Is my Morso under-performing?

[jtcedinburgh]

Hi folks. I have a Morso Owl which is a 7kW stove, so perhaps medium in size, and I mainly burn logs, though I am experimenting with smokeless anthracite briquettes right now as well.

I am wondering whether my stove is under-performing, but I have nothing to measure against.

Problem is this: I find it difficult to get the stove up to temperature (i.e. stovepipe surface temp of ~300F 18" above the stove) - can take as long as an hour to get this far, though occasionally I can do it more quickly - and without 'babying' and regular feeding it goes out quite quickly.

Two examples:

Wood: If I load up with three or four 14" splits, maybe 5" at thickest point (which is around the max I can hold in the firebox) it will need refilling in around two or two and a half hours at the very longest, normally well within an hour and a half. Even with the air down, from very very hot at 11pm it is stone cold by 6am. I read here about overnight burns, and I'm lucky to get a burn long enough to last half that time.

Smokeless fuel: reputed to burn for 18 hours, with air at 3/4 (bearing in mind this is a multi-fuel stove) I left the house at 10am this morning to take my son to his swimming lesson. Inside house temperature was around 16C (60F) which is its current 'unheated ambient' temperature. After a couple of hours from 8am I got the stove going with wood and added coals to get the room temperature to around 20C (68F) before leaving - with around 12 or 15 red hot coals going nicely. Returned at 2pm, the coals have gone cold - no sign of heat, stove has a vague warmth to it but is far from being hot. Room is back at 16C.

I grant you I may be expecting too much of a modest-sized stove, but surely to goodness expecting smokeless briquettes to burn for four hours (when they are rated to at least 12 hours burn-time) isn't too much to ask?

Chimney is ~15m Victorian clay-lined, with 316-grade liner inside into stove (top-exhaust). Room is admittedly slightly bigger and lossier than the stove would prefer - actually three open plan rooms, 16'x14'x12' joined onto 12'x10'x12' onto 12'x12'x7' - but it doesn't explain the poor performance, surely?

Any help appreciated. It was a fairly expensive stove and was professionally installed, chimney was swept less than 8 weeks ago and is not used 24/7, wood is largely dry hardwood (occasional damp bits do find their way into the stove I admit) and it's what you'd call in the US 'EPA style' - i.e. it's super efficient and clean-burning, with primary air (underneath I believe), secondary airwash to the glass, and tertiary 'afterburner' vents to the rear. So, not by any token a 'cheap and nasty' stove.

Thanks,

John

 

[jtcedinburgh]

Just for clarification, I should have specified its heat output in BTU - AIUI 7kW is roughly 24k BTU. Still keen for any and all advice here...

 

[begreen]

Have you placed the thermometer on the stove for a few days and tracked the stove top temperature? It could be the stove is working fine and that it's just very efficient. If the stove is working well, you want the flue temps to stay moderate while the stove top temps go up. This is the way our Jotul F400 worked. It often had cooler stack temps than the PE T6, but respectable stove top temps. As soon as I cut back the air supply and secondary burn increases, even the T6's flue temps will decline below 400F (pipe interior temp) while the stove top temps increase to about 550-600F (surface).

 

[jtcedinburgh]

Hi - I shall try that, but it doesn't really explain the short burn times and the fact that the solid fuel went out despite having 3/4 primary air...

 

[precaud]

One of the confusing things about these stove output numbers is that it is a "maximum output rate per hour," not an "average hourly output." And it doesn't specify the period of time over which the stove is actually capable of producing that rate of output. That small a stove may only be able to produce 24k btus/hr for 15 minutes, at the very peak of each load's burn cycle; the rest of the burn cycle it is well below that rate. The smaller the stove, the larger this discrepancy is. And I think that is what has led people to, in general, opt for one size larger stove than the numbers might suggest.

I think it is more realistic to take the weight of each load of wood, convert to btus, multiply by .65 for approx. stove efficiency, then divide by the time between loads. That will give you a better number for hourly output. For a small stove, 5 pounds of wood with 2.5 hours between loads is only going to give you AT MOST 11.2k btus per hour.

It would be really nice to see something like this in every stove's manual:

"To maintain an average output of xxx btus per hour, you must feed the stove yy pounds of seasoned wood every zz hours."

I know, I'm dreaming...

 

[begreen]

Precaud's explanation is right on. 7kw is not a big stove. If it's having to be pushed hard to keep up with the heating task, wood consumption is going higher than if the stove was throttled back and running at a lower temp. That doesn't mean that anything is wrong. Our Jotul could get 6 hr burns when the outside temps were in the mid 40's, but when they dropped down to the mid-20's we were feeding it every 3 hrs and having to run it with a stove top temp about 150-200 degrees hotter.