Combi vs heat only for old pipework?

[IrvWorth]

Hello - I've had different advice from different heating engineers and whilst I suspect the answer comes down to personal preference, it would be helpful to have some more opinions.

We are replacing the regular boiler but not radiators in our 3/4 bed semi and have been advised by one engineer that the higher pressure in the system caused by a combi could result in leaks or other issues. Another engineer is telling me that no one replaces with regular these days and a combi will be fine. Current systems we think is >15 years old.

We are two working adults with two young kids - hopefully we'll be here for a long time (i.e. with teenagers). We currently have one bathroom but might add another shower at some point.

What do you think?

 

[bernardgreen]

If you have a working set of radiators with header tank in the loft and a hot water cylinder then replace only the boiler with a heat only ( not a combi ) boiler.

That is my personal ( non preofessional ) opinion based on the experiences of friends, relatives and neighbours who have been persuaded that a combi was the best option for them. More than half of them regretted having had a combi installed.

 

[DP]

A regular boiler could be a system boiler ( it has a pump and expansion vessel within boiler case), open vented boiler ( afore mentioned components external to boiler) connected to radiators and cylinder
A system boiler subjects the pipe work to pressure of about 1.5 bar, whereas open vented uses the feed and expansion cistern in the loft
A combi is just a system boiler with hot water generating components within the case
Having a cylinder means if boiler goes belly up, you still get hot water
A combi can be connected to a cylinder for hot water from two sources
A combi will have more components within case, so chances of it failing are greater

Person who says no one replaces 'regular' boiler for similar is talking rubbish
By the way you can have a combi that does not subject pipe work to higher pressure

 

[Dan Robinson]

As above.... An Intergas will run open vented on the heating (and only has one extra part in combi form).

Any installer that defaults to a combi option "because everyone has them now innit" should be shown the door.

If you're happy with your pressure on the hot and cold them there are advantages to keeping the cylinder.

A decent installer will consider the options, discuss your plans then steer you to a decision.

FWIW, out of the last 10 boilers we've installed only 4 have been commissioned as Combi's.

 

[IrvWorth]

Thanks everyone, really helpful answers (love this forum!)

Current pressure is fine but we have an electric shower and would be looking to get a mixer when we eventually get the bathroom done. Yes - we have an existing loft tank, would this be enough pressure for that kind of shower (upstairs bathroom)?

So sounds like pressure increase within the existing system is not a major concern?

If the answer to both the above is yes then i guess the decision is:

• heat only - keep the airing cupboard (which we like), immersion backup if boiler fails, and i guess we could upgrade to a high recovery cylinder in future if we find we are running out of water after showers etc.

vs.

• combi - hot water on demand but at the expense of heating, space gain (we'd put it in the airing cupboard freeing up the kitchen wall)

Or is there anything else to consider?

 

[bernardgreen]

Yes - we have an existing loft tank, would this be enough pressure for that kind of shower (upstairs bathroom)?

The answer depends on the tpe of shower you prefer. Skin ripping high pressure jets or gentle skin soothing rain fall.

Mains pressure electric heated showers may be mains pressure at the cold water inlet but are seldom mains pressure at the shower head due to flow restriction inside the shower unit. A header tank a feet feet above the loft floor can provide a pressure at the shower head that is similar to that from an electrically heated shower.

 

[stem]

I have had experience of both types and here are my personal observations.

For a new install, combi's don't need header tanks and the wiring is much simpler, installers like these because it makes the installation cheaper, and can avoid the need for them to employ the services of an electrician. However, in your case if these items are existing and there is sufficient control with motorised valve(s) etc., it doesn't apply.

Because Combi’s have more components inside, they have greater possibility of a failure, however, boilers, are not suitable for DIY work. Repairs should only be carried out by suitably qualified people and with parts from the manufacturer. On the other hand, most heat only boilers have a lot of components externally mounted. Pump, motorised valve(s), cylinder thermostat, programmer etc., these can be easily replaced by a competent DIYer or local plumber, and with any manufacturers parts obtainable from local plumbers merchants, or DIY sheds.

To generate hot water directly from the cold mains supply, combi boilers generally have a larger heating capacity than a heat only boiler. Because of this, you may have to have a larger diameter gas pipe to the boiler installed. In my case this unsightly and highly visible pipe was routed around the outside of the property all the way back to the gas meter.

Combi’s can be very slow to produce hot water if the boiler is starting from cold. First the boiler has to heat up, then a heat exchanger has to heat up, then the heat is transferred from the heat exchanger to the incoming cold water, only then can hot water begin to make its way to the tap. Some boilers have a facility to keep themselves preheated by firing up the boiler every so often to keep it warm just in case you need some hot water, but that only covers the first stage of the delay. So, if you are on a water meter, you may find consumption increases whilst you wait for hot water to arrive.

 

[Agile]

To be fair some water is wasted waiting for hot from a cylinder as well and this is more as pipework is usually 22 mm.

But I always advise a cylinder if more than 1/2 people living there.

Tony

 

[Madrab]

So sounds like pressure increase within the existing system is not a major concern?

The only issue you may have with pressure is if you go from an open vented central heating system to a sealed/pressurised system. The existing radiators and pipework will need to be pressure tested to ensure that there are no weak points in you system prior to going sealed, any found would then need to be corrected.

As the others have mentioned though your CH system can stay as open vent (low pressure) with the new boiler and if you find the cold cistern in the loft with the hot water cylinder doesn't give you enough hot/cold showering pressure, a shower pump can be added to boost. An unvented HW cylinder could also be installed, that will supply mains pressure HW.

A separate HW cylinder is much more preferable for a family IMO.

 

[IrvWorth]

Ok, sounds like heat only is the way to go for now, and then we can update cylinder in future it required.

Thanks again

 

[IrvWorth]

Ok, sounds like heat only is the way to go for now, and then we can update cylinder in future it required.

Thanks again