My first foray into the forum!
I am seeking some wise advice on a proposed heating, cooling and hot water system.
I live in the south west of France and have been working on a renovation scheme to what were (and are still in part) very dilapidated farm buildings since 2004!
I am recently retired and now resident here and seeking to finish the project while I still have a little energy left!
Holiday time over this 15 yr period has allowed us to restore one 2 bed house in which we currently live. It is served by a 300l Akvaterm Solar sealed thermal store on the first floor, heated by a single solar panel (no room for another) and a wood burning stove in the living room below (with Laddomat), with 2 x 3kw immersion heater back up.
Whilst down in the south west, it can get very hot here in the Summer and seriously cold in the Winter.
The system works reasonably well, but is let down in the Spring and Autumn months when it is either too warm for the stove or not quite warm enough for the ST panel particularly when the sun is not shining – causing undesirable and expensive need for electricity back up!
I am now turning my attention to the proposed adjacent principal residence conversion – a property on 3 levels – planned for 3 bedrooms and two bathrooms on first floor, living rooms and shower room on ground floor (with semi basement below) and kitchen/dining on lower ground floor to the rear.
The first floor and ground floor are both suspended timber.
As the other house, the walls are 600mm solid stone, that I am lining with insulated studwork.
Many years ago I and the wife laid a piped underfloor heating array to the insulated concrete kitchen floor before covering with a screed – the pipes are still sticking up waiting for the manifold that is currently sitting in a box under the bed!
I want a more flexible, controllable and efficient system here and am scratching my head a little as to the way to go.
Gas is most effective/instant, but it would have to be LPG and of course it is not green - and its days are arguably limited and LPG is expensive – nevertheless, an option even if only a back up when it is really cold.
Solar Thermal - plenty of space and good aspect on this roof, so definite potential.
Solar Voltaic – don’t know much about these or how they connect into/form part of a combined system?
Air Source Heat Pumps – seem very popular around here for the small square newbuilds, but have picked up on lots of bad press as to (lack of) performance, very high maintenance costs and disappointing running costs, enormous radiator sizing – not convinced that this is the solution on a property of this size and construction?
Cooling in the summer (40 degrees) is also a consideration - I think these ASHPs can work in reverse?
Ground Source Heat Pump – too late, external works now sorted!
Wood Stoves with back boilers – we will have a stove in the living room and down in the kitchen, but loading them with logs for reliance on hot water supply is hard work in the Winter and we are not getting any younger, so I do not see these as being planned as a fundamental part of the CH/HW system. I had also understood that a thermal store had to be physically above such stoves to allow gravity circulation in powercuts – that will not be achievable here as I would ideally like to locate the hot water cylinder in the semi basement.
So, my initial thoughts are as to a very large pressurised thermal store (TS) or hot water cylinder in the basement, fed by any/all of the above (save the stoves), providing mains pressure hot water, with heating to radiators generally and to the underfloor heating in the kitchen only – whatever the solution, it has to be reasonably simple, cost efficient and low maintenance.
I’d be very interested and grateful for advice on what combinations the forum members have found to work well or not so well, so here’s hoping!
I guess that this ground will have been trod before so any pointers to earlier similar discussions would also be appreciated.
Thanks.

Thanks Mike - really very helpful!

Can I just pick your brains a little further.

All floors (except the kitchen) are timber suspended with new floorboards.
Can underfloor heating pipes be efficiently run below these existing boards with insulation then fixed below it. I can't really interfere with existing floor to ceiling heights due to existing door threshold heights.
It may be more tricky to fit, but there will be suspended mf ceilings below the existing joists with a c50mm gap that allows general services distribution - that could facilitate continuity of heating pipe circuits between each joist void?
Would you know if there is any reckoner for sizing an ASHP and any leading manufacturers who have got this technology right?

I have a couple of boxed ST panels sitting in the workshop for years now that I had planned to make use of.
The PV route is interesting though. Do you know if there is some gadgetry for directing PV power into the TS immersion heaters when there is availability and call for it; or back into the grid when it doesn't - and how is this control married to the trigger for the ASHP so that pump doesn't run (and use (albeit relatively little) mains power) when its not needed?

Understanding/designing the overall control/coordination of the heat source inputs and ultimately the heating distribution and temperature control is the most mind-numbing for me!

Getting there and again thanks for your help.

Charenteman
Whether you use (or need) the PV controllers as suggested by Mike1 could depend upon how your grid tie works, - or rather the billing.
Over here such controllers are not used because the billing for grid use when you have PV is an annual reconciliation. When the PV produces more than you need the surplus goes to the grid and the meter runs backwards. When the PV produces less than needed the grid makes up the difference and the meter runs forwards. At the end of the year the meter is read and a reconciliation is made. The trick here is to get the right amount of PV to match annual usage because what you are paid for over production is very uneconomical against the price of extra PV panels.
I'm not sure what system for grid tied PV is used in France

To couple an ASHP with space heating, a small buffer tank is often used so that the heat pump doesn't keep turning on an off with small fluctuations in temperature. It would be possible to add a PV-connected immersion heater to heat the buffer, if you're generating lots surplus electricity.

ASHPs are normally only used for space heating, as they operate most efficiently at lower temperatures - which is why they're good for underfloor heating.

A thermal store would often be used for hot water, and in your case heated by PV electricity when available, or from the grid when not. If you have plenty of sunshine you could go big enough for a couple of days supply, but it will start to get expensive for more than that. In a smaller property one downside of that would be that the heat given off in summer would contribute to overheating of the property, but if you have a cool basement to put it in that's probably not a problem for you.

Having said that, you could run everything off a thermal store, heated by the ASHP + PV. Or use the ASHP to pre-heat the hot water in the thermal store. It depends how complex you want to make it / how the financial payback works out.

BTW, note that really effective airtightness can significantly reduce the space heating requirement - but that involves much more than just draft exclusion at doors and windows - it involves sealing the entire structure.

Thanks both, much appreciated!

I have Heated the water with the immersion heater this afternoon - it took 4.9 kWh to heat the water up to 46 degC in an hour and 45 minutes.

Typically the heat pump has used approx 1.5 - 1.6 kwh to do a similar job (in winter nearer 1.8 to 2.0 typically) in 45 minutes.

That gives a COP of approx 3.0 now and 2.5 in winter - better than I expected. I see no reason why an ASHP should not be used for DHW with a suitably sized cylinder.

Using the ASHP It costs me the equivalent of less than 5p per KWh to heat the water in summer (7p in winter) - I have to put up with some fan noise for 45 minutes a day - but that is all.

GE, thanks for sharing your data!

You have a good strategy to heat the tank only when it is properly cold (rather than keeping it constantly nearly-warmed) as this is as nearly as possible equivalent to countercurrent heat exchange, which is thermodynamically more efficient (less irreversible). The key point is that the bottom of the tank gets very cold, so it can it accept heat from the ashp when running at low temperature. When the bottom of the tank gets hotter, the ashp efficiency falls off, though if yours never goes above 43degC it's never too bad.

For it to work as you describe, your daily DHW usage has to be enough to nearly-but-not-quite empty (cool) the tank over a day. You have presumably tweaked the storage temperature to make it so, and you don't consume very much DHW (5kWh/d = ~150litres/day at shower temperature), so you can get by with keeping the tank pretty cool.

If your tank were/is a standard one, the standing losses would account for a good chunk (>1kWh) of your daily usage. It could be argued that you could have installed a smaller tank, as if you are not using solar or overnight E7/E10 there's no need to store a whole day's worth of hot water. You'd use up the smaller tank and then reheat it several times during a day, eg after each person showers - so perform the same cycle you currently use, but more often. Smaller tank= less standing loss and cheaper to build and quicker to reheat.

Another strategy is to have two small tanks (or one tank divided up). Use one as the 'tepid' feed tank, and heat it very efficiently with the ashp at low CH/UFH temperature. From there, tepid water goes into the delivery tank, which gets an occasional blast with the ashp or immersion running at high temperature to top it up to usable temperature. I am thinking about using the solar coil region in the bottom of our tank for the 'tepid' heating.

Sterilise the whole system by heating up weekly if you are bothered about legionella at the temperatures you describe.

That's a good idea! Or perhaps heat the whole tank several times a day, but to a lower temperature (limit the ashp at say 45degC to improve its CoP). Good for a shower but maybe not for washing up.

GE, have you tried running the same cycle but at 2am instead of 2pm? This might reduce the CoP, but you could use a cheap smart-meter tariff and lower-carbon nighttime electricity. Interesting if that would work out better/worse overall.

Full disclosure: like Charenteman we have an old stone house and am planning how to improve the heating while not starting from the ideal ingredients. I'm certainly not looking to pick holes in GE's setup, sorry if sounded like that, instead I'm mulling out loud what to do with ours.

Charenteman, I am thinking along the same lines as Mike1, using low temperature ashp for heating. Hot water pretty much decoupled from space heating, so no thermal store, but interested in GE's experience. Log stove, but not coupled into heating system. This setup worked very well in our previous old stone house renovation!

Big decision is whether to: add underfloor heating (big job but we might lift some floors anyway to insulate); or just fit bigger radiators; or go for air-to-air heat pumps (aka Aircon units) and no piped system.

Where I live, I can't make the numbers work for adding domestic solar (thermal or PV) - its much more effective to buy energy from utility-scale solar farms.

We are planning to insulate the stone walls first before fixing the heating system, so will be a few years before we commit. (We have lived in the house for one winter and used much less energy than expected, there's a body of opinion that stone rubble walls are more insulating than we used to think, when made airtight.)

The previous owner fitted a 250l hot water cylinder with two coils. We are only using the upper coil, leaving the lower half of the cylinder unheated, we heat the cylinder twice a day for family of 5.

Since a thermal store (providing DHW and heating) warms the cold feed water for DHW starting at the base, and gradually getting hotter as the coils winds up through the TS, the low grade water at the base is used as part of the DHW production. If you split cylinders into a DHW cyl and effectively a buffer tank, you lose that benefit.

It also fixes the potential usage of that heated water, with no flexibility over the DHW to heating ratio.

PiH, it would be good to see the figures that show it is generally accepted that you should not combine DHW and heating in a TS. I think the range of possible scenarios for energy inputs and outputs make that statement too general. Probably true if you have a big old energy guzzling building. Not true if you have a well insulated set up.

Posted By: GreenPaddyPiH, it would be good to see the figures that show it is generally accepted that you should not combine DHW and heating in a TS. I think the range of possible scenarios for energy inputs and outputs make that statement too general. Probably true if you have a big old energy guzzling building. Not true if you have a well insulated set up.

I don't have figures, - and yes it's a general statement predicated by the notion that DHW and CH have different heat requirements and temperatures and those heat demands occur at different times. Given the options for obtaining heat - wood fired, ST, PV, ASHP, GSHP and electricity with all its various billing options IMO it will generally be better to separate the demands of DHW and CH. If you are storing your heat and you generally need more stored heat for CH than DHW there will usually be a loss of efficiency during the non-heating periods due to increased standing losses if you only have one heat store for both DHW and CH.
A few of us have big old energy guzzling building, a few of us have a well insulated set ups, most of us have something between the two either because of architectural, planning or cash limitations and each building will have its own 'best fit' options but as a general rule I would argue that separating DHW and CH is better.

Thats right, and the pressure ratings are usually different, and the number of nozzles, and the materials, size, weight, design regs and the price. But for the purpose of GP's question

conceptually they both store heated water.

so

You choose one that suits your needs for storing heat. (Or both, or neither).

Posted By: Mike1Thermal stores and unvented hot water cylinders don't store hot water under pressure

I'm sure it's just a typo but, to avoid any confusion, it's vented hot water cylinders (i.e., usually ones fed from a tank in the loft with an expansion pipe going back up to that tank) which don't store hot water under pressure. Unvented cylinders store the water under pressure.

Hi Goodevans, thanks again for sharing your info, very helpful! Impressive electricity consumption. Thanks for the pointer on the gen5.

Hi Mike1, good point but I would advise that the safety devices on both vented and unvented systems should be regularly maintained, including the thermal cutouts as well the safety valves. This very tragic accident involved the F&E tank of a vented system where the immersion did not cut out.
http://www.dailymail.co.uk/news/article-506604/Water-tank-fault-scalded-baby-girl-death-known-unreliable.html

Posted By: WillInAberdeenIn discussion, he thought they'd be of more interest to a plumber, whereas he considered himself to be a 'heating engineer' so he doesn't concern himself with inspecting those bits.

I wonder if there are standards that are supposed to be followed for describing and executing such services? And whether those standards are optional or mandatory, and whether as a 'heating engineer' he's a member of any professional association that subscribes to those standards?

I confess I'm fairly pleased that all the water storage in our house is in a single cupboard downstairs, apart from WC cisterns, for I expect we'll get some plumbing problems eventually. We have had two floods in the garage - once when it was all new and the plumber hadn't tightened up one connection fully and again recently when I turned the outside tap back on without closing the drain valve fully. The first time there was a couple of inches over the whole floor by the time we noticed; this time just a six foot diameter puddle.

There are no requirements for any testing / inspection of mains pressure DHW tanks in domestic installations here - and I can't recall hearing of any problems either. The only problem I have had with mine (at one time I had 4) is that about every 5 years I had to take out the base plate (containing the immersion heater, thermostat socket, safety thermostat socket and sacrificial anode) and dump the whole lot in the bin and replace with new due to limescale build up, done on a fix when broke schedule.

Given the millions (literally) of mains pressure DHW tanks there are here and the total lack of issues (other than limescale) IMO the UK requirement for annual inspection is a bit OTT