Advice on thermal energy input, storage and output please.

[Culver]

I'm after some advice please on an upgrade project to our space and water heating requirements.

First of all let me describe our current setup:

1. 15kW Wood Pellet Boiler through a standard S-Plan into radiators and a dual coil cylinder.
2. East-West Solar with 24 tubes on each roof face. (Installed by cowboys for the previous owner)
3. Wood Burning Stove in the living room.

What are the problems?:

1. The boiler is in a lean to on the back of the house which has practically no insulation so we're losing far too much heat.
2. The solar needs a bit more fettling as it's not really performing as well as it should. (Although at least now it's safe!)
3. We need a WBS that can integrate into the rest of the system.

What are my proposed solutions so far?:

1. Build a proper utility/boiler room to replace the lean to.
2. Install a thermal store/heat bank/accumulator.
3. Get a shiny new WBS which has a boiler that can feed the storage cylinder.

What questions do I have?:

1. I should have plenty of space for the storage cylinder, I reckon I could even fit a 5000l Akvaterm. What issues are there with going too big? I'd like to be able to store enough heat for only running the WBS every couple or so days.
2. I know that at one point I had a link to a spreadsheet that could calculate the space heating requirements. Can anyone point me in the correct direction?
3. Any recommendations on things I should do when planning the utility/boiler room? Spending £1000s to save a few pence on heating isn't an option but as the main wall of where I propose to put it faces north-west I reckon that there must be some clever/simple/cheap ideas.

Cheers

Phil

[dhaslam]

1. I should have plenty of space for the storage cylinder, I reckon I could even fit a 5000l Akvaterm. What issues are there with going too big? I'd like to be able to store enough heat for only running the WBS every couple or so days.

There are two problems with going too big, apart from cost.   The first is that the boiler may not be able to heat it fully.     Unfortunately there is  a recurring theme across many  renewable energy appliances that they don't perform to expectations.   My stove only produces one third of its rated output never heats  more than one third of the buffer tank.     The second problem is heat loss.  If you go to 5000 litres the heat loss is not five times more than a 1000 litre  store but the extra size does make quite a difference.   One problem with large stores is that even if in theory  they can store two days heat  the second day is going to be colder so  you  probably will need to use the stove each day anyway.     


3. Any recommendations on things I should do when planning the utility/boiler room? Spending £1000s to save a few pence on heating isn't an option but as the main wall of where I propose to put it faces north-west I reckon that there must be some clever/simple/cheap ideas.

The important thing if you are  using manually fed boiler is to have  easy access from the house, preferably without going outside  and with space  to store fuel ready to use.     Apart from  frost protection I don't think insulation is that important but   you need to think of doubling up the  insulation on standard tanks and pipe runs need  to be very well insulated.    There may be problems feeding the store from a woodburner in the house  if it cannot feed by gravity.

The more heat input you can have from solar the better.  After using a large  wood burner for nearly twenty years  I now have a house that is largely solar heated.   With some changes next year  I should get close to eliminating all energy costs.   

[Brandon]

in answer to the 4th 1.


The bigger the store, (and therefore the longer storage period) the greater the standing losses, if heating it with the WBS, then for xxkWh of heat required, you will burn more wood, as you will have to account for the losses.

To produce xxkWh of heat you need to burn a given volume of fuel, so either a large boiler for small periods, or a smaller boiler for longer periods, if storing that heat, then the losses are deducted, and therefore you need to burn x% more fuel.

You only need that sort of storage if you are batch burning in a gasifying boiler, in which case the whole gambit changes.

Given that your pellet boiler will fire on demand, I feel that you do not need that size of storage, unless of course you want 40m² of solar.

[Culver]

Some clarifications:

With the WBS I was planning on either choosing a model that is designed to work with a sealed system or a pumped system with a dedicated heat exchanger and a gravity safety load dump. Pure gravity is definitely not an option due to the house being a bungalow and the distance between the WBS and the store.

Is batch burning an option with a WBS as that was what I was hoping for? Whilst my wife is generally a master fire maker, there are some nights where its just too much bother and burning every other day would be good. I would intend to size radiators (or maybe underfloor) so that I can use a TMV to set the flow temp down at 40-50°C. With a TMV on the DHW as well, then surely the reduction on temperature for the second day shouldn't be noticeable if the store has been sized correctly? I intend that the utility/boiler room will be highly insulated (U value significantly less than 0.2 for all elements if I can achieve it for reasonable money) and will potentially add insulated boxing in around the store if required.

I think I've lined up a free source of logs (although I'll be harvesting them so I guess they're not truly free) and we have plenty of space to store them for seasoning, so the pellet boiler would be relegated to a backup. The boiler room part of the new utility will have an area for storage of bags of pellet, logs and kindling and will be accessible without having to leave the comfort of the house.

Is the quoted 40m2 of solar split between east and west faces or is it per face? We've got nowhere near that at the moment, though but i'd be interested to hear how well large quantities of solar work during the depths of winter and how to cope with the load dump complications at the height of summer.

Sorry if this now getting complicated.

Phil

[Brandon]

sorry the 40m² was a semi throw away to highlight the point, and it would be south facing.

When I refer to batch burning, I mean a gasification boiler A thermo syphon will work in a bungalow, take the return under the floor, and ensure that the return tapping on the store is higher than that on the boiler.

UFH will run at 30^o.  But that does not negate the cooling of the store if you want DHW out of it.

If you can poke up with the wait for the water then it will save you kWh...

What do you expect your heat load to be? How much hot water do you require?

Lets start from that end and then we can see what will or will not be practical / sensible.

I have talked clients that have come to me for 50kW gasifying boilers and 5000l akvaterms out of it as it was not what they needed.  It is big bucks to spend just to be able to brag about the size of your store 

[Culver]

Unfortunately due to the concrete floors in half the house, running the return in the floor is not really an option.

As for the required heat load, we will have about 120sq.m of floor/roof area. The walls have retro-fitted cavity wall insulation and I have increased the roof insulation to 300mm of mineral wool. Approximately half the floor area is concrete of 1975 vintage and the rest is suspended tongue and groove with an uninsulated vented void. The house is double glazed throughout, but judging from the condensation that we get I don't believe that they are inert gas filled and the gap is rather smaller than in our previous house. I'd appreciate guidance in properly calculating the heat load.

Our current usage (assuming that the pellet manufacturer has quoted a true value for the heat output and the boiler is running at the quoted 80% efficiency) is about 50 to 80kWh per day which taking account of boiler efficiency converts to 40 to 65kWh of heat input into the house, of which a maximum of 5kWh per day is currently for hot water.

Over the summer I'd like to not have to (frequently) use either immersions or the pellet boiler for producing hot water so the store has to retain sufficient heat from solar input to cope with periods of bad weather.

[dhaslam]

Working out the heat loss just means adding up the areas multiplied  by the u-value.  For example your ceiling 120sq metres  would have a u-value  of about .1 so  it would use 12 watts  for each degree  difference between inside and outside  temperature, or in other words  practically nothing.    Your walls are probably about  .4  and windows  3.0.  Concrete floors  lose most around the perimeter, something like  1.0 average.     For ventilation loss I use .36 X volume in cubic metres X  airchanges per hour  X degrees C.       On the heat gain side there is  gain from south facing windows only in winter.  I don't have a figure but perhaps  an average of .5kwh per sq metre per day.    There are also gains from electrical appliances etc  and occupants.    If you have a calculation in a spreadsheet you can adjust the figures a bit to match your actual usage. 

Solar gain doesn't have a very regular pattern.  You can get a sunny week followed by two dull weeks so it isn't easy to regulate.  The ratio can be about 4:1.  However in summer dull days  solar rarely produce less than  1kwh per day per square metre.   To be able to  be sure of hot water all summer   means having quite a lot of surplus at times and  to avoid waste heat  would need a very large storage volume.  However I think this is the best way to go if you have space for a  large low cost insulated store.      The other plus of having oversize solar collectors is that there will be a large heat   gain in winter.                       

[Culver]

What's the recommended external design temperature for south-east England then? We generally use a living room temperature of around 18C so the heat load difference between external design temperatures of 0C and -5C would be quite substantial. The U value of 1.0 for concrete floors seems absolutely horrendous, is this just for the zone around the edges and if not is there anything that I can do about it? For the ventilated floor area how on earth do I calculate the air changes per hour figure?

Sorry to ask so many questions but it's my engineering background rearing its head and demanding more input parameters....... 

Phil