System upgrade - Combi, Unvented Tank or Gravity fed system - HELP!

k1rkyc

Hi everyone,
I'm in need of some opinions please

My partner and I currently live in a 1980's house with a 20 year old oil Grant boiler. We have 1 bathroom and one downstairs toilet and we don't have any children, We have a gravity fed system at present, with a cold water tank and expansion tank in the loft, and a vented hot water cylinder that's also around 20 years old (if not more!).

We've had a few issues recently with the valve's needing to be replaced in the loft tanks, and also a leaky hot water pipe under the concrete downstairs (now sorted!). We think the time is nearing to have it all replaced.

We've had a few plumbers warn us against Combi Boilers, saying that there's a lot of issues with oil fed ones. We've also been told that a Combi Boiler or an Unvented tank would put extra pressure on the system and potentially cause more leaks in old pipework in the concrete under the floors. Our cold water pressure is around 2.5 bar I believe.

We guess a Combi boiler would probably be the most efficient out of the 3 options, but we've also been glad of the tanks in the loft recently as they alerted us to the underfloor leak.... we would have been blissfully unaware for months... until our water bill arrived!

Just wanted to come on here and get some more advice/opinions from you all please

Thanks in advance for anyone taking the time to read this and/or reply

ThisIsWeird

Hmm, tricky, as a few different factors involved here.
Yes, a sealed/unvented/pressurised system will put more stress on the pipework than a 'gravity/vented' type, so that should be a consideration if the pipes are dodgy.
Having said that, a 'sealed' system has a fixed volume, so if a pipe to a rad bursts, it'll only dump a couple of gallons in your house and then stop; a 'vented' system with its refilling tank in the loft will pour and pour...
How many dodgy pipes  in concrete will you be replacing?
I don't know much about oil combis, but for small households, combis in general can make sense. Also cheaper to install rather than also replacing a hot cylinder.

k1rkyc

ThisIsWeird said:

Hmm, tricky, as a few different factors involved here.
Yes, a sealed/unvented/pressurised system will put more stress on the pipework than a 'gravity/vented' type, so that should be a consideration if the pipes are dodgy.
Having said that, a 'sealed' system has a fixed volume, so if a pipe to a rad bursts, it'll only dump a couple of gallons in your house and then stop; a 'vented' system with its refilling tank in the loft will pour and pour...
How many dodgy pipes  in concrete will you be replacing?
I don't know much about oil combis, but for small households, combis in general can make sense. Also cheaper to install rather than also replacing a hot cylinder.

Thanks for the reply! That's an interesting thought about the fixed volume vs tank refilling.
Well, we've had to 'cap' 2 off and re-route the pipes as they were the cold & hot feeds between downstairs loo and kitchen. There was a leak somewhere along it! Not sure how many other lie beneath!
Decisions, decisions!

ThisIsWeird

Are you laying new pipes for these services?
And is it 'only' hot and cold supplies, or are CH pipes buried in concrete too?
Are these bare copper pipes buried in concrete? 

FreeBear

Copper pipes buried in concrete is just a ticking time bomb - The cement attacks the copper and you end up with a pipe full of pinhole leaks. Sleeving the pipes in plastic reduces the problem, but you need to make sure all the bends are protected as well. You've already had one leak, so more are likely to develop. Realistically, a full replumb should be on the cards when you change the boiler. It would give you the opportunity to fit larger radiators and perhaps even place them on alternative walls.

Rather than sticking a new oil boiler in, I'd seriously consider looking at a heat pump. The difference in cost wouldn't be as much as you might think, and with the government's Boiler Upgrade Scheme grant (currently £7500), you might find a heat pump is cheaper. Pop over to https://forums.moneysavingexpert.com/categories/heat-pumps and ask for opinions there. You could also try getting a quote from someone like Octopus and see what they quote (if they install in your area).

Albermarle

We've had a few plumbers warn us against Combi Boilers, saying that there's a lot of issues with oil fed ones.

Done a quick google on this and can not find anything specific about oil fired combis being a big issue. However I am not an expert !

Section62

k1rkyc said:

I'm in need of some opinions please

I'd agree with Freebear that thinking about a heat pump rather than a replacement oil boiler should be one of the first considerations, but before that, asking yourself whether the oil boiler actually needs replacing yet.  For an oil boiler made in the early 2000's I'd suggest 20 years means it probably still has a fair amount of life left in it - normal/system oil-fired boilers have a lot less to go wrong with them than modern combi boilers and tend to be more robust.  They are often modular in construction so when parts (such as the burner) fail it is often possible to replace them with a compatible parts (if the originals are no longer available).

If you switch to a heat pump then ideally you'll need to have a hot water cylinder.  So removing what you have and going combi will (probably) mean switching back to a hot water cylinder in the future. If you are sticking with oil for the time being then having an electric immersion heater in the hot water cylinder (if you don't have one already) means you'll have a backup heating method for hot water if the boiler breaks down or you run out of oil (more important in remote rural areas where flooding or snow may mean the tanker can't get through to you).  A combi usually means not having a hot water cylinder and immersion heater backup.

Although there's a tendency for people to treat electric water heating as something to avoid at all (very high) costs, if you don't use much hot water and you have the right tariff, heating some of your hot water by electric in the summer months can work out cheaper than only using the oil boiler.  If you have PV panels then combining those with immersion heating of your hot water can be the ideal solution.

So in your situation I'd probably prioritise sorting the pipework and radiators - you'll probably need upgrades to both to be suitable for newer lower flow temperature systems, not just to deal with the leaks.  And consider whether it is worth upgrading the hot water cylinder to one with better insulation (or just buy a jacket).  If there is no visible sign of corrosion or leakage from the original cylinder then you don't need to replace it just due to age.

The impetus to replace old boilers is very much sold on the newer ones being more efficient.  But against that you need to consider whether the capital cost of a new boiler (and installation) will be recovered in savings from a more efficient system before it becomes life expired.  Sadly I expect a lot of people have had new boilers thinking it would save them money, but in reality what they have spent in having the replacement will never be recovered by savings in energy use.

If you had pressure on living space and needed to free up the area used by the hot water cylinder, or were considering a loft conversion and the water tanks were in the way, then changing system may make sense.  But if you don't have those problems and you are happy with the performance of the gravity system then making changes would possibly be changing for the sake of change.

coffeehound

Did they measure what maximum flowrate you could get from the main?  2.5 bar is quite low so this could end up dictating your choices

ThisIsWeird

2.5bar, whilst not wonderful, should be perfectly fine for most combi use. The delivered flow should also be measured, tho'.

Many combis are being run on less than 2 bar.

FreeBear

Section62 said: The impetus to replace old boilers is very much sold on the newer ones being more efficient.  But against that you need to consider whether the capital cost of a new boiler (and installation) will be recovered in savings from a more efficient system before it becomes life expired.  Sadly I expect a lot of people have had new boilers thinking it would save them money, but in reality what they have spent in having the replacement will never be recovered by savings in energy use.

Went from an ancient Baxi back boiler with a SAP rating of 65% efficiency to a Viessmann 050 combi (SAP rating 89%). Had been using around 3300kWh per year of gas with the Baxi, and now looking at just 2800kWh. A £20 per year saving - That is going to take a very, very long time to recover the cost of having a new boiler (like, never).

But having replumbed the system and fitted larger radiators, the place is easier to heat. Also in a better position for the day when heat pumps are the only viable option.

Albermarle

ThisIsWeird said:

2.5bar, whilst not wonderful, should be perfectly fine for most combi use. The delivered flow should also be measured, tho'.

Many combis are being run on less than 2 bar.

Yes, I would be very happy to just have 2 bar !