Hi All,

Looking for some advice on how best to approach a new heating system - primarily for DHW but perhaps later for SH via UFH.

We live in a small single storey house with a flat roof in the mountains in southern Spain. We are currently working on getting it properly insulated (from the outside!) but the bureaucracy involved means it's probably going to take a while. We currently have 3 woodburning stoves - two small freestanding ones and a larger glass-fronted one built into the old chimney in the living room. For DHW we have a decrepit on-demand gas boiler connected to a cylinder outside.

Our DHW demands are fairly limited. Normally we'd have between 2 and 4 people living in the house, but we're all grubby hippies and we probably shower a maximum of 2.5 times a week on average, and we probably don't use more than 30l of water when we do - so that's 300l of shower water a week - of which not all will be from the hot water system. The rest is washing up/domestic cleaning, so I'd guess we're not normally using more than 100l of DHW / day at the absolute maximum - and usually less.

The DHW system needs changing urgently. Our water comes from our own springs, and we don't have a huge head on it - currently only a few metres - might end up being more soon if we can get another tank commissioned a bit higher up - but pressure isn't great. We're having loads of problems with insufficient pressure causing the boiler to cut in and out when having a shower + the boiler is old and not very good anyway. We have a lot of free wood available and an aversion both to on-demand gas boilers - which I've never liked - and using fossil fuels for heating generally.

We'd like to move to heating our water with wood + PV electric (we have a 3.75k solar array whose output we are rarely using to the max) + possibly some sort of simple solar collector. For the time being we're going to try and continue to do space heating using the wood burners but it's possible that at some point in the future we'd like to have the option to do something with UFH if we have the time/money to redo all the floors :-/

The obvious simple solution in a UK property would be a vented DHW cylinder with two coils, one from a wood boiler and one from the solar (with the option to add a buffer for UFH later). The problem with this is that, living in a single storey building with a flat roof, the venting/expansion requirement is tricky to do. I don't want to have to have a pump just to get adequate DHW pressure. Is there a way round this that I'm missing?

From a safety perspective, I'm pretty uncomfortable about a sealed system with a solid fuel boiler. Even with pressure relief valves and maybe a heat-dump towel rail or somesuch, the prospect of having a potential bomb that could flatten my house if I overstoke the boiler makes me twitchy. Also, pressurised cylinders seem like an important potential point of failure even in "normal usage" - I believe in the UK they need to be inspected annually for this reason? Am I being unduly paranoid about this? Is there a way to use a sealed system with a solid fuel stove safely?

That leaves me with some sort of thermal store option - dumping the heat from boiler + solar + electric into a big tank of water and then extracting it into DHW via an internal coil/internal tank/external PHX. In the future we could potentially use the store water to run UFH as well. On paper this looks like the most suitable option for us, but it's also going to be expensive + I'm worried that for the quantity of DHW we need it's pretty wasteful heating a big thermal store. I'm also lost on the whole internal coil vs external PHX thing - I'm obviously keen to maintain as much stratification as possible so that we can get sensible temp DHW with a minimum size store, but I can't work out which system is better for that. PHXs can return cool water to the bottom of the tank, but only if the hot water demand is enough to coil the primary circuit water significantly; coils give you a certain amount of stored hot water + they're simpler + their impact on the cylinder water isn't dependent on speed of DHW demand, but I'm guessing that even the best designed coil is going to destratify the cylinder more than a PHX system operating in ideal conditions...

Other points to note (in no particular order):

* We're at 1200m altitude and it does freeze here occasionally - solar collector probably needs to be indirect with glycol I guess
* We get a pretty decent amount of sun most of the year - our small DHW water needs can probably be delivered from solar almost all of the time if it's done right

Any thoughts/pointers gratefully received!

Cheers,

Posted By: djh…if not then PV is a lot simpler and I believe is cost competitive.

From my doodling on the subject:

1) If the sun is bright then solar thermal beats PV on cost and area.

2) If you only want low-temperature water (e.g., for UFH in a reasonably well-insulated house) then solar thermal beats PV on cost and area.

3) If there's only weak sun and you want hot water (e.g., DHW) then solar thermal more-or-less doesn't work whereas PV at leasts works a bit and well if you've got enough area.

Since most of Europe has conditions for 3 for a significant part of the year (a few months, at least) then that seems to dominate. In addition, as DJH says, PV is simpler. It's also more flexible: you can make use of the spare electricity in summer (run a car or whatever) whereas excess hot water is just a nuisance and potentially a dangerous one.

Posted By: djhOr a drainback system, or a freeze-proof silicone-tubed system.

Or cycle a bit of water through the collectors to warm them during cold nights. It's what I'm planning to do, combined with an emergency drainback which needs to be reset and refilled manually.

Thanks for the input guys. It seems like the thermal store option is probably the way to go.

On PV vs Solar Thermal
We are completely off grid (we're right out in the boondocks) so we have a moderately large system (3.75kw nominal) but no grid backup. During the winter we will probably not have a huge amount of spare PV power (our solar day is quite short thanks to a steep rise to the west) - certainly not enough to do more than a top-up heat of a store. I could get more panels but we already have 15 so space is a little bit of an issue. For the summer I'm also thinking of doing something a little bit mad and bonding some copper piping to the back of my PV panels and running some of the store water through it. The rear of the panels gets to about 75-80 degrees in the hottest parts of summer, which really drops their efficiency. If I can dump some of that heat out into water, I can get a double benefit - more PV output + some extra hot water! For the winter (when the PV panels won't get warm enough to do anything useful) I was thinking of making a home-made collector - we have a good and v. cheap metalworks in the village so I think it shouldn't be too hard to get a basic flat-plate collector working without exorbitant expense. Unlike the UK, we have enough sun here that hyper-efficiency of the collector isn't such a big deal.

Stores
Being something of a tinkerer, and given the price of purpose-designed thermal stores, I'm seriously considering making my own. But the feasibility of that depends on what solution I aim for. If I use an external PHX, then all I need is a large tank with a solar coil and a decent number of ports at useful heights. Then I can insert a few temperature sensors and wire the whole thing up to e.g. a Raspberry PI. I can control some valves with the RPI to optimise where the various heat inputs are directed based on their flow temp + the temp of the store at various heights. If I want an internal coil for DHW my choices may be a little bit more limited - a lot of the tanks with a well-designed internal DHW coil + a solar coil already have all the pumps/electronics built on, and I don't really want to pay several grand for someone else's R&D - which I can't alter if it's broken/unsuitable for my use-case. The other thing I don't know about is whether there are tanks available with good stratifying baffles on various inputs but without all the electronic gubbins that I don't want to pay for.... Any pointers/previous experience on this gratefully received!

Heat Exchangers
djh - I'm not sure exactly what you mean about your PHX - are you saying that it keeps the secondary water inside the PHX pre-heated to 30? And that this is causing the primary circuit to run more than it needs to? This is exactly the sort of reason why I want to build me own control logic! We recycle grey water + have very short pipe runs so warm-up times aren't a huge issue for us. If your PHX is returning water that is overly warm to the bottom of the store, that would suggest that the PHX is incorrectly sized for the application, no? Obviously it's not going to be as cold as mains input (and you probably wouldn't want it to be) - but it should be extracting enough heat to get you down to around 30C, shouldn't it?

I'd love to be convinced that a well-designed coil was the answer here, because it means I don't need an extra pump and a flow switch (not to mention the PHX itself) - but I'm still nervous about pumping mains-temperature water (which can be bloody cold here in winter) through the middle of the store - it seems like the internal cooling effect is much more likely to destratify things than a properly configured return from a PHX - but then I guess the key is in that "properly configured" bit! Can anyone with a modern TS with a full-height coil share their experiences in this regard?

Posted By: Chris P BaconDidn't the Spanish government apply some mad tax so that you effectively had to pay the same amount for electricity whether you produced it yourself or got it from the grid? Or has that been reversed?

AFAIK this is only relevant if you're grid-tied. It got knocked back by the European Court but I didn't follow up on what happened next because it doesn't matter for us. The problem that Endesa (Spanish electric co) have is that so many people are doing PV here (it's a no-brainer) that they say they are struggling to pay for the cost of the grid infrastructure (i.e. their profits are less outrageous than they are accustomed to!) In the long run it will be a real problem. If people have grid connections that they almost never use - but which they rely on as a fallback for emergencies - then the funding of electric supply here will have to change. I don't know enough about the regulatory framework of the Spanish electricity market to understand why that needed legislative intervention (if indeed it did) - I would have thought a slightly updated pricing structure with increased standing charges would have fixed the problem. But perhaps that would have been very unpopular/not allowed. In the end what really needs to happen is that everyone needs to wake up to the fact that national-level power generation is no longer necessary, appropriate or economical, particular in climates where solar is so reliable - and we can have local micro-grids which sell to their neighbours as appropriate. But I don't see Endesa taking that lying down!

Posted By Herodotus: “For the winter (when the PV panels won't get warm enough to do anything useful) I was thinking of making a home-made collector…”

But have a think how much useful a home-made collector would do. Without direct sun I think you'll find that at DHW temperatures losses from the collector will pretty much balance gains from the sun leaving very little heat to go to your thermal store. I did a calculator to help with this sort of thing:

https://edavies.me.uk/2012/11/pv-dhw/

Commercial evacuated-tube (ET) collectors aren't that expensive and would, I think, work a lot better in dull conditions. Well, they're not that expensive in the UK - I imagine there are cheapish suppliers in Spain but with those things transport costs tend to be significant as you don't want standard couriers chucking the tubes around. Best of all if you can go and collect them from a supplier.

I'm very much with you on the DIY design which you can fiddle with - particularly the control logic. My plan for DHW is to have two PHXs in series. The first's primary circuit would be heated by the very large space-heating thermal store, the second by a smaller DHW-only thermal store. (The space-heating TS will be heated from ETs, the DHW TS from PV.) Rather than keeping the PHX permanently warm (as per DJH's system) I'll press a button or something to signal that they should be heated for five minutes or until hot water is no longer being drawn off.

The issue with coils is that they need to be very large if you want a high flow rate of hot water. PHX come into their own when you wish to run more than two showers at the same time.

However with a PHX the water that is returned to the bottom of the thermal store is hardly ever under 30c.
Some companies fit two large coils in their thermal store, one at the bottom and a 2nd one at the top, this seems to work as well as one large coil that goes from the bottom to the top of the tank.

Posted By: djhJust remembered, another option is a tank-in-tank design. They're common in Denmark I believe. I don't know what the pros and cons are though.

They do not help unless you are willing to use a "sealed" DWH system, or have a location high up for the header tank. Think of them as a "normal" DWH tank that has a coil with a very large surface area connected to the boiler. (They allow a small DWH tank to take all the output a large boiler can give it.)

Posted By: djh

Posted By: ringiThey do not help unless you are willing to use a "sealed" DWH system, or have a location high up for the header tank.

I don't understand why the requirements would be any different to a thermal store with a coil or PHE?

The coil in a thermal store and a PHE contains very little water, therefore there is very little risk of a large explosion when this water expands. The "inner tank" in a "tank in tank" often contains at least 100l of water, making it no different to a normal 100l DWH tank as far as the risk of it exploding etc.

People like thermal stores due to:
a) very little DHW is stored, therefore no issues if it is not kept at above 60c.
b) the pressure in the thermal stores has no effect on the water pressure at the shower, therefore the thermal store can be "open vented" with header tank directly on top of it.

Personally I have no issues with just using a normal mains pressure DHW tank, and putting on a 2nd pressure safety set if you don't trust them. (But you are more likely to die in a car crashing going to get the 2nd pressure safety set, then a correctly installed one is to fail.)

Posted By: HerodotusFrom a safety perspective, I'm pretty uncomfortable about a sealed system with a solid fuel boiler. Even with pressure relief valves and maybe a heat-dump towel rail or somesuch, the prospect of having a potential bomb that could flatten my house if I overstoke the boiler makes me twitchy.

That is no real worry, any one with a solid fuel boiler/TS gets to learn how to operate it. You have a couple (or 4) temp gauges on the tank and you soon learn to look at the gauges and know how much wood you can load up. People just don't fill up the stove with the gauges reading 95 top to bottom. (or at least only once )

You are contemplating UFH, which will necessitate 'doing' the floors, how about do the floors but with extra insulation in place of the UFH and spend some of the central heating money on more insulation (and maybe MVHR) put a bit more insulation on the outside and roof and then do away with heating except for a small wood stove for a few times a year. - Then all you have to sort is the DHW, the need for a TS goes away you just need a well insulated DHW tank.

If you are going to be stuck with a TS then PHX are not very kind to the stratification in a TS. The water goes back too hot and too fast. My system has a separate indirect DHW tank next to the TS and is heated by the TS in the same way as any other load e.g. the CH. The DHW tank also has an immersion heater so when electricity is used to heat the DHW I don't have to heat the TS, just the DHW tank. It works well. When I put together my TS I designed the return to aid the maintenance of stratification, it works well - see- http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13082&page=1#Item_8

With regard to increasing the pressure of the DHW without a pump, the only way that I know is height, you have to raise the header tank supplying the HDW tank up. Could you put it on the roof, on legs, but then you need to take the insulation around it at least as good as the roof insulation or better. (i.e. include it in the heated envelope) Another solution is to bury the header tank in the ground up hill from the house and have an under ground pipe feeding the house, but for that you need a hill and a supply that can trickle into the header tank. - otherwise I think you are stuck with a pump.

The issues in the UK are:

If a large thermal store is inside the house, then heating it in summer can overheat the house.

If a large thermal store is outside of the house, loses in winter are large.

It is hard to get enough heat from solar thermal to heat a large thermal store in the autumn (when space heating is not needed), without having lots of issues with the solar thermal panels overheating in summer.

It is hard to get enough output from PV to cover the electrical needs of a property and the heat loses form a large thermal store. But we can get enough PV output to cover the electrical needs of a property and to heat a normal sized DHW tank (outside of CH season).

Night time electricity is cheap enough to make it a reasonable option for DHW, but not if you have the loses from the large thermal store.

Hence it seems best to separate the DWH tank from the thermal store.

Most homes that need lot of hot water, want it all at the same time, and hence put in a 300l DWH tank. This gives enough hot water for 5 people to have showers. Therefore it is not an issue if the DWH tank takes 40 minutes to reheat using the water in the thermal store.

Tank-in-tank can be great when you need to heat lots of hot water quickly using a very large boiler and have a large regular need for hot water, for example the showers in a public swimming pool, when they all get used every 20 minutes due to swimming lessons.

Tank-in-tank can also be great when you have then in a large thermal store and you don’t care about the losses from the thermal store.

I think the main issue in the UK, is that using a 2500l thermal store is not a good way to heat DHW outside of the space heating season regardless of how it is done. Hence most people put in a separate DHW tank, or small thermal store for the DWH.

Tank-in-tank are expensive in the UK, and if they are drained down by someone that does not understand them, it can result in damage to the inner tank. Most plumbers in the UK are not used to them. The building regs in the UK don’t allow DIY on mains pressure DHW tank, but do allow DIY on open vented thermal stores.

ringi,

Most concise! I can see why tank-in-tank is perfect for me but not a good idea for the vast majority of UK households.