You say heat requirement is 12kW but at what external/internal temp? - this might be important if it was based on zero deg C outside and 18 deg inside and it was often -5 deg C outside and you (well let's say your live-in parents) want 22 deg C inside.

If, your house was at the stated max temp difference and the heat demand figure was correct then your 12kW solid fuel boiler would have to run 24/7, leaving you no chance for it to cool down, be cleaned and heat up again EVER whilst it was cold outside and of course you need 1-2kW of heat output for your DHW - which has priority, guaranteeing that you would be cold unless you were in the bath! The heat leak radiator is irrelevant in the context of your question but 1-2 kW for DHW on a 12kW system is relevant.

You have not mentioned a tank where hot water is stored for you radiators, do you have one - I do hope so. You have not mentioned why you think your heating system is incorrectly specified but even from the scant details you have given I fear it is obvious: you have been cold this winter.

I would expect the heat requirement for your house to have been calculated at an external temp difference that is typically exceeded no more than 5% of the time. For my house I use -5deg, it gets down that low for no more than say 3 weeks a year, it does get colder but only for a day or 3 per year. IF, and it is a big IF, yours is 12kW then I would expect a 24kW wood burner feeding into a 1000 liter tank (assuming by solid fuel you don't mean pellets).

Sadly the short answer to your question is No the figures do not look right.

Oh hell - I just re-read your post - your radiators: what is the temp difference from which they give 12kW heat output, normally this would be 50 deg, meaning 70 deg water in them, meaning when the temp in your tank fell below 70 deg, in my experience 75% of the time, the radiators cannot heat your house adequately when it is cold outside. These figures would be marginal at best for a Gas Boiler and are utterly inappropriate for a wood burner. What next? I can't say, but post more details of your system is the first step. Sorry. BTW, were you planning to add extra insulation and improve air-tightness this summer?

If your boiler can only deliver 12kW then your rads can only deliver <12kW no matter how big they are. You are right in that there is no way for the system to generate 12kW + 1.5kW.

They are partly wrong about the DHW. If the heat loss from the house is 12kW (at whatever temperature difference is specified) then the boiler output to the rooms also needs to average 12kW or the rooms will be colder than specified. If the max output is 12kW and for some of the time the heating is off (eg when it's feeding the DHW) then the average delivered to the heating cannot be 12kW. However the average is NOT as low as 12kW-1.5kW = 10.5kW. Lets say the DHW is being heated for 1 hour a day. Then the average available for heating is...

12 * 23/24 = 11.5kW

But that assumes the boiler is blazing away at 12kW for 24/7.

It also sounds like the rads might need balancing? Compare the temperature of the rads in those problem rooms with the others.

none of the rads have a TRV. I was told the system design wasn't suitable for use of TRVs?

If your plumbing really looks like that, you're probably also losing a measurable amount of heat from the expansion pipes.

Is your installer a member of a trade association? If so, I expect they have a code of conduct and I expect it requires a more detailed analysis of the requirements than you have told us about so far.

Posted By: dgd17none of the rads have a TRV. I was told the system design wasn't suitable for use of TRVs?

I'm not an expert but isn't such a design against building regs? Perhaps an expert can confirm.

DBSCG (Domestic Building Services Compliance Guide) : All rads except heatsinks and references should have TRVs.
HETAS refers to flues and chimneys, not really CH system design.

I suspect not much 'design' has gone into the installation....

Posted By: dgd17so as i understand it the sizing of the rads should allow an extra 10 or 20% on top of the heat req figure to allow for heat loss?

Yes, but more important is that the boiler should be bigger as well so it doesn't have to run flat out 24 hours a day.

One thing you haven't mentioned is whether the boiler also outputs significant heat directly to the room that it is in? If so, that is additional heat that hasn't been taken into account in everybody's analysis so far.

Posted By: DarylPDBSCG (Domestic Building Services Compliance Guide) : All rads except heatsinks and references should have TRVs.

Sadly for dgd17 that's a 'should' rather than a 'must'/'shall', so there's wiggle room if that's the only statement.

just spoke with hetas re what calculation are required. the only requirement seems to be that number is calculated for the heat requirement. no particular methodology recommended or required.

There are no dum Qs dgd17! To answer your question what is the rated MPG for your car - do you achieve it - no, never not even once! When will you clean your boiler and how long will it take to cool down, clean, reset, and warm up the rads to the temp they need to give you max hear output. If you are a slave to your boiler, work hard and add wood in the small hours and buy only very well seasoned oak, you might get an average of 8/9kW from your boiler over say a week. That's why one always, as in 'always', doubles the heat output of a wood burner over the house's heat demand. A wood burner is a batch heater, designed to work in batch cycles and store heat in a battery (TS) for when it is not 'on'.

OK your house needs 12kW sometimes, 13.5 peak incl DHW water. Your heating delivers 15kW (13.5 via rads and 2kW via direct heating from boiler) via a wood burner with no thermal store and no TRVs. This is absolutely inadequate and will result in you being cold.....as you have obviously noticed. So it is time to talk about what to do next:

1/ Reducing your heat load by insulating and making things more airtight (with ventilation as required) will always help but will not SOLVE your problem unless you have a high mass house, you insulate externally and you get the load down to about half ie more like 6kW. We can bounce around this idea, talk about the generalizations contained therein, compromises, options etc but after all that you are going to say it is expensive and very difficult for you practically. It is the ideal solution, it is also the most costly solution and highly unlikely; nevertheless you should at least explore this option first and there may be a few very low cost things you can do with some silicone and a few rolls of rockwool that will help in small but significant ways.

2/ You can bin your wood burner and fit a very low cost 20kW gas boiler, this will SOLVE your problem for little capital cost but will cost you an arm and a leg forever more. Or fit a big pellet burner instead, pain in the bum with the pellets but possible.

3/You can change your wood burner and add a TS, but you need a lot of room, a fair bit of cash and the head of your last engineer on a spike out side your door so that whoever says they can do the work understands that they had better mean it. As I first mentioned and Peter-I-H reiterated with a wood burner you need 25kW ish plus a 1000-1500 litre tank.

4/There is also probably a clever solution with a small tank (operating as a neutral point) and an additional gas burner operating in tandem with your wood burner and some sophisticated controls and a damn good heating engineer but aside from knowing it should be possible that's as far as my knowledge goes.

5/ You need TRVs on all but one radiator (or, better, on all rads with pressure bypass) BUT only an expert can comment on whether this is safe because your wood burner is an uncontrolled heat source and were it to be warm and you fill it up with wood, would the one radiator wouldn't leak enough heat, I think not - what other safety controls exist etc etc. TRVs will help you be warmer in any compromise scenario and save you money if you solve your problem.

thanks everybody for the input.

on closer inspection of the calculations i've been given i have just noticed they haven't included the hallway and landing in the heat requirements!

i will go do as dantenz suggest and establish heat load of each room as a first step

here is the results of my evenings work with a tape measure and an online heat calculator:

total heat requirement: 12.7kw
output of current rads on main heating circuit: 11.58 kw
heat leak rad : 1.5 kw

stove output 14kw - 12kw to hot water, 2 kw to room

https://docs.google.com/spreadsheet/pub?key=0AupO5VvgkKENdERPMWZPUjFVRTFhWVBXNUJzTzVWRlE&output=html

all suggestions on how best to proceed are most welcome:)

i'm a bit puzzled gotanewlife,
you say that a woodburner is designed to be a batch heater and use a thermal store. i understand that this is the best arrangment. but the boiler stove we have does not seem to be designed for this. the installation is as per the manufacturers instructions which make no mention of a thermal store?

ah ok. i understand the point you are making now.

to be honest i am reasonably relaxed about not being warm 24/7. house was previously heated by stoves and open fires in each room and a rayburn in the kitchen. i'm ok with waking up cold and having to light the stove to heat the place. its not ideal but i can live with it.

but i would like to be able to get the place when warm when the heating is going. that's the main issue i would like to address.

Good point that Chris.
You can also have a look at your daily weather and decide when your boiler is not producing enough.

Right, you need to know about kW and kWh.

If your burner is a name plate 12 kW model and it runs at 70% efficiency for 10 hours then that is:

12 (kW) x 10 (h) x 0.7 (efficiency) = 84 kWh

A kilo of timber has around 4 kWh/kg

so
84 (kWh) / 4 (kWh/kg) = 21 kg of timber needed.

Now if your house has a heat load of 12 kW at say a temperature difference of 15 °C
then you will need:

12 (kW) x 24 (h) x (1/0.7) (efficiency) = 411 kWh/day

411 (kWh) / 4 (kWh/kg) = 102 kg of timber per day

Your room by room heat load calculations can be changed for different temperatures giving you a new heat load.

Yes, the efficiency is a tricky one, all depends where it is measured. It should be the primary energy to delivered energy ratio, or the overall efficiency. Trouble is that will vary for each installation.

So if we rearrange my calculations then we get:

12 (kW) x 24 (h)= 288 kWh/day

288 (kWh) / (4 (kWh/kg) x 0.7 (efficiency) ) = 102 kg of timber per day