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[mcarter]

Monitoring with ET-732, I record a supply temperature @ 172 and return of 169.  Without heat on, I see a 40 degree drop on return when hot water is used.  Any feedback or comments?

Michael

[willieG]

40 degree drop is more than the "experts" would like to see. i say if it works and your  stove recovers its ok

if you think its not ok perhaps you could speed the delivery rate up some?


i fyou are topped out at 172 and you drop it 40 degrees to 132 then by the time your stove kicks in and gets burning yoru stove could (i say could) drop down quite a bit?

[mcarter]

Thanks willieG,

I realize the sweet spot is no more than a 20 degree (farenheit) differential for maximum efficiency.  I can increase my delivery rate and see if there is a notable difference.  I wonder if the size of the DHWX can influence the draw.  I need to see what the temp is at the OWB during hot water use.  It appears to be a 8 to 9 degree difference between OWB control reading and the supply line reading from the Maverick. 

[willieG]

Thanks willieG,

I realize the sweet spot is no more than a 20 degree (farenheit) differential for maximum efficiency.  I can increase my delivery rate and see if there is a notable difference.  I wonder if the size of the DHWX can influence the draw.  I need to see what the temp is at the OWB during hot water use.  It appears to be a 8 to 9 degree difference between OWB control reading and the supply line reading from the Maverick.

you could (if you don't have allready) put in a tempering valve to control the temp of the water leaving the domestic tank, this would (if your OWB is hotter than you need your domestic water) slow down the amount of btu going into your exchanger which would put less of a draw on your OWB

or if you have a valve on the line going to your domestic exchanger slow the feed rate to control the temp of your domestic water there by also lowering the demand of draw on your OWB

these are things you can do if your flow rate is not adjustable from the pump

[mcarter]

Going to look and see if I have a tempering valve this evening, I hope I have one, it would be nice to decrease the water temp a bit and if it helps on the draw, that would be excellent!

Thanks for that idea!

Michael

[mcarter]

I dont have a tempering valve but i do have a shutoff valve on both lines going into my hot water heater.  Would it be possible to partially close one of these valves to achieve the same effect or would that ot be advised?

Thanks,

Michael

[willieG]

i dont recomend that you slow the supply of water to your domestic hot water tank but you could slow the amount going through the exchanger if you had a tee in the line with 2 valves you could  send some cold (supply)  water directly to the domestic tank  by , bypassing the exchanger and some cold (supply) water to the exchanger.

this is just a thought.it may not be the proper way but i think it could work with some variation in water temps being felt but likely bearable

[mcarter]

40 degree drop is more than the "experts" would like to see. i say if it works and your  stove recovers its ok

if you think its not ok perhaps you could speed the delivery rate up some?


i fyou are topped out at 172 and you drop it 40 degrees to 132 then by the time your stove kicks in and gets burning yoru stove could (i say could) drop down quite a bit?

I dont see the temps on the pex supply line drop below 159F(with an 8 degree correction, that would be 167F at the boiler) during a 40F degree differential with the return pex line.  I think based on this my stove recovers fine. 

When we discuss a 20 degree differential as optimal, is that relative only to the supply/return temps, or would that possibly just refer to the differential at the OWB alone? 

I think the only thing that would really concern me is the less warm water entering the boiler and its effect (if any) on the expansion/contraction characteristics of the steel.  This is probably a non-issue but I find it interesting to discuss.

[willieG]

what is meant with the 20 degree  difference recomended by most is that the temp at which water enters your house ( before is enters any appliances) should be at leasts 20 degrees warmer than the water leaving your house returning to the OWB .

so if your water is entering your house at 159 the experts say your water returning (leaving the house) should be no less than 139

 if i read your post properly you say you are losing 8 degrees  between teh boiler and the house? if that is correct how many feet of underground line do you have. Most store bought pipe like thermopex brag of only a 1 degree loss or less per one hundred feet of pipe (im not sure at what gpm they are using)

like i said you could speed up delivery of the water and that should raise your return temps if you think you need to

im not big on understanding how the 20 degree drop or less helps with efficiency, perhaps someone else on here knows the reasoning?  perhaps they think a big swing in temps could casue a stainless tank to contract and expand too much but  or it may casue yoru boiler not to recover?

[Ridgekid]

I could be wrong but we have two different factors. Heat loss with nothing using the OWF water and delta t across a hx using the water.

For the first one, we all want to see the least amount of heat loss from and to the OWF. We have heard from others (including my self) that we loose one degree from the OWF and maybe 3* more across a hx. We can't control it, but we do know what our loss is.

For the second part, CB for example, states that the delta T should not exceed 30* when a heat exchanger is in use. As you recall with the Grundfos I was getting 20-25* across my furnace hx. With the TACO I get 15*.  Here we learned increasing water flow decreases delta T.

Does this make sense or did I make it more confusing? 

[willieG]

makes sense to me speed up water flow you increase btu delivery but you dont increase the amount your exchanger is drawing out so return temps rise

we cant (after we put our pipes in the ground) control our heat loss from those underground pipes but can see we are losing btu's. the goal of us all (or it should be) is to keep this loss to a minimum but that can also be driven by cost. we all do what we think is best for our situation without really thinking about the draw that that lost heat may put on our stoves or our wood pile. as i have stated before that uinderground pipe you put in the ground is likley just as important as the stove you choose becasue they work hand in hand on the performance of your stove and may also form your opinion of how good your stove is operating

[mcarter]

Sorry, to clarify for you willie, I am comparing the controller temp at OWB with the temp I am measuring with the Maverick on the inside of the house on the supply pex line.  OWB controller reads 181F and supply line inside home reads 173F, the 8F difference.  Was just using that to roughly correct for the difference of water temp inside the pex and the temperature reading outside the pex with the Maverick.

The 20 degree differential is something I read about some time ago but it concerned industrial steam boilers and residential steam boilers where radiators were used.  Some of the old engineering articles (early 1900s) that you can dig up with Google elude to this differential as being optimally efficient for the fuel source (coal or wood, I assume).  I don't know for sure if it has relevance to OWBs, since they are not steam but I guess that based on the 10 degree differential programmed on the controller in my OWB, which would allow for some temp drop before full recovery, it seems like they are trying to keep it in that range perhaps?

It would be nice to see a link to some information on that as it pertains to OWBs specifically. 

I get 15  - 20 across HX and 35 - 40 on DHWX (30 plate).

[willieG]

i guess CB is a big name in the OWB business and they say 30 degrees is fine? i say it your OWB is recovering on those cold nights then anything is fine as long as the fire can get thetemp back up

i have seen  my  stove going to the house and if the doemstic and the furnace are both using heat and the "big water user/little women" is taking one of them 30 minute showers, my return water going back at 130 (it only comes in at 160) and when teh fan kicks on at 150 at the stove i have seen the stove temp drop to 145 or a bit less before the fire gets going and starts to catch up..but it does and i never worry about it.

as i get a little older and retired, i may have more time to tweak the stove and try and get a little more out of it. it has heated the house for 10 years (this is 11) and i guess i can afford the time to try and cut down on teh wood usage a bit.