Complete redo of heating system, where to begin?

[andrewindevon]

We own a 6 year old theoretically "low energy" house.  It presently has DHW provided by a small (0.7 kw) air source heat pump with immersion backup.  Space heating is provided by a separate Air to Air heat pump (0.7 kw) working through a MHRV system.  Without boring everyone with the details we are using large quantities of electricity, house is fairly cold and want to replace the DHW system and supplement or replace the ASHP side of the MHRV.  Floor area is about 220 m^2 volume about 450m^3. Room in roof design with lots of windows.
Average annual electric consumption has been about 10,000 kWh costing £1,400 with a green electric tariff.  The children are growing up fast and I expect out DHW use will rocket as they do!  There is no mains gas (at the moment).  November last year we fitted a 3.6kw PV system to a south facing out building.  There is a large unshaded east roof, a somewhat shaded west roof and a part shaded large south facing dormer.
At the moment the space heating is off (trying to save money and it's pretty useless anyhow) giving room temperatures of about 15c with an outside air temp of 3 c and overcast skies.  The lounge is a pleasant 19c with a 5 kw woodburner that's been on for the last 3 hours.

We've had some ideas as to what to change but I'd be interested in peoples ideas..so what would you do?

To get gas connected to the house would be about £2,500.
The MHRV and it's ducting is undersized for the house so quite a bit of wall is coming down to replace the ducting.

[Ted]

Replace the 5kW WBS with a bigger one with back boiler and run rads and DHW from it. Integrate with solar thermal for summer DHW with immersion backup.

I assume you are also the poster at http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=8095&page=1#Item_0 - if so, have you sorted the air-tightness out? Getting rid of drafts is #1 priority. WBS will need a dedicated air supply.

[Richard Owen]

If you had plentiful and easy access to wood, I'd be tempted to put in a RHI compliant wood batch boiler rather than a WBS with back boiler. Big thermal store to store hot water.

If you don't have easy and plentiful access to wood, I'd pay to have gas put in and use the most efficient condensing boiler I could find.

I'd make sure I had a hot water storage system that was solar thermal compatible. Which means that combi boilers are out unless you can find one that will take a hot water input.

Once you've chosen how to heat your water, you could then put a water jacket heater on the MVHR to boost heating.

[spaces]

Having sorted draughts (does the entrance let in bucketfuls of cold air directly from outside whn in use?), heat your water with solar if the south-facing outbuilding is close by - I assume your house's south facing roof is heavily shaded - and with wood. If you want to live 'green' then 15-16C should be acceptable if there is a wood stove emitting some bone-warming radiation. Woolly jumpers and log-splitting are the best sort of central heating.

[Baz]

If you are using 10k kWh per year I think that you need to find the hidden cable your neighbours are using to tap into your electricity.

[dhaslam]

It doesn't sound good that  the woodstove is only bringing the living room to 19C.   My living room is 21C  today with the stove  and underfloor heating off.      Is it possible that the insulation was not properly installed?   

[HalcyonRichard]

Hi Andrewindevon,
                          It seems your costs are about average for a combined utility bill. I would try to find out what is used on space heating, water heating and electrical applainces/lighting.

For example  the average electrical consumption in the UK is 10 kWh/day or 3650 kWh per year. If your home is the same then over 1/3 of your bill is still going to be there if you use gas or wood. This would mean your water and heating is costing you £ 889 per year.

If your heat pumps run at an average COP of 3 (another guess) and the cost of gas is 1/3 that of electricity then the cost for gas and electricity will be the same(at 100% efficiency).

To install a gas boiler and radiators will cost at least £ 2,500. Add the gas connection cost @ £2,500 and it will cost you £5,000 before you can make any savings at all - that is if gas saves anything at all.

My personal feeling is to use wood if you can obtain it cheaply. Then look into a Central heating and hot water version of an efficient WBS. You could keep the heat pump(s) as backup or protection when you are not there for any period of time.

It would also be interesting to know the power used by th MHRV and how efficient it is. Does it run all the time ?

A 220 m2 house is quite large so even if it is well insulated I would expect larger than average heating requirements.

Regards Richard

[martin]

"If your heat pumps run at an average COP of 3" -  - average? - peak, perhaps............

[andrewindevon]

Yeah that's me... I'd rather not bring that discussion into this one, different emphasis.

There's no water at the end of the building with the WBS and no simple way of getting any there unfortunately.  No cheap wood, all bought in from the usual sort of sources.

Things I have considered so far...

Do nothing, as long as the outside temp is above 0c then an internal temp of 15c is not too bad and the lounge heats up quickly. Pros cheap!  Cons: as the children get older they will want to sit in their rooms and will probably insist on ( or sneak in) fan heaters! Running costs.  Just had to bung on electric underfloor heating to dry some boots after being for a walk in the sleet.

Log gassification boiler to thermal store. (seperate DHW cylinder for Solar thermal) Pro: Low carbon, in line with concept when building the house.  RHPP of £950, possible RHI.  cons: cost (about 15-25K depending on model used and amount of work) certainty of fuel supply quality. Issues of exhaust particulate. RHPP is looking fairly unlikely. Resale of the house...

Gas boiler to solar thermal equipped cylinder Pro: simple tech understood by every plumber in the town, low cost of boiler/water tank con: Cost of connecting gas to the house About £2K. Goes way against the "green" plan when building the house.

Some other Heat pump tech; pro - green electric , con  never ever buying one of them again.

Add another wbs (or pellet boiler) to the north end of the house (the end with the water) and from there to a DHW cylinder/rads. Pro - simpleish con:  No obvious place for a stove, pellet stoves expensive.

I don't know what the COP is,but in the winter I suspect its less than 1... which is depressing! 

[regen]

If you are going to buy all your wood then in the long run i think you would be better off moving to gas for the main heating system. Wood is great if you can get it for free, have plenty of dry storage space and actually enjoy collecting it.  However wood burners are dusty, require constant attention, purchased wood is not always reliable and installation with a flue etc will be more than a gas boiler.  The gas boiler can probably be arranged so that it only tops up when the other heat scources are not sufficient. What ever you go for you will probably need a hot water cylinder and rads.

Good luck

Regen

[Baz]

Given awkwardness of wbs and gas how about a GSHP with borehole instead of surface loop. Will give a better COP.
You mentioned a lot of windows so how about some internal DIY thermal shutters. Also have a look at how the MHVR is working and if it is taking cold air from unheated rooms and pulling warm air from the rest of the house into those rooms unnecessarily. The rooms you are not heating may be still taking out a lot of heat from other rooms so if the internal walls are stud you could insulate them.

[dhaslam]

I don't think that  MHVR is  very effective  at transferring heat.  When I run  the condensing  dryer in the utility room  that puts out over 1 kW of heat  I cannot detect any temperature rise in the other rooms.    Air just isn't much use  at  transferring heat.   If you add a new boiler to supplement the heat  into the ventilation system  it probably wouldn't  be very efficient.       

How many rooms have electric underfloor heating?   Could this be used with off peak  electricity?       

[HalcyonRichard]

Hi Andrewindevon,
                          I would check and measure the system you have. If it's not performing try to find out why. If you look at :-

http://www.genvex.dk/uploads/GB_GE_PREMIUM_1_1L(2).pdf. The bottom graph for the L version suggests a COP of 4 @ 10 C
and a COP of 2 @ -5C or about 3 on average for a temperature range of -5C to 10C.

I would try and heat the whole house to the same temperature any differential heating will cause condensation problems. If there are any rooms not heated then keep them sealed from the rest of the house by keeping the doors shut.

Really the information needed is how much energy is needed to heat the house in kWh/year. Then work out the costs for various options. A very good point about the ease of use and maintainence of various systems. As for being green - this is your home and familly you must do what is right for you and your situation.

Regards Richard

[qeipl]

Andrew,

As others have said I think you need to work out how much electricity each bit of your installation is guzzling.

Space heating should be easy - your summer bills will show what's being used when no heating's required.

Separating DHW from everything else might involve a bit of guesswork but will give you an idea.

I have an ASHP DHW cylinder that also does 70m^2 of UFH which appears to be very economical to run, but I've heard of some versions that are a nightmare because the immersion 'back-up' is kicking in regularly.
If the capacity of the cylinder is too small for the usage pattern then the immersion heater is on a lot of the time to make sure recovery times are short.
This could be a major contributor to your high electricity consumption but you won't know for sure until you work out how much each bit of your system is consuming.

When you have some numbers you can then start to consider the options.

Malcolm

[billi]

I would say  as well ,to have a closer look   into  the details of the system , if 10,000 kWh  for a whole year is  all.....   for all  electricity needs ....then i do not find this  too frightening

First  question  should be ,how to improve the existing system  in my opinion .......... like  is it possible to pree-warm the incoming  air  from outside  to the heatpumps with a  soil-type heat-exchanger ,  or  a solar hot air collector  , .....perhaps


Perhaps a hot air  Wood burner could be integrated
 .... or a masonry  stove  can be integrated in the Heatrecovery  system to spread heat in all rooms



I cant  see the point of  redo all,and replace with something else  before not  finding out the problems or how to improve

But perhaps i have missed something

Billi