Solar for Central Heating

[kristen]

I'm finding this complicated to sort out, so I apologise if I am less than succinct in asking my questions.

We've been in this house for 9 months. Our boiler is dying a death, and the hot water tank next to it looks pretty ancient.  So an opportunity for a change.

Out of the blue I had an unsolicited call from www.solarhome.tv and they appear to have what I want.  However, I have not found anyone to offer me a comparison quote - plenty of folk offering Solar for DHW, apparently none offering Solar for central heating.

Please note that I do realise that in January I will be lucky to get modest assistance with the DHW, let alone Central Heating  . But even in January the figures suggest 1.15 kWh / day, and given 4 panels that's like having a couple of kettles on for an hour (have I got that right?) and that's definitely more than nothing!
In essence my understanding is:

The SolarHome folk stick some solar tubes (I reckon they are Navitron, they look to be the same) on the roof, plus a thermal store.  Mains pressure cold water goes through a coil in the top of the thermal store to provide DHW.  The existing (or New in my case!) boiler heats the water in the [top of the] thermal store, and a lower-loop is heated by Solar.  Obviously there is an option to have immersion too (which seems prudent in the case of the boiler failing, as happened to us this for a cold one-week period this winter!)

The water in the thermal store is an integral part of the central heating circuit.

When the central heating comes on the hot water goes straight out of the thermal store into the rads.  Strikes me that would warm them up pretty quickly, but they would return a large volume of cold water straight into the thermal store (for the boiler to then heat up).

I have no idea if that is better / different or efficient / inefficient compared to the boiler heating the water for the Rads. directly.
We have a swimming pool and that could benefit from the Solar in the Summer.  So it may make sense for me to have a more-than-normal number of solar panels to get some extra gain in the Winter, and yet have a suitable heat-dump for the summer.

I'm struggling with the calculations and comparison a bit.  Some help would be appreciated, as well as any opinions you may have.

The house is 300 sq.m (i.e. 600 sq.m for the two stories)
The utility / garage block is 45 sq.m (single story)
Pool is about 70 sq. m - say 110 cu.m of water. (Its enclosed in a sort-of-greenhouse arrangement - it didn't need any heating from Jul-Sep last year (but there were spells that would have benefited from "free" heat!!)

The utility / garage block has a flat roof, orientated South, and this strikes me as a very convenient place to mount the collectors.  Short pipe runs, easy access.

The "dying" [oil fired] boiler is 250,000 btu. (it does the central heating and hot water, and is not a condensing boiler!).  Central heating system is cast-iron skirting-board rads (circa. 1960) [single circuit for both floors, no thermostat, no individual Rad stats  - although each has a "tap" to minimise the flow e.g. in unused rooms].  We ran the system for about 2.5 hours morning and evening in the cold of winter, and occasionally hit the extra-hour button at lunchtime.   Now we run them for 1 hour in the morning, and on a "cold" day another hour in the evening.   (The house is occupied all day).

The house was built in 1961, is a substantial build - lots of concrete, block-and-beam.  The original flat roof (on the two-storey part) has had a pitched roof added, with about 8" of loft-insulation.  This is on top of the block-and-beam original roof.

The walls are not insulated.  I have had a quote of about £ 800 for cavity fill.

The swimming pool has a separate oil-fired boiler. The folk that service it say it is in good condition.  (It burns oil at a phenomenal rate, BUT it does raise the pool temperature relatively quickly;  Solar would need days & weeks to do its job, but the reality is that until the air temperature is up enough it is not comfortable to swim, which means mid April at the earliest, and thus I think that there is a fair bit of March and half of April for the Solar to do some of the work before the Oil does the rest).  Topping up the heat through to end October would probably be Solar-only, and thereafter the air temperature is becoming too unpleasant to swim.

Fuel usage (per day)

	Oil	Electritiy		Water
Period	Litres	Day kW	Night kW	Cu.M?
Dec-Jan	30	55	15	2.5
Feb	33	50	14	5.5
Mar	20	55	17	5

Edit: Error in oil usage - now reduced by a factor of 10!!

Questions
Am I barking mad? 

What sized replacement boiler would I expect to use? Will it be smaller than the original as its job is to heat the thermal store, rather than needing to be able to heat the significant amount of cold water in the Rads (i.e. as per the current system)

Would 2 x half-sized boiler be better? (e.g. at the ends of the season I can run a single unit, perhaps more efficiently than a bigger unit at 50% output?)  Half-sized, in my circumstances, would perhaps be a pair of bog-standard boilers, rather than a more specialist single unit?

I'm assuming a new condensing boiler, and that this will use a "Flue" direct through the flat roof above, rather than the chimney (up side of building to second storey) as at present?

(This is a rural location, no mains gas available)

Is the Solar likely to save me some money?  Can I calculate roughly how much?

How does Economy-7 night-rate compare to Oil price?  (e.g. using immersion instead of boiler) - I'm just curious on this one!

I have considered a ground source heat pump.  Its not viable to retro-fit underfloor heating.  The capital cost of the equipment seems too high to be justified in my circumstances - e.g. I might well need an oil-fired boiler too.

Would I be better off with a DIY installation? (I have a handy-man who can help)

Sorry for all the lengthy questions.  I would appreciate any opinions and suggestions and potential suppliers.

Many thanks,

Kristen

[Loir]

1. Am I barking mad? 

2. What sized replacement boiler would I expect to use? Will it be smaller than the original as its job is to heat the thermal store, rather than needing to be able to heat the significant amount of cold water in the Rads (i.e. as per the current system)

3. Would 2 x half-sized boiler be better? (e.g. at the ends of the season I can run a single unit, perhaps more efficiently than a bigger unit at 50% output?)  Half-sized, in my circumstances, would perhaps be a pair of bog-standard boilers, rather than a more specialist single unit?

4. I'm assuming a new condensing boiler, and that this will use a "Flue" direct through the flat roof above, rather than the chimney (up side of building to second storey) as at present?

(This is a rural location, no mains gas available)

5. Is the Solar likely to save me some money?  Can I calculate roughly how much?

6. How does Economy-7 night-rate compare to Oil price?  (e.g. using immersion instead of boiler) - I'm just curious on this one!

7. I have considered a ground source heat pump.  Its not viable to retro-fit underfloor heating.  The capital cost of the equipment seems too high to be justified in my circumstances - e.g. I might well need an oil-fired boiler too.

8. Would I be better off with a DIY installation? (I have a handy-man who can help)

Hi Kristen,

A lot of questions. I will attempt some answers to some.

1. No, it makes sense to use solar both financially (in the long term anyway) and for the planet.
2. No, the boiler needs to be sized to heat the house and supply your hot water needs. Its size can be calculated by a competent boiler installer. You cannot assume that you will get much solar input during the cold winter months. So the boiler must be able to cope.
3. No, it will add considerably to the cost with no great advantage. The boiler, whatever size it is, will run to keep the thermal store at a fixed temperature then it will stop. Probably it will not fire up at all during a hot period in the summer.
4. Sorry, not my field of expertise.
5. This will depend on how many solar panels you install. There is a table on the Solarhome website (a copy of the one on this site) which shows how much heat in watts will be absorbed on average per day. If you use all this heat you will save the cost of running the boiler to create this amount of heat. The full spec of a boiler will state the amount of fuel required to generate so much heat (units may need converting). With the price of fuel you can calculate how much you will save.
6. Sorry, don't know.
7. No comment. Perhaps others can comment..?
8. You really need a qualified plumber to install the boiler and heat store. Perhaps the solar panel installation can be done by a competent DIYer if it uses a separate coil in the thermal store.

You also asked about using the thermal store to heat the radiators. This is very slightly less efficient than heating the radiators directly from the boiler. But you have the advantage of using any available solar gain for CH and well as HW. Please make sure all the radiators have thermostatic valves on them.

The pool will provide a very useful heat dump (see assorted discussions on this forum) allowing you to have more solar panels and not worry about too much heat in the summer.

I do not know SolarHome but there are many other plumbers who have experience of thermal stores. Not so many who have experience of solar panels. However there is a place on this forum to ask about installers in your area.

Loir

[Paulh_Boats]

1. Am I barking mad? 

3. Would 2 x half-sized boiler be better? (e.g. at the ends of the season I can run a single unit, perhaps more efficiently than a bigger unit at 50% output?)  Half-sized, in my circumstances, would perhaps be a pair of bog-standard boilers, rather than a more specialist single unit?

Kristen,

1. Maybe, join the club. 
3. Probably no. The better condensing boilers have clever software inside them. They can automatically adjust their output to efficiently match the load - the technical term to look for is "fully modulating". For example, they can automatically vary their gas flame to generate anywhere between 9kw to 28kw (e.g. 30% to full power), so I don't think you need 2 boilers. Some of them even adjust the pump speed automatically to match the load, slow in summer fast in winter etc.

[kristen]

Very helpful, thanks Loir.

"The full spec of a boiler will state the amount of fuel required to generate so much heat (units may need converting)."

<Slaps-forehead> !

Yes, obviously.  Thanks.  I (or Google!) should be able to cope with the units-conversion.

"Please make sure all the radiators have thermostatic valves on [the rads]"

Sadly I don't think that is possible.  There is a little trap-door at the end of each skirting-board-radiator where the adjustment valve is.  It has less than 1.4 turn of useful adjustment, and doesn't fully turn the radiator off.  There isn't much that we can find in the way of by-pass pipework (e.g. such that we could fit a room stat which controlled a motorised valve).

I think the best we can hope for is to fit a wireless thermostat so that at least there is some feedback to the boiler other than me thinking its too hot/cold and going out to flick-a-switch    I'd be happy for someone to tell me otherwise though.

"there are many other plumbers who have experience of thermal stores. Not so many who have experience of solar panels"

Thermal Store for both DHW and Central Heating plus Solar does seem to be a bit of a challenge

"there is a place on this forum to ask about installers in your area"

Thanks, I'll check that out.

I spoke to John @ Navitron today.  He was very helpful.

The bits I need (excluding pipework and installation) come to around £5,000.  Excluding a boiler.  Solarhome's quote is similar, but is for 2 panels whereas John's "back of envelope" suggested 8 panels.  It would be easy enough to bolt-some-more-on if I go down a more DIY approach.

John also gave me details of an installer in Ipswich who is coming out tomorrow to have a look.

"fully modulating ... adjust the pump speed automatically"

Clever stuff, thanks Paulh_Boats being armed with that sort of info should help me make a more informed choice.

Kristen

[shambles]

I am using something similar to what you are describing - 300L thermal store, direct connection to 5 radiators, hot water via a heat exchanger, single 20 tube solar panel heating the thermal store via a coil at the bottom, direct connection to gravity fed woodburner, 3kw immersion - no gas or oil. Small 2 bed terrace house.

We have had our first electricity bill for the winter period January - March and we are spending £10.00 per week on electricity to cover everything including lighting and cooking as well as heating, using economy 7 to heat the store overnight. During November - December, when we were on a standard day tarif with no solar, we were spending £13.00 per week. I need to look at the figures to work out if the saving is due to weather, the tarif or solar or all three - it isn't easy to tell.

Hot water is never a problem. Heating is more problematic because it takes much longer to charge the store than it takes to deplete it through the radiators. I think we only have about 90 minutes of radiator use per day if we just rely on economy 7 and the woodburner. I don't have a problem with this - the house hasn't got too cold, but my partner has whinged a little about the kitchen and bedroom being cold at 17C. Some pictures of Polar Bears struggling to walk over melting ice has helped a bit with that. if we run the heating for much longer, it begins to impact on the hot water...

The solution has to be added insulation - especially the windows, which are single glazed Georgian jobbies. I'm quite happy with it, but if I were to have my time again, I'd have gone for a two-tank setup, keeping the hot water seperate so that I could run the heating until the heating tank is completely cold. I think...

Oh, I forgot. Nightime electricity is 4p per unit, daytime is 10p... We use in the ratio of 2:1

[kristen]

So I might be able to have a somewhat smaller boiler and night-rate immersion (for the colder months) ...

"I'd have gone for a two-tank setup, keeping the hot water seperate"

My plan is to have a mains-pressure feed through an exchanger at the top of the thermal store for the DHW - and "reserve" the top X% of the store for that.  I have no idea what the correct value for X is, yet!  Thus the Rads. won't be able to steal/share that heat, as such.

One thing I've been meaning to ask:

Is it feasible to have the return water [from the Rads] be returned to the thermal store according to the return temperature?  It sounds complicated, but I'm thinking of a motorised valve and, say, 3 return positions - "bottom", "bit higher up", and "even higher up"?

If the water is coming back close to the temperature it started at it strikes me that putting it back in at the bottom will cause some mixing - and might raise the water temperature at the very bottom to the point where the Solar doesn't get activated [e.g. mid Winter].

But my understanding of such things is small, and I'm prone to trying to complicate everything ....

Kristen

[shambles]

I have to admit to being very new to this as well, but...

Here's a scenario. it's a cold day - you have your lovely thermal store heated up in the morning to a toasty 70C. Then your heating comes on for a couple of hours. The radiators take all the heat from the bottom two thirds of the tank (through an exit pipe a third of the way down the tank), dropping the temperature down there to 30C and warming your house. The top third of your tank stays hot, but in contact with the cooler water below. How long does it take for the top of the tank to cool down? Will there be enough hot water for your morning ablutions? What about the evening, when you get back from work (or whatever?)

I haven't tested this to destruction (but I will do, now it is warmer), but I suspect that the answer might be 'not very long'. Despite stratification etc, I suspect that the top of the tank might cool quicker than one might expect... hopefully, someone with a greater knowledge will be able to tell you about this.

[kristen]

"Will there be enough hot water for your morning ablutions? "

The red-hot salesman promised me it would!!!! 

I believe there is something in the thermal store to prevent/reduce mixing of the layers.

If the thermal-store temperature falls below 80o the boiler will kick in - won't that then, in effect, give priority to the ablutions layer?

Kristen

[shambles]

Yes - that sounds right... But search the forums - I'm sure there are a few threads on this particular subject. My biggest concern for you (I went through this dilemma myself) is that you might as well, if it is possible, have a system design that will allow you to completely shut down the oil-burner side of the equation and still leave you with something that will work reasonably well. You can be sure that the price of oil is only going one way in the long-term and it is going that way quicker than the alternatives... I opted out of it altogether, but there are obviously other ways of looking at it.

[kristen]

I had a very useful meeting today with Robert Elmer an Ipswich-based installer for Navitron.

He looked at the schematic from the Solar Home folk and whilst he liked the principle he thought it desperately underpowered.

(In fairness all I have is the Salesman's Proposal and I am awaiting a proposal from their engineer who came along later).

His main concern is that I have a huge amount of water in the CH radiators, and only, say, 300L in the thermal store.  So the moment that lot gets into the Rads the boiler is going to come on - so I'm not really "storing" very much heat.

But there again I suppose if the CH is "on" all day then any benefit from the Solar collectors can go straight into the CH circuit.

He favoured some sort of heat bank.  The most obvious seemed some bricks of the type used for night storage heaters, run the solar through that, and then draw it out as needed.  When it is exhausted then the boiler will be doing all the work.

I quite like the idea because its all in series.  Take the return water from the CH through the heat bank IF the differential is worthwhile.  I could also have some 6kW electric heaters in series - that would cover for any boiler failure, or for an alternative source of electricity (e.g. combined Power & Heat or, who knows!, a Wind turbine if prices fall enough in the future)

He thought it worthwhile having as many solar panels as the roof I have would hold (its a 100sq.m flat roof).  I'm still a bit anxious about how much heat we will actually need for the swimming pool in the summer, so I'm more inclined to install a "conservative" number for now, and then after a summer's experience with the pool increase that number if necessary.  Having the swimming pool at 80F is lovely, not so lovely at 90F !!

But obviously the more panels the pool can take in the summer then the more that are available for CH during the winter.

I was also quite pleased that he estimates that I can reduce the boiler size from 250,000 BTUs down to perhaps 150,000.  That moves it from being a more specialist boiler to being run-of-the-mill, and the boiler will work longer hours.  I think the current boiler has had a short life by being worked hard for only very short periods of time.

We also talked about combined Power and Heat, and me going round to the local chippy for a 6p bag and 1,000L of chip fat!  The suggestion was a 200 kVa device and either flog the excess back to PowerGen, or to my neighbour.  I'm using around 65kW per day, I expect my neighbour is similar, and assuming my electricity is cheaper than the Lecy Board he'd probably be game.  If my generator fails hes no worse off, but after a storm the likelihood is that he will have power from me, whereas he might be off for several days otherwise.

Hes gone away for a think, I'll report back as the story develops if folk are interested.

Kristen

[ecogeorge]

You're using 65Kwh a day !!!!!
I hope  you've  had the wheel on your electricity meter balanced to turn at that speed !

[kristen]

"You're using 65Kwh a day !!!!! "

I'm a bit thick with the units of such things - but based on weekly meter readings I am using 65 units a day (15 at night, 50 during the day).  Is units = kWh? and if so is that 65kWh or have I got the wrong end of the S.I. units?!

I'd like to reduce the electricity use (indeed we have since we moved in 9 months ago - we were using 90 units a day then), but I'm not sure how 

We have mostly low-energy bulbs; all standby-type equipment has power-blocks so that it can be turned off at a single switch (TV/DVD/Video/etc, 10xComputers.  We have thrown out one of the American-style fridge-freezers the previous folk left behind, the other one is costing £5 per week, but there are two small fridges and a freezer.  We cook-electric here (had an oil-fired Range before) so that's several ring-hours per day, and the oven is on for 30-60 minutes a day I suppose. We have the washing machine on pretty much every day; a clothes line has cut down the use of the tumble drier.  Because the boiler is playing up the immersion has been on a fair bit, and that doesn't have a time clock [so gets left on sometimes], but that will get fixed as part of the New Heating System Sortout!  The other thing I don't know about is the circulation pump for the pool;  that has a man-sized on-off switch, so I assume it is a hefty electricity user? and that doesn't have a timer either, but that's due to be fixed before the swimming season.

Kristen

[KenB]

Kristen,

65kWh per day is a lot of electricity for a domestic household.

In monetary terms it is about £45 per week, or approaching £2400 per year.

The washing machine, tumble drier and immersion heater are likely to be the main culprits, drawing about 3kW each when they are on and heating.  I suspect that the immersion heater is left on unnecessarily for long periods of time.  By way of an experiment, try turning it off for a day and see how much you save.

I suggest that you get a timeswitch fitted ASAP to your immersion heater.  It will cost about £20, and it will save you that in about a week.

The immersion heater probably has an adjustable thermostat on the top of it, underneath the plastic or metal top cap.  It is advisable to turn this down to no more than 55 degrees C.  Any hotter than this and you are just wasting power making excessively hot water - too hot for washing.

I also had a boiler failure about 3 years ago, and had to heat my water with the immersion heater for the summer (until I got solar panels), I found that it was using an extra 8kWh per day on top of my usual consumption.  I was glad to get the boiler fixed and never use the immersion again.

By way of comparison, my household uses about 8kWh of electricity per day.  The national average (used by the utilities for the purposes of illustrating average bills)  is normally taken as 3300kWh electricity (9kWh/day)  and 20500 kWh gas or equivalent heating oil.


Ken

[kristen]

Thanks Ken.  I don't think its the immersion heater that's the problem.  We've had months when we haven't used it at all; the electricity usage has gone up about 15kWh a day for the last week and I'm sure that's the immersion!, as well as guests over the Easter weekend.

We did an experiment a month or so back and turned everything in the house off - and the meter stopped - so I don't think there is some equipment that we don't know about.

I also bought a meter (13-Amp plug-in monitoring device) and that I've used to measure fridge usage, and so on.  I think I need to have another round of that - checking the computers and stuff too.  Perhaps we need to get more serious about turning things off, but what I really can't understand is that we seem to be using much more electricity than at our old house, and we have bought most of the electrical devices with us, so its hard to see what has changed.

The differences I can think of are:

Oil Fired Range previously, now electric hobs and oven.  About to replace old knackered Halogen hob with Induction.
Old knackered boiler, much wider-bore & longer pipework; are the circulation pumps likely to be using much electricity?
Old knackered immersion heater.  About to change that.
Two, old, American style fridges left by previous owner.  Have thrown one out.

My consumption = 6 times the national average is pretty depressing!  Its a big house, we are in it all day long (work from home), but its still only 2 adults and 2 children.

Kristen

[KenB]

Kristen,

As you work from home, you probably have PCs turned on a lot of the time.  If these are older models with CRT monitors, they can use about 6kWh if left on for 24 hours.

It might be worth switching off each circuit in turn at the meter/consumer unit/fuse box to see what is using the most electricity.

You might even want to invest in an Electrisave power monitor, which clips onto one of the meter leads and has a wireless display of how much instantaneous  power you are consuming. They are about £65, but sound like it might pay for itself in a couple of months.

Circulation pumps only use about 100W, and fridges only use about 1kWh per day (unless they are very badly iced-up).   It almost sounds like you have some device plugged in thats drawing a continuous 2kW, a heater of some sort, greenhouse, pool heater possibly?

Happy Hunting,



Ken