Converting gravity fed system to unvented

dr_bogenbroom

Hi,

I’ve moved into a 30 year old house which uses a gravity fed system with cold and F&E tanks in the roof space.

We have a 14 year old oil boiler which works fine but will probably need replaced in the next few years so it’s at this point I’ll consider other options.

I’d like mains pressure throughout the house for both hot and cold water taps; is it possible to do this with pressurising the central heating system? If this is possible this would reduce the chances of leaks appearing due to the pressure increase on the 30 year old system.

This is assuming our mains pressure and flow rate are suitable. The rising main is fed by 20mm MDPE and has a flow rate of around 20l / min. 

We may consider a heat pump in the future but I’m not quite sold on this yet and oil here in NI is 35% cheaper than natural gas. 

Thanks.

Section62

dr_bogenbroom said:

I’d like mains pressure throughout the house for both hot and cold water taps; is it possible to do this with pressurising the central heating system? If this is possible this would reduce the chances of leaks appearing due to the pressure increase on the 30 year old system.

Did you mean 'without'?

An "indirect" hot water cylinder means the hot water supply system can be at a different pressure to the water circulating in the HW/central heating circulation pipework.  It should be possible to operate an unvented indirect cylinder on a vented heating system... but there are a lot of other factors to take into account and you might be best getting a heating engineer to have a look round and advise you on what would work in your situation, as well as future-proofing as much as possible.

ThisIsWeird

dr_bogenbroom said:

Hi,
I’ve moved into a 30 year old house which uses a gravity fed system with cold and F&E tanks in the roof space.
We have a 14 year old oil boiler which works fine but will probably need replaced in the next few years so it’s at this point I’ll consider other options.
I’d like mains pressure throughout the house for both hot and cold water taps; is it possible to do this with pressurising the central heating system? If this is possible this would reduce the chances of leaks appearing due to the pressure increase on the 30 year old system.
This is assuming our mains pressure and flow rate are suitable. The rising main is fed by 20mm MDPE and has a flow rate of around 20l / min.
We may consider a heat pump in the future but I’m not quite sold on this yet and oil here in NI is 35% cheaper than natural gas. Thanks.

Hi Dr B.

A few wee points to start with;

1) A 30-year old house should have plumbing in very decent condition, so should cope with the large increase in pressure from going 'unvented' with no issues. Of course, they could always be a rogue joint that will be exposed by this, but you'd have to be really unlucky. Do you know if the pipework is even plastic?

2) 20lpm is decent, and should be fine for an unvented system. What matters equally, tho', is the pressure it is being delivered at. Has this been measured at all? The difference it could make would be, for example, if you open two outlets at the same time, say a shower and flushing loo; with a low delivery pressure (say, 2bar or below), you can expect the flow to each outlet to drop quite noticeably. With a good 3+ bar pressure, the 20lpm should be nearly maintained.

3) You are out in t'sticks? Are you saying you have mains Natural Gas available, but oil is 30% cheaper than this?! Or by 'natgas' do you mean LPG in a tank?

4) I don't blame you for being cautious about Heat Pumps, and it's usually best to hold back as long as you can (ie, for as long as your current boiler is still working), as they are constantly being improved, and prices coming down. I am assuming that you still plan to retain a 'hot cylinder', tho' changing to an unvented type? Ie - you ain't going 'combi' oil boiler?

Ok, your current boiler is working ok at the mo'? And you are planning the transition to 'unvented' only when this boiler goes kaput? If so, I'd agree with that - don't change your system with your current boiler - just leave it all, and keep researching and checking your options and stuff out - like your mains pressure.

Almost certainly you can nurse your current boiler along for a few good years, and I think you will have arrived at the best way forward at this point. A reasonable chance it'll be HP :-)

I understand that HPs require a certain type of hot cylinder, probably more like a 'Thermal Store', but i'm not well versed in this. So it would be silly to go unvented now, fitting a conventional 'Megaflo' type hot cylinder, only to find it isn't suitable for HPs later on - a waste of money.

Nurse what you have, and keep your options as open as possible.

Yes, you can go unvented, but do check your mains pressure too. (There are always ways to boost this, tho', if needed.)

FreeBear

ThisIsWeird said: I understand that HPs require a certain type of hot cylinder, probably more like a 'Thermal Store', but i'm not well versed in this. So it would be silly to go unvented now, fitting a conventional 'Megaflo' type hot cylinder, only to find it isn't suitable for HPs later on - a waste of money.

DHW cylinders for a HP have a huge coil inside, way bigger than you'd use with a gas or oil boiler. Heat Geeks have developed a small thermal store cylinder which they claim will replace a conventional DHW tank and take up a lot less space - Opinions are divided as to whether this is a game changing product or if it lives up to the claims being made.

If the OP has microbore pipework, it will probably need ripping out and replacing with 15/22/28mm pipes. Certainly, the radiators will need replacing with much, much bigger ones. Depending on room sizes, perhaps doubling up on radiators.

Heat pumps are likely to be the future for most of us. But it is going to be expensive to install (for some, the £7500 boiler upgrade grant will make it reasonable). For the majority, the installation will be disruptive, especially where pipework has to be replaced. If possible, it makes sense to make the system heat pump ready if/when components need changing - I've already replaced most of my plumbing here with 15 & 22mm pipes and routed them with a HP in mind. Also fitted larger radiators, but these will probably still be too small for a HP. Got rid of the DHW tank (went for a combi) - The original location would have been unsuitable for a HP due to the length of pipework required, so have earmarked an alternative location.

Kiran

I'm not sure what the combinations of coils are that you can get in a modern hot water cylinder, but you may well be able to get one with a heat pump coil and leave it for future use. I was at one point looking at installing a thermal store that would be heated via, wood, oil, electric and solar thermal. The wood fired heat source was going to be installed at a later date do the coil was going to be left for future use. In the end I did away with the wood heat source due to the cost of a wood gasification boiler plus the uplift cost of the store over the multi coil cylinder I went with in the end. The extra over cost will be installing a large stove centrally in the house to push the heat around more than the one which takes a while to get he heat around downstairs due to the solid stone walls (the room that it is in being part of the old cottage) acting as a thermal store. 

There's lots of permutations and combinations and advice given over a forum is best of the posters ability without seeing the whole picture. Definitely worth getting advice from someone who can survey and properly design 

dr_bogenbroom

I’ll speak to the boiler service guy when he comes in a couple of weeks. This I suppose leads me on to my next debate - oil vs gas (and future proofing for heat pump tech).

Here in NI oil at the moment is 58p per litre, so this works out at about 5.6p per kWh. Natural gas is expensive here at about 9p per kWh. Of course oil can fluctuate more and isn’t regulated and did go up to 99p per litre during the energy crisis. It does get more expensive during the colder months due to supply and demand.

I can get connected to gas for free - I’d just just need to pay for the boiler and other upgrades if going with unvented etc. I know gas boilers are slightly more efficient as they can modulate unlike oil boilers which are on/off, and with the oil I have a big ugly tank in the garden which will need replaced one day… 

This is one for the future as I’m going to upgrade our insulation from 100mm to 300mm soon but any thoughts are welcome.

Just to add the house has an EPC D, cavity walls and solid internal walls with concrete lower floors in which pipe work is buried in. It’s a detached 4 bed house with double glazing but this is 30 years old now, but seems ok for now! 

ThisIsWeird

dr_bogenbroom said:

I’ll speak to the boiler service guy when he comes in a couple of weeks. This I suppose leads me on to my next debate - oil vs gas (and future proofing for heat pump tech).
Here in NI oil at the moment is 58p per litre, so this works out at about 5.6p per kWh. Natural gas is expensive here at about 9p per kWh. Of course oil can fluctuate more and isn’t regulated and did go up to 99p per litre during the energy crisis. It does get more expensive during the colder months due to supply and demand.
I can get connected to gas for free - I’d just just need to pay for the boiler and other upgrades if going with unvented etc. I know gas boilers are slightly more efficient as they can modulate unlike oil boilers which are on/off, and with the oil I have a big ugly tank in the garden which will need replaced one day…
This is one for the future as I’m going to upgrade our insulation from 100mm to 300mm soon but any thoughts are welcome.
Just to add the house has an EPC D, cavity walls and solid internal walls with concrete lower floors in which pipe work is buried in. It’s a detached 4 bed house with double glazing but this is 30 years old now, but seems ok for now! 

To recap - don't do anything until you actually have to. Yes, ask the service guy for his thoughts, and use it to build up a pic. Always bear in mind that it'll just be his (theoretically informed) opinion, but all folks have their biases.

By the time your boiler goes kaput, the choice between oil and NG may have become more clear. Certainly, as you say, NG means much smaller and a bit more efficient boilers, the loss of the oil tank, less noise. It may also be ready for converting for use with H gas, or any other development in that area. Oil will be oil, tho' perhaps bio-oils will be developed further, who knows? And Heat Pumps will surely continue to improve. So leave the decision for as long as you can. :-)

A domestic CH/DHW system consists of two sections; one includes the boiler, radiators, rad pipes, and the heating coil to the hot water cylinder. The water in this part is kept completely separate from the hot and cold domestic water supplies - wot comes out your taps. This boiler 'system' can be 'vented' or 'unvented', the former requiring that wee F&E tank in the loft, and the latter doing away with that, and instead having an Expansion Vessel and a pressure gauge. That's the boiler system. It has no bearing on how the H and C water is supplied to your taps.

Then there's the H and C supplies, the H is currently coming from your DHWT (hot cylinder) which is supplied under gravity by your CWS in the loft - the big tank - and the C comes directly from that same tank. This, too, can be made unvented, by losing the CWS in the loft, and fitting a DHWT that can withstand mains pressure. 

You can even have them mixed - the boiler system being unvented, and the DHW fed from a CWS. Or even vicky-verka, tho' that would be daft. Usually both sections are the same.

An unvented, sealed, boiler system makes sense, as you can lose that wee F&E tank, and the system water tends to remain in better condition 'cos it isn't open to the air. An unvented DHW system can make sense as you can lose the big CWS in the loft, and can have a good, mains-powered flow to all your taps. But that comes down to how good your mains is.

FreeBear

dr_bogenbroom said: Just to add the house has an EPC D, cavity walls and solid internal walls with concrete lower floors in which pipe work is buried in.

Copper pipes buried in concrete is a recipe for disaster unless the pipes have been sleeved in plastic. The cement in concrete attacks copper, and eventually, pinhole leaks start to appear. Depending on the flooring you have down, you will be blissfully unaware of any leaks unless the heating system is pressurised.

As part of your long term planning, have a think about where pipes can run in the ceiling & upstairs floor. Preferred locations for pipes to drop down to ground floor radiators, and radiator location.

When I replumbed my heating system, I opted to run a pair of 22mm pipes from front to back (the upstairs joists run in the same direction). Radiators are mounted on internal walls as close to the feed/return pipes as practical and connected with 15mm pipes. Radiators are traditionally mounted under windows in the belief that the rising heat will counteract cold air forming around the windows. In my opinion, with modern double/triple glazing, the radiators no longer have to be under windows.

ThisIsWeird

DrB, are any of the original plumbing pipes made of plastic? Especially the ones coming up from the floor to the rads. 

dr_bogenbroom

House was bought early nineties so not sure if that will determine whether or not the pipes buried in concerts will be in conduit or not.

The pipes coming up from the floor are copper and 15mm