Immersion heated cylinder - space is tight

[jamse]

Hi,

Apologies in advance if I'm asking daft questions - I appreciate that some of what I'm trying to achieve might be mutually incompatible, but I'm trying to suss out the best system possible.

We are in the process of refurbishing a small cottage to move into. The space heating will all be electric, so this query is only re. water heating. We had been thinking of going down the route of an unvented system (and of course getting it professionally installed by someone who's G3 compliant). However my understanding is now that our water pressure is not up to providing adequate performance unvented... so, I'm looking at vented systems, and as a result thinking that it might be feasible to DIY it.

As far as the installation goes, there will be a kitchen sink, bath, bathroom basin and mixer shower, all installed on the ground floor. The airing cupboard space is also located on the ground floor.

I understand that a typical vented system would have a cold water tank in the loft. That would be pretty tricky in the cottage. So, I'm wondering whether I could get away with installing a small cold water tank immediately above the cylinder in the airing cupboard - my understanding is that a setup like that would completely clobber the chance of getting decent hot water pressure from the cylinder, as that comes from the height that the cold supply is above. But if it's otherwise compatible I think we would be happy with resolving that by using a pump. (All the cold taps can be supplied off the mains anyway - I wouldn't be looking to supply them off the cold tank.)

So, my question is - would that setup work? I think what I'm describing might be roughly equivalent to a fortic / combination cylinder, but from my reading I can't quite work out whether they are compatible with pumps either. If they are - would that be a better way of going than my Heath-Robinson version?!

Is there anything else I need to be watching out for? What sort of size of cold water tank do I need to stop it running out if I can pump hot water? Is that any different if I can work out a way of getting the tank into the loft? (The pipe runs are a major issue, with lots of immovable timbers causing problems.)

Another thing - if possible I'd like to specify a cylinder that is 'solar ready' - we probably won't be able to put in solar HW at the moment, but I'd like to at some point relatively soon, and don't want to have to replace the tank at that stage if I can avoid it. Does anyone have any advice on specifying a cylinder that has preferably 2 immersion heaters for use now, good insulation, and a solar coil ready for a later upgrade?

Thanks in advance,
Jamse

 

[doitall]

How you go about it all depends on the storage tank. You need 75-100gallons for a pump, especially if the recovery is low.

If you had a cellar or space for an unvented cylinder and a large storage tank you could install a pump set and pump into the cylinder.

 

[JohnD]

this is not the question you asked, but, as you appear to have gas available, and if you are interested enough in energy efficiency to be thinking about solar hw, why are you going for electric space heating?

 

[jamse]

Hi doitall and JohnD,

Thanks for such quick replies!

@doitall:

We had been thinking for a small cottage, with just the two of us at the moment, and only one bathroom, of going with something like 150L cylinder (so about 30 ish gallons). Is the issue with the pump on a smaller cylinder simply that you'll use up all the hot water?

If so, could we get away with spec'ing the cylinder based on how much we want to use? E.g., we know the size of the bath - say 125 L - so a 150 L cylinder would be more than enough for that. We could also spec the pump so that it doesn't make the flow rate on the shower too high - say 10 L per minute - then 150 L would be good for a 15 minute shower. (And I think I've built some margin into those as they don't take into account the fact that the water used will actually be a mix of H and C, so my hot water would go a bit further.)

On the issue of the cold supply to the cylinder - what happens if I'm pumping hot water out at, say, 10L per minute and, due to having a very small head, gravity would only supply the cold water at a lower rate? Does the cylinder 'suck' cold water down from the header tank? Is there a risk if the header tank is small of that being emptied, and air getting into the cylinder? That sounds like it could be a bad thing.

@JohnD:

Are you assuming gas because of our previous thoughts on an unvented system? It's, hmm, not quite as simple as that! I'd tried to give the edited highlights to keep things simple, but what the heck... ! Here's a brief summary of how we got here:

We were never keen on the idea of gas for any heating for environmental reasons - it's more of a consideration about carbon emissions than just energy efficiency. We get our electricity from Good Energy, so that actually makes our electrical space heaters 'green', if not cheap or efficient, to run.

On the hot water side we had been thinking of solar hot water and got people in to quote. Various problems were pointed out to us, including pipe runs (same issue as our being able to get a cold header tank in the loft, really) and the insufficiency of our rising main to be able to run an unvented cylinder. However, one of the people quoting pointed out how much easier it would be to run a wire for solar PV down to our consumer unit than to run 50mm pipes to our airing cupboard. Solar PV would then be able to contribute to our HW, space heating, and of course any other appliances we were running. The quote came back surprisingly close to the price for Solar HW, and with the Feed in Tariff we might be able to make it stack up financially.

A pipe run from the loft is not completely impossible - it'd require a fair amount of extra work and some putting up with exposed pipe work in our bedroom (which of course I could box in, but even that would be quite an issue). If the feedback I get here is that there's no way of making my system work without a header tank in the loft I may need to accept that - and of course if I accept that then it's probably also back to the drawing board on whether to go with the solar PV or solar HW! (Though I guess there's a difference between a 22mm (?) pipe run for a header tank to the cylinder and a pair of solar HW pipes that are 50mm with their insulation, plus the 22mm as I guess I'd still want a header tank as my rising main still isn't up to powering an unvented solar cylinder...)

As an aside, and for completeness, there is a gas main run to the property, but it is terminated in a meter box with no meter. We are thinking about using gas for our cooker since both the wife and I much prefer to cook on a gas hob, but that would be setting our green principles aside a little, and we're not sure about the cost of getting a meter fitted, and having a gas pipe run to the kitchen. At least that pipe run might be a little easier than the one from the loft!

Hope that summarises our thinking a little. Any thoughts would of course be welcomed!

Kind regards,
Jamse

 

[muggles]

OK, first things first, you should spec the cylinder to the house and any possible future requirements rather than just what you need now, so look at the possibility of the bath and shower being run at the same time for example (unless it's a shower over the bath of course) and also consider things like whether your washing machine draws off the cylinder too.

The problem with using a pump with an undersized loft tank is that you will run it dry, meaning no water until it's refilled, possible airlocks in the system, and pump cavitation leading to premature failure. None of these things are good.


I'm intrigued by your environmental argument against using gas; the framework for gas supply is already in place and has a very long life expectancy, but every time a few more people sign up to so-called 'green' electricity, more wind turbines / other power generation sources must be manufactured, meaning raw materials are mined, transported across countries to manufacturing plants, then transported again to their final destination, and particularly in the case of wind turbines their life expectancy is relatively short, meaning that this process is repeated to create new ones. I would doubt that there is much, if any, environmental benefit in this when you consider all the extra pollution being created by the production of these extra units against the use of an existing system, but maybe that's an argument for another day. I don't want to seem like I'm knocking your attempts to be environmentally friendly, it really is commendable, but I've come across too many eco-warrior types who have just jumped on the bandwagon without looking at the full picture (don't even get me started on Prius drivers...)

IIRC it costs typically 40% more to heat your house using electricity, presumably you're paying a premium for your 'green' alternative so it'll cost you more than that, I hope you have a lot of money to throw away. As for making it stack up financially, you're going to need to produce a very large amount of electricity in order to be able to feed in to the Grid as well as supplying your own needs, and this will of course incur both up-front costs and the loss of interest on the money that is no longer in your bank account. Just make sure you really can afford it before you pitch in!


Having said all that, if you really have got your hearts set on using electricity, then it can be done. I'd be more inclined in your particular situation to go for PV panels and use the electricity generated to power the immersion heaters, a 'wet' solar system would still need backing up with the electric from time to time and there is also wasted energy when the cylinder gets up to temperature and the solar switches off, whereas any extra electricity generated by PV could be used to power other things. If the installation price is similar this is the more sensible option.

As for cooking, look at electric induction cookers, they're very fast to react and might be a suitable alternative to gas for you.

 

[Whitespirit66]

don't even get me started on Prius drivers....

Shouldn't that be spelt Pious!

I'm afraid I don't really understand the green arguments that you put forward. You seem to infer that funds aren't limitless, but you are prepared to invest an inordinate amount of money in a system that may or may not provide the energy you need.

You are against using clean and highly efficient modern gas boilers to heat your home, but have a preference for cooking with gas???

With China opening a new coal fired power station every week, wringing your hands about using clean gas to heat a small cottage seems pointless.

You can tie yourself up in knots and spend all of your hard-earned in the process, but you need to get real, and whack in a combi!

 

[jamse]

Thanks for those comments muggles and Whitespirit66.

On the practical side with the system it sounds like a large enough cold water tank is pretty essential to avoid those problems you describe, which will mean finding a way to get the pipe run from the loft... (The bath is over the shower, btw.)

On the financial side, the Feed In Tariffs, which came into force in April, are very generous for solar PV. They're also something of a misnomer as you don't just get paid for what you feed into the grid, you get paid even if you use the electricity yourself. Roughly speaking I'd be expecting a 10% payback p.a., so after 10 years the kit has paid for itself (quicker if you factor in savings on buying in electricity). And the payback continues for 25 years. We are fortunate to be in relatively well-paid jobs, so paying higher ongoing costs is an option for us, and something we can choose to do by cutting back a little on other 'treats' we might have rather than putting pressure on us to afford the essentials. However, having just bought the cottage and having to spend a fair amount on making it habitable, keeping the expenditure now down would be a good thing.

I completely understand the arguments over the "embodied energy" in installing hardware - that's got to be taken into account for a whole-life analysis of a piece of kit. However, gas is a fossil fuel, and that means year in year out you keep chucking CO2 into the environment when you burn it - not to mention concerns over the security of supply. It's also something we can do to send signals, to governments and energy companies and the like, saying that this is something we're interested in. It's not perfect, but it's a mechanism we have at our disposal.

I don't really buy into the whole attitude of shrugging because there are bigger emitters out there. I want to do my bit, even though that's a small bit on the grand scale of things. If enough people do their small bit then it adds up to a big bit. (And of course we in the west are complicit in a lot of the coal that the Chinese are using - much of it will go into making products to send over here.)

As for cooking on gas - I certainly don't claim to be perfect or an eco-warrior. I want to keep my emissions down, but I'm nowhere near zero. I'll definitely take a look into induction hobs though...

Thanks again,
Jamse

 

[doitall]

Jamse.

The problem with a pump is the storage tank must be large enough to supply hot and cold for a balanced shower, too small and you'll run dry, damaging the pump.

If you have a low mains pressure the combi suggestion is not one you should take seriously.

 

[jamse]

Thanks for that doitall.

I'm going to see if I can lay my hands on a pressure meter so I can get an accurate measure of the water pressure - what level should I be looking out for to count as high enough to make particular solutions viable? (E.g. combination tank, or even going back to the original idea of having someone install an unvented tank for us.) Does flow rate also come into it too? (I guess I can easily measure that with a bucket and a watch!)

My intention had been to run the cold taps and the cold side of the shower off the mains rather than through a cold water storage tank - is there any problem with this? If that's OK, am I right in thinking that effectively I'd be speccing the tank to be large enough to keep the HW cylinder full?

In fact - let me know if this is crazy talk - would the following work? Instead of having a cold water tank supplying the hot water cylinder, could I feed the HW cylinder inlet direct off the mains too? I guess I would need a one-way valve to prevent contamination of the mains. And, as this would still be a vented system, I would need a small vessel that would be performing the 'catching vented HW' role usually played by the CW tank, but it would only be playing that role. (I'd obviously need to work out a suitable way of getting the vented water out...). How does that sound - is it totally daft?!

Thanks again to all for the great input and help on this!

Best,
Jamse

 

[Boilerman2]

It's not only unvented Cylinders that have to be notified, so do vented types under Blg regs part L1 - so why do you think you can DIY, if you do don't forget to pay LABC about £100 to inspect it!!

 

[jamse]

Thanks for the tipoff on the notification aspect, Boilerman2. I didn't know that it was necessary for vented systems. If I do go ahead with it I'll make sure I contact the local council if needed.

Everything I've read does still lead me to believe that installing a vented cylinder is more feasible as a DIY job than an unvented one. Of course I don't know everything about it - but if I did I wouldn't be on here asking for pointers!

What do you think about the idea of getting the cold water source for a vented cylinder direct from the mains (via a one-way valve), and just using the 'header' tank for venting?

Thanks,
Jamse

 

[muggles]

You can't direct-feed a vented cylinder, for a start there's no way of shutting off the water supply when it's full, you need to run the main the the header tank then a pipe back down to the cylinder

 

[jamse]

Thanks muggles - fair enough, I knew what I was suggesting was 'out of the ordinary'!

Thanks,
Jamse