Radiators not piping hot last year

Vitor

A potential downside of any chemical flushing of an older CH system is opening up pinholes in the copper piping, leading to more cost to locate and repair.

WIAWSNB

Vitor said:

A potential downside of any chemical flushing of an older CH system is opening up pinholes in the copper piping, leading to more cost to locate and repair.

Always a risk, but that's not really a 'fault' of the process, but a side effect of removing the sludge that was protecting and blocking these holes, which would almost certainly leak in the future in any case. 

A power-flush could expose this, but that would allow its repair. If the pipes are fine after a PF, then it should remain so. 

w00519773

WIAWSNB said:

WIAWSNB said:

You won't need the 35kW in order to heat your home - your CH will draw a lot less than this - but the extra 5kW over the 30kW will give you noticeably better showers and faster bath fills, if that's important? 

However, you will need a matching cold water mains supply in order to benefit. Any idea what your mains flow and pressure are like?

Hang on - is your Potterton one of these large floor-mounted jobbies with a built-in unvented hot cylinder? And the WB replacement is a straight forward combi? If so, then the WB combi will almost certainly be a 'flow' disappointment compared to the Pottie, as it has to heat the water instantly on demand. If that is the case, then I'd suggest the 35kW. 

Could you answer the info required, please?

Yes, it is one of those big floor-standing boilers with a built-in unvented hot water cylinder.  I believe the Powermax range was designed as a storage combi: it has both a gas boiler and an integrated unvented hot water cylinder in one tall, floor-mounted unit.

WIAWSNB

w00519773 said:

WIAWSNB said:

WIAWSNB said:

You won't need the 35kW in order to heat your home - your CH will draw a lot less than this - but the extra 5kW over the 30kW will give you noticeably better showers and faster bath fills, if that's important? 

However, you will need a matching cold water mains supply in order to benefit. Any idea what your mains flow and pressure are like?

Hang on - is your Potterton one of these large floor-mounted jobbies with a built-in unvented hot cylinder? And the WB replacement is a straight forward combi? If so, then the WB combi will almost certainly be a 'flow' disappointment compared to the Pottie, as it has to heat the water instantly on demand. If that is the case, then I'd suggest the 35kW. 

Could you answer the info required, please?

Yes, it is one of those big floor-standing boilers with a built-in unvented hot water cylinder.  I believe the Powermax range was designed as a storage combi: it has both a gas boiler and an integrated unvented hot water cylinder in one tall, floor-mounted unit.

I'm familiar with them. They give a cracking hot water flow, as the water has been preheated. It's max flow is dependent on the incoming mains supply.
The WB is a combi, so that needs to heat the water instantly, on demand. That's why it needs to be powerful - 30 or 35kW. It only fires to that level when providing hot water, and only for as long as the tap is open - it should save a wee bit on running costs.
Again, a combi is dependent on the incoming mains flow, so it's worth knowing what this is - any idea?
So, in essence there's little/nothing to choose between the two for CH, but the 35 (or 36 or even 38) will provide a larger hot water flow, probably not far from what you've been used to.
I have a 30kW combi, and it gives great showers. Running a bath, however, is painful, which is why I haven't done so for more than a decade...
Anyhoo, test yer incoming mains, and compare the DHW flow rates of the two combi sizes, and decide if it's important.
To test your incoming supply, run your kitchen cold tap, full on, into a container for 6 seconds. Measure the litres produced, and times it by ten = litres per minute. (Better still, use a garden tap that comes off the mains).

Vitor

A 35kW combi will be fine for instant hot water but you really must pair it with a controller that has outside weather compensation and full modulation of the gas burner or it won't run in most efficient condensing mode which saves 15% on the gas bill

w00519773

I sent this to the plumber:

"
The reason I’m having the boiler replaced is that a few of my radiators are not working, and most of them do not get fully hot. In addition, the boiler itself is quite old.
I’ve done some research and understand that a standard system cleanse with inhibitor (which I was informed is included) may not be sufficient in my case — do you agree? It seems that a powerflush might be required to properly remove any sludge build-up.
Could you advise whether it would be best to arrange a powerflush before the new boiler is installed, or if this can be done as part of the installation process? Also, if it is possible to do it beforehand, would you be able to carry this out as a separate job prior to the boiler replacement?"

and received this back:

"
If your radiators are cold, it is a sign that there is sludge build up in the system, if you were to go ahead with a power flush you would be advised to do it before installing a new boiler, otherwise the sludge could circulate into the new boiler and cause damage.

If the system is quite old, a chemical flush may advisable instead as a power flush can cause leaks in old rads/pipework. The chemicals would be introduced and we would leave it for a few days before draining and flushing.
"

Not sure what to do.

Albermarle

w00519773 said:

I sent this to the plumber:

"
The reason I’m having the boiler replaced is that a few of my radiators are not working, and most of them do not get fully hot. In addition, the boiler itself is quite old.
I’ve done some research and understand that a standard system cleanse with inhibitor (which I was informed is included) may not be sufficient in my case — do you agree? It seems that a powerflush might be required to properly remove any sludge build-up.
Could you advise whether it would be best to arrange a powerflush before the new boiler is installed, or if this can be done as part of the installation process? Also, if it is possible to do it beforehand, would you be able to carry this out as a separate job prior to the boiler replacement?"

and received this back:

"
If your radiators are cold, it is a sign that there is sludge build up in the system, if you were to go ahead with a power flush you would be advised to do it before installing a new boiler, otherwise the sludge could circulate into the new boiler and cause damage.

If the system is quite old, a chemical flush may advisable instead as a power flush can cause leaks in old rads/pipework. The chemicals would be introduced and we would leave it for a few days before draining and flushing.
"

Not sure what to do.

Toss a coin.

WIAWSNB

Wil you be having annual services in order to maintain the warranty - I presume around 10 years? If so, you are 'warranted', so I'd choose your preferred installer, or do as Albe suggest

FreeBear

Vitor said:

A 35kW combi will be fine for instant hot water but you really must pair it with a controller that has outside weather compensation and full modulation of the gas burner or it won't run in most efficient condensing mode which saves 15% on the gas bill

Depending on actual cold water flow rate, a 30kW, or even a 24kW combi will be sufficient. Few homes really need a 35kW unless the cold water flow rate is exceptionally high.

If the OP wants a storage combi, the Viessmann 111 has a 46l storage tank. It will also modulate down to ~3kW which is plenty to maintain the temperature in many 3 bed properties. You'd need a bit more to actually heat the property up (typically, 6-8kW).

If you are fitting a new boiler, it is well worth doing a proper heat loss calculation and replace some of the radiators with larger ones. Fitting larger radiators means you can run at a lower flow temperature which means higher efficiency from the boiler. Coupled with a programmable thermostat that can modulate the boiler, you have potential to cut your gas consumption. Unlikely you'll achieve 15% saving, even with weather compensation (more likely 5-10%).

But do consider fitting a heat pump. If you don't have any other gas appliances (gas oven), you'd save on not having to pay the standing charge. the cost of running a HP would be comparable to gas, perhaps even lower if the system is specified and tuned properly. Wish I'd gone for a HP when I replaced my boiler a couple of years ago...