Domestic Options

[muldonach]

We live in a detached rural property approx 80m2 ground footprint and on two levels so total 160m2 floor area. The property is reasonably well insulated - half of it is stone construction with an internal dry lining which has 4" insulation fitted, the other half is of block construction with internal cavity wall the loft has 6" insulation, the modern part is double glazed and the older part has secondary glazing so we are happy enough with that.

 

For the last 25 years we have heated it with a Hunter Midi 20 multifuel with wraparound boiler and a central heating system which has been fine. The stove is in the lounge and exhausts into an old stone chimney (unlined) via 2m of 6" stainless faired into the chimney with a weak mortar mix. A second chimney runs up alongside this (presently unused). There is no other heat source available i.e. no mains gas and no currently fitted stand-alone gas or oil tank. The only heat storage presently available is a domestic hot water tank - well insulated but low capacity and no real heat exchange capability to the central heating.

 

The stove is well past its normal life expectancy and due for a replacement and the weakness of the system is the lack of heat availability in the early morning or evening if we have been out all day (less of an issue) so I am planning to change the stove and rebuild the chimney to a modern standard.

I would also like to fit a buffer tank to allow heat storage and provide for early morning heating, there is no space in the existingchouse to fit a decent sized tank nor is the house presently engineered to accommodate it or get it into place.

I have my own supply of timber and plenty of kit to process transport and store firewood, although the present stove is multifuel it has not seen anything but timber since the kids believed in santa - more than 20 years. We do not have sufficient space available for a chip store and feed system

 

I have space to build a gable- end extension of about 5 x 5metres and have previously had PP to do so, I am intending to renew that permission and fit  a buffer tank in the new extension. I am pretty confident that a decent sized stove in the lounge would provide adequate heat for the entire house but I am not sure that this is the best option going forward - I have the option to install a gasification log burner in the extension and use that as a primary heat source with a much  smaller stove in the lounge as either a roomheater or as a secondary heat source to the buffer tank. The extension would be built as a workshop with a heating system in it rather than a living space

So - questions:-

 

1. Given that a nominal 20KW stove (and in practice a good bit less) has given satisfactory results I am confident that that rating is plenty and we should be looking a buffer tank of min 1000 ltr with a natural tendency to oversize a bit I would probably be looking at 1500ltr but would appreciate any comments on capacity requirements especially from comparable houses.

 

2. If I go for a stand- alone stove in the lounge does anyone have any recommendations in the 15-20Kw range?

 

3. Any recommendations (or otherwise) on gasification boilers? I would be looking at 20Kw size?

 

4. If I go with a boiler in the extension I would be looking at an approx 5Kw stove in lounge as roomheater or a larger stove with boiler hooked up to buffer. They would be serviced by separate chimneys, - comments welcome.

 

4. Does anyone have any experience of hooking up two fueled heat sources to a buffer tank - any problems to head off?

 

5. Anyone out there got a case study of any similar project?

Edited May 11, 2020 by muldonach
mod.

[openspaceman]

2 hours ago, muldonach said:

1. Given that a nominal 20KW stove (and in practice a good bit less) has given satisfactory results I am confident that that rating is plenty and we should be looking a buffer tank of min 1000 ltr with a natural tendency to oversize a bit I would probably be looking at 1500ltr but would appreciate any comments on capacity requirements especially from comparable houses.

The big question is how do you intend to transfer the heat in the buffer tank to the house? If you use radiators you will have less heat available compared with underfloor as the underfloor heating (besides providing loads of heat storage in the slab) will take the tank down to a lower temperature, like 25C, and still heat the house while the radiators will lose effectiveness as they drop below 50C.

 

If the heat demand for the house in cold weather averages out at 12kW but you only fire the stove (which must then work at 18kW) for 16hours then the buffer must provide 96kWh into the house. In practice the house stores some heat, especially the chimney breast so the heat buffer is only required to start reheating the house in the morning before the stove is lit.

 

A heat store of 1500ltr with a top temperature of 90C and a lower temperature of 40C will store 87kWh with no losses

 

Our little semi of 80m2 over two floors in sunny Surrey seems to cope with an input of 4-5kW for 16 hours to keep the ground floor to 20C with no heating on upstairs and that's with 9" brick walls but good double glazing.

[muldonach]

Central heating is via radiators - no underfloor at present and would be pretty major surgery to fit.

[openspaceman]

39 minutes ago, muldonach said:

Central heating is via radiators - no underfloor at present and would be pretty major surgery to fit.

No it isn't a thing to retrofit. How hot is the water in your central heating circuit? If it is to be the same primary circuit water as the heat store this will limit how hot the heat store can get and in turn, with the radiators limiting the heat that can be extracted from the store, affects the size of the buffer tank.

 

Heating the tank becomes a dilemma as when you re light the stove(s) you want all the initial heat to go to the house and then, when house is up to temperature, just bleed off heat into the buffer tank but keep it stratified so the high temperature is at the top and it heats from top down and not waste heat by keeping the bottom of the tank tank hotter than necessary.

 

A laddermate type device that keeps the water in the stove boiler above about 60C in the same way a thermostat keeps an engine warm will prevent cold water from the store affecting the stove.

 

 

[muldonach]

9 minutes ago, openspaceman said:

No it isn't a thing to retrofit. How hot is the water in your central heating circuit? If it is to be the same primary circuit water as the heat store this will limit how hot the heat store can get and in turn, with the radiators limiting the heat that can be extracted from the store, affects the size of the buffer tank.

 

Heating the tank becomes a dilemma as when you re light the stove(s) you want all the initial heat to go to the house and then, when house is up to temperature, just bleed off heat into the buffer tank but keep it stratified so the high temperature is at the top and it heats from top down and not waste heat by keeping the bottom of the tank tank hotter than necessary.

 

A laddermate type device that keeps the water in the stove boiler above about 60C in the same way a thermostat keeps an engine warm will prevent cold water from the store affecting the stove.

 

 

We are a little bit at cross purposes I think - the only components of the system that I would prefer not to change if possible is the piping to the radiators - everything else can be changed without too much hassle. So we are looking at a brand new install as far as major system compontents and control goes

 

The present stove has 4 ports so one pair services the DHW cylinder as I understand it and the other pair service the central heating circuit although both outlets are at same temp. There is a thermostat on the side of the DHW tank which switches on the central heating circ pump at a set temp (or rather there should be - it has not worked in years and is manually switched). I have no absolute temp available - but the circuit gets PBH to the extent that the stove needs to be shut in at times - so boiler outlet temp can reach into 90's no problem.

 

Not sure where you are coming from with comments on stove lighting etc - I would regard that as design detail - but yes it would need to be addressed. The house is permanently occupied so the system is not going to be started from total cold on many occasions.

 

[openspaceman]

5 minutes ago, muldonach said:

33 minutes ago, openspaceman said:

We are a little bit at cross purposes I think - the only components of the system that I would prefer not to change if possible is the piping to the radiators

Not much more to say really, I'm not familiar with your specific requirements so tried to give a general concept of deploying a heat buffer tank in conjunction with a wood fired boiler.

 

You could increase the size and heat exchange efficiency of some of your radiators to be able to run down the heat store further as underfloor is not a possibility.

 

I've been out of the business for over 10 years now.

[muldonach]

2 minutes ago, openspaceman said:

Not much more to say really, I'm not familiar with your specific requirements so tried to give a general concept of deploying a heat buffer tank in conjunction with a wood fired boiler.

 

You could increase the size and heat exchange efficiency of some of your radiators to be able to run down the heat store further as underfloor is not a possibility.

 

I've been out of the business for over 10 years now.

Appreciate your comments thanks - and good one on the radiators although to be honest most of the upstairs ones are throttled right in so would possibly just need TRVs at one end and the other end opened right up.

Would I be right in thinking bigger is better for the thermal store?

[openspaceman]

43 minutes ago, muldonach said:

Would I be right in thinking bigger is better for the thermal store?

I don't think so as the heat losses from the store will go up with size, even though they are well insulated. Also you have to consider the cost of corrosion inhibitors, on our commercial units we used none as the systems were pressurised ( i.e. not open to air to let more oxygen in)