Navitron Renewable Energy and Sustainability Forum

BIOMASS => General => Topic started by: iank on October 07, 2010, 03:58:16 PM



Title: Advice for beginner please re WBS+PV
Post by: iank on October 07, 2010, 03:58:16 PM
Dear all

we are total newcomers to all this, and more i read about this the more confused i become.

We have just bought a house near Scarborough, built 1930s.

It currently has a LPG fired AGA in the kitchen and LPG gas fire in living room and also will heat the water and CH  and oil fired boiler for water and CH.

Our plan was to remove both of these, install a 4 kWp PV system and a log burning stove in the living room with back boiler to do the CH with a decent accumulator tank. The house heating needs have been assessed as 35 kW. We anticipate running the WBS in winter but not in summer.

My queries are:

Can we rely on an electical immersion heater as a back up in winter in case the WBS fails or I break my leg? We could retain the oil boiler, but this would mean combining an unvented with an unvented system - is this correct and are there problems with this?

We need some automated CH system for when we go away - presumably this would be possible using an electrical immersion heater and pump for the CH?

We were told by one installler that if we have a WBS sufficient to heat the house, the living room  would be unbearably hot. As far as I can tell, the back boiler stoves put out only a similar amount of heat to the room as ordinary stoves. Surely this comment is wrong?

Our needs for DHW are minimal in the summer (at the moment we use none, we use an electric shower and kettles as needed). Is it possible to heat the top layer of water in the tank with electricity for this?

We are not going to install cavity wall insulation because of the problems i have read concerning damp penetration.

Any advice welcome

IanK



Title: Re: Advice for beginner please re WBS+PV
Post by: A.L. on October 07, 2010, 05:20:03 PM
hello iank,

35kw is a large heating load, even for a house without cavity fill, how big is your house and how was the load assesed?

Can I encourage you to get cavity insulation, specify the polystyrene bead type as under extreme exposure conditions it does have a greater "safety margin"

Presume you are going to max out the loft insulation. Is the 35kw with loft insulation?

I will leave the WBS to those who know and will be along shortly

Immersion heaters are typically 2-3kw so you are going to need a lot of them if you really have a 35kw demand  :)


Title: Re: Advice for beginner please re WBS+PV
Post by: mikey9 on October 07, 2010, 08:58:35 PM
Not quite what you are looking at as we only use the WBS and solar thermal for DHW but......

We have a 4 coil 210 litre tank (Macdonalds - Dunfermline?)
Solar at the bottom, WBS (0.8kW back boiler in Clearview Pioneer 5kW WBS) coli and oil boiler coil up the sides and immersion one at the top.

Only use the Solar (summer and shoulder) and WBS coil shoulder and winter. We have had two babies and get through DHW more than some but have always had more than enough for our needs.


Title: Re: Advice for beginner please re WBS+PV
Post by: desperate on October 07, 2010, 09:16:37 PM
Hi Iank welcome

first thing to do as AL says is to install as much insulation as poss and then calculate a realistic heat requirement, 35kw is bonkers. our 1930s leaky 3 bed semi needs about 14kw in the coldest weather plus say 4 for hot water. You are sort of right that a WBS will overheat the living room if it can supply the whole house in theory, it seems hard to get more out of a back boiler than from the body of the stove into the room. our clearview 16kw with 8 kw backboiler seems to deliver about 25% to the water and the rest to the room. There is no chance of using immersion heaters as a back up for the heating, I would strongly suggest you keep the Oiler as a back up.

If you can post a rough plan of the house with window sizes and type I will do a heatloss calc for you.

Good luck in Scarborough

Desperate


Title: Re: Advice for beginner please re WBS+PV
Post by: dhaslam on October 08, 2010, 01:46:01 AM
Electricity is fine for backup heat and even as the main heat source  but you need to find an alternative energy supplier  that has a good night rate.     There is a  I am  sceptical about using wood for anything other than occasional  room heat.    The situation in England  is a bit different  from  other  countries because  there isn't enough alternative electricity to go around  and there is quite a lot of wate wood but if you are producing  daytime  electricity, that is even more scarce,  it does  shift the balance a bit.
 
In an older design house  that doesn't have much solar gain  you would get a lot of value from  extra solar panels  dedicated to heating.   Older houses need  heating up to the end of May usually.   March to May  solar gain  is a big proportion of the annual  total.     Last heating season  09/10 wasn't a typical one but there was very high solar gain during the  extended  frosty conditions.   My average solar daily gain in  February 2010 was only slightly lower than July and August 09.   This is what prompted me to go for an almost  entirely solar system.     

Of course the heat you would get from solar would disappear very quickly if  you don't have adequate insulation and  up to a point it is cheaper to insulate than to add extra heating capacity.     


Title: Re: Advice for beginner please re WBS+PV
Post by: iank on October 08, 2010, 12:01:23 PM
Dear all

thank you for these replies. I obtained the figure of 35 kW from a stove manufacturer, who provided an estimate from details of room sizes etc. I have attached the estate agents figures and floor plans below.

I am struggling to reconcile what desperate says

There is no chance of using immersion heaters as a back up for the heating, I would strongly suggest you keep the Oiler as a back up.  

and what dhaslam says

Electricity is fine for backup heat and even as the main heat source

Am I missing something obvious?

IanK


Title: Re: Advice for beginner please re WBS+PV
Post by: MarkB on October 08, 2010, 12:30:14 PM
Hi Iank welcome

first thing to do as AL says is to install as much insulation as poss and then calculate a realistic heat requirement, 35kw is bonkers. our 1930s leaky 3 bed semi needs about 14kw in the coldest weather plus say 4 for hot water. You are sort of right that a WBS will overheat the living room if it can supply the whole house in theory, it seems hard to get more out of a back boiler than from the body of the stove into the room. our clearview 16kw with 8 kw backboiler seems to deliver about 25% to the water and the rest to the room. There is no chance of using immersion heaters as a back up for the heating, I would strongly suggest you keep the Oiler as a back up.

If you can post a rough plan of the house with window sizes and type I will do a heatloss calc for you.

Good luck in Scarborough

Desperate

There are stoves, including wood pellet stoves, that will supply relatively little heat to a room and plenty of heat to the water. 35kW is a very big stove though, and I'd recommend a traditional boiler (of whatever fuel) to supply the peak heat. You certainly don't want to be stoking a log stove to provide this level of heat - you're probably looking at around 10kg of wood per hour.

An immersion is OK as a backup for DHW but you'll never supply 35kW for heating through an immersion. Your mains supply to the house is probably limited at 100Amp, which gives you around 24kW but the cost of this would be prohibitive and it is very unfriendly to be using such high grade energy to provide low-grade heat. Please don't do it. Your PV at best will only supply 11% of your peak heating requirements, and much less than this in practice in winter when you need the heat.

You're best bet is:
  • Insulate
  • Insulate
  • Draught Seal
  • Insulate
  • Smallish room sealed stove in living room (optional)
  • Large boiler to supply main heating (condensing gas/wood pellet)
  • Thermostatic valves on radiators, preferably with a night time setback


Title: Re: Advice for beginner please re WBS+PV
Post by: A.L. on October 08, 2010, 02:05:43 PM
hello again, some questions

1. Is the second floor bedroom a later room in roof extension? how much insulation in the ceiling above it (or in the sloping roof) and what is the construction of the bedroom walls? particularly amount of any insulation.

2. How much insulation in ceilings of first floor which are not below the second floor bedroom?

3. Single or double glazing? Approximate total area of glazing?

4. Is the balcony original or is it part of a later lounge extension.

5. What is the floor-ceiling height?

Can attempt "as is" and "possible" calculations at weekend


Title: Re: Advice for beginner please re WBS+PV
Post by: iank on October 08, 2010, 03:36:47 PM
As best I can estimate

1. Is the second floor bedroom a later room in roof extension? how much insulation in the ceiling above it (or in the sloping roof) and what is the construction of the bedroom walls? particularly amount of any insulation.

Yes an extension, minimal insulation in the roof above (the underside of the tiles is visible), walls are plasterboard. I intend to insulate these AMAP

2. How much insulation in ceilings of first floor which are not below the second floor bedroom?

Do not know[/iPossibly a reasonable amount as these ceilings have been lowered at some stage, the room heights are about 8 - 9 feet here.

3. Single or double glazing? Approximate total area of glazing?

All double glazed, quite a lot of windows, guess 15%

4. Is the balcony original or is it part of a later lounge extension.

Part of an extension, with flat roof. We are going to roof in this balcony, and will insulate the roof and walls well

5. What is the floor-ceiling height?

Mostly about 10 feet

Thanks


Title: Re: Advice for beginner please re WBS+PV
Post by: desperate on October 08, 2010, 06:30:18 PM
Dear all

thank you for these replies. I obtained the figure of 35 kW from a stove manufacturer, who provided an estimate from details of room sizes etc. I have attached the estate agents figures and floor plans below.

I am struggling to reconcile what desperate says

There is no chance of using immersion heaters as a back up for the heating, I would strongly suggest you keep the Oiler as a back up.  

and what dhaslam says

Electricity is fine for backup heat and even as the main heat source

Am I missing something obvious?

IanK

Iank

Leccy can be used for heat, but not from an immersion heater, dedicated leccy boiler or storage heater maybe, but first work out how much a kWh costs in carbon footprint and money, then decide what you need to do. I,ll do your heatloss later on,

Ta ta

Desp


Title: Re: Advice for beginner please re WBS+PV
Post by: desperate on October 08, 2010, 07:34:42 PM
Iank


just  had a look at your roomn sizes but didnt see a floor plan, need a bit more info to do a proper heatloss calc, basically the external wall area and material, the roof area and material, ditto for the floor and window area and material. You could do a rough and ready calc by taking each room volume in cubic feet, multiply by 5 to give BThU output for each room, add the lot up and add 10,000 for dhw and there you go. If I remember correct 3600BThU =1kW, i will check that conversion.

Desp


Title: Re: Advice for beginner please re WBS+PV
Post by: A.L. on October 08, 2010, 07:51:29 PM
hello desperate, perhaps we should co-operate on this  ;D

there is a floor plan on page 2 of the attached .doc but it seems a little temperamental about appearing, try clicking the mouse on page 2 or just lifting off the up/down scroll bar


Title: Re: Advice for beginner please re WBS+PV
Post by: desperate on October 08, 2010, 08:03:55 PM
cheers A.L found it thank you,  nice straight forward one to do eh     facepalm


Desp


Title: Re: Advice for beginner please re WBS+PV
Post by: dhaslam on October 08, 2010, 08:29:00 PM
Electricity is  competitive on price with oil  provided you use  off peak prices.     If you take roughly 40kWh as being equivalent to a gallon of oil.   Prices  in Ireland presently are  2.90  for  oil  and  2.94 for  off peak electricity.   There is a lot of variation in the output from wood burners  but if you managed to get one that output  10 kW + 2kW to the room for   say 12 hours daily it  would use  at least 36 kilos of wood and probably more like 50 kilos at  160 per tonne  and  would cost  8 per day,   electricity   and oil  10.50.   As others have mentioned  there would be a problem getting  144 kWh electricity into a nine hour off peak period because that would be  16 kW average,    needing  67 amps.   There  also  would be a  limitation of the   accumulator tank  that  would have to be massive.  


                


Title: Re: Advice for beginner please re WBS+PV
Post by: A.L. on October 09, 2010, 04:20:11 PM
desperate, that'll teach us to volunteer!  ;D

iank,

I have assumed that any future heat source takes combustion air from outside and reasonable draughtproofing gives an infiltration of one air change per hour

Case 1 no insulation anywhere - design heat loss 28kw

Case 2 assume 50mm mineral wool in lounge extension ceiling and 2nd floor bedroom ceiling with 100mm mineral wool in first floor heatloss ceiling - design heatloss 22kw

Case 3 cavity wall insulation and 250mm mineral wool in 1st & 2nd floor ceilings with 100mm mineral wool in 2nd floor bedroom walls - design heatloss 15kw

You should then add a safety margin of 15-20% and 3kw for hot water, so a range from 22kw-37kw boiler size


Title: Re: Advice for beginner please re WBS+PV
Post by: iank on October 11, 2010, 09:00:45 PM
Many thanks for this usefull advice which has saved us from 2 bad errors

Firstly, not putting in a massive WBS that requires an almost full time job feeding it. I envisage doing more with my recent retirement than spending it at the saw bench.  The calculations given to us by AL will be helpful in deciding the size of stove. We generally like a colder house, and only felt cold when internal temperature got to about 12degrees when we lived in glasgow.
Is there anywhere I can find out the estimated wood requirements for different size stoves? I think we will go for a reasonable size stove that will not break our backs, the biggest accumulator tank we can fit in, run the stove flat out for a bit, and hopefully have a couple of days off.

Secondly, not having a back up heating scource. I guess we will stick with the oil boiler, as it and tank are already there, although LPG would be preferable environmentally and for cooking, but would need to buy amd install a new boiler.


Title: Re: Advice for beginner please re WBS+PV
Post by: Brandon on October 11, 2010, 11:40:49 PM
Is there anywhere I can find out the estimated wood requirements for different size stoves? I think we will go for a reasonable size stove that will not break our backs, the biggest accumulator tank we can fit in, run the stove flat out for a bit, and hopefully have a couple of days off.


The estimate wood consumption of a stove is simple, as you take the rated output in kW (bear in mind that this is in lab conditions) and divide by 4 (due to wood below 20% MC having an energy density of about 4kWh/kg) to give kg/h (the volume will vary with wood species, but the mass required will be pretty steady)

A stove that is not providing an excess of heat to the property will NOT be able to make use of an accumulator, as there will not be any energy to accumulate.  If you have your own supply of timber then it may be worth considering a proper wood boiler and accumulator, even then, if you are using 25kW, then a 50kW boiler will only store 1 hours worth of heat for every hour of burn.

A wood processing effort on par with justme's (http://www.navitron.org.uk/forum/index.php/topic,10025.0.html) will be required to heat solely with wood on that scale.

As has been said before, insulate, insulate, insulate, I would advocate a minimum of 100mm of phenolic (or equivalent as a minimum on walls and floor, and 200mm in the roof.  Then start your draught proofing.

HTH

Brandon