

Just checked the pipe at the stop cock, and it's 15mm. How much of an issue is this? Pressure seems fine, measured flow seems fine....?

 

[dcawkwell]

Just checked the pipe at the stop cock, and it's 15mm. How much of an issue is this? Pressure seems fine, measured flow seems fine....?

Should be fine. 22mm would improve things if there was poor flow but
if it is fine as is go for it.

 

[Smurfin]

To be honest, that's my interpretation of "fine" rather than anything qualified. Cold water tap in kitchen is 3 bar fully open, downstairs w/c which is right next to the stop cockc and the outside tap much higher pressure (not measured though). Outside tap measured 22 litres per minute, but I didn't try it with other taps open - should I check this as well?

 

[Dan Robinson]

Yes try it. Measure the pressure with a couple of mains taps open.

15mm will be fine for one outlet at a time. But not more.

I have 6 to 7 bar static pressure and a 25mm main, with 22mm internal plumbing up to where things split off to the various rooms - 15mm pretty much from there on to the appliances.

Two taps work fine.

But only just.