What Happens To Boiler Outlet Temperature When Load Is Not Being Met At Max Output?
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Given your question, I would assume if flow rate through the boiler are a constant, heat input is a constant (max firing rate), then delta T through the boiler from return to supply is also a constant. Say the delta T is 15 degrees at your flow rate, add that to the return temp to get the supply temp. Cold start at 135 return water, supply is 150. One hour later when the loops are returning hot water at 155, supply water is at 170. As HWS temp exceeds setpoint, the boiler starts to turn down the firing rate (if a modulating fire burner).
So in that scenario, if the boiler does not meet the load, wait one hour for the boiler to catch up with the deficiency.
My guess would be the load is only not met at cold start or recovering from a night setback temp. You would have the recovery load plus the maintenance load. Once the structure , the load, is up to temp, boiler capacity would exceed demand and the boiler would modulate down heat output or cycle on/off. If the boiler cannot heat the structure under maintenance conditions (load not met) the problem is not with the boiler but either with the distribution (getting the heat out into the house) or heat loss (insulation windows siding).
So in that scenario, if the boiler does not meet the load, wait one hour for the boiler to catch up with the deficiency.
My guess would be the load is only not met at cold start or recovering from a night setback temp. You would have the recovery load plus the maintenance load. Once the structure , the load, is up to temp, boiler capacity would exceed demand and the boiler would modulate down heat output or cycle on/off. If the boiler cannot heat the structure under maintenance conditions (load not met) the problem is not with the boiler but either with the distribution (getting the heat out into the house) or heat loss (insulation windows siding).
Thanks for the thoughtful reply.
My actual scenario is that I am derating my 10/15 KW pellet boiler to 10 KW so that it will run longer with the idea that this is better for efficiency and equipment life. An oil boiler will supplement the pellet boiler's output during times when it can't meet load. I am trying to come up with a scheme to control this.
My current plan is to use what I already have and looks like this:
-Ecobee thermostat senses it is time for stage two heat AND
-It is less than X degrees outside
or
-Two hours of heat has been called for by thermostat, AND
-The boiler water has not got up to temperature Y AND
-It is less than X degrees outside
A limitation for sole thermostat control of stage 2 heat is comfort. It will call for stage 2 heat during recovery from setback unless setback is minimal, which might be all that can be expected from a "weak" boiler during cold, yet not coldest, conditions. Maybe it is indeed desirable to fire the boiler during a deep setback recovery, especially when it is less than X degrees.
This situation mirrors somewhat a dual fuel setup with a heat pump, only no lockout is required on minimum temperature for the heat pump and the pellet boiler's output is not tempered by low outside temperature.
Note that when either boiler starts, it will run to its aquastat satisfaction, and there is a 120 gallon buffer tank.
Thanks again for any more thoughts on the subject.
My actual scenario is that I am derating my 10/15 KW pellet boiler to 10 KW so that it will run longer with the idea that this is better for efficiency and equipment life. An oil boiler will supplement the pellet boiler's output during times when it can't meet load. I am trying to come up with a scheme to control this.
My current plan is to use what I already have and looks like this:
-Ecobee thermostat senses it is time for stage two heat AND
-It is less than X degrees outside
or
-Two hours of heat has been called for by thermostat, AND
-The boiler water has not got up to temperature Y AND
-It is less than X degrees outside
A limitation for sole thermostat control of stage 2 heat is comfort. It will call for stage 2 heat during recovery from setback unless setback is minimal, which might be all that can be expected from a "weak" boiler during cold, yet not coldest, conditions. Maybe it is indeed desirable to fire the boiler during a deep setback recovery, especially when it is less than X degrees.
This situation mirrors somewhat a dual fuel setup with a heat pump, only no lockout is required on minimum temperature for the heat pump and the pellet boiler's output is not tempered by low outside temperature.
Note that when either boiler starts, it will run to its aquastat satisfaction, and there is a 120 gallon buffer tank.
Thanks again for any more thoughts on the subject.
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It would stay at 135 plus however much it heats up going threw the boiler depending on your flow rate.
Thanks. I was kind of thinking that too. Boiler protection would kick in. I'm thinking if it was still at 135 or so after a certain amount of time, kick in the oil boiler. Thing is with the time lag, on the next cycle it would drop again to 135 as the pellet boiler slowly drops in temperature as it can't keep up and the time lag would begin again. Maybe more temperature swing in the house than is comfortable.
I would not consider trying to implement two stage heat in a single family residence. If you have the load, I would first just turn up the Windhager to its higher rated output. Then try adjusting the setback scenario, either higher setback temp or earlier recovery time.
Heat through the boiler and heat out to the distribution zones can be two different independent variables. That is how I would implement the control scenario. Primary secondary loops with outdoor reset of the distribution loop water temp is the only thing I would consider for the application, unless you have forced hot air and need the hot boiler water temp at the blower.
By resetting the loop water temp lower based on outdoor air temp, which is a proxy for the structure heat demand, the distribution loop stays on longer at the lower water temp, and consequentially the boiler will achieve setpoint HWS faster and dial down the firing rate to meet demand, staying on longer at a lower firing rate as the loop stays on longer at a lower distribution temp. Both water temps respond independently to different control requirements.
If you have conventional hydronic baseboard and are running 180 deg boiler water though it, I would expect short cycling of the loop and then the boiler, which is to be avoided in the wood burning appliance. The Windhager is made to turn down the firing rate, which is the ideal control scenario. It is necessary to maintain constant load to do this, constant load but lower instantaneous heat output. If you had cast iron radiators or radiant at 180 deg water in the loop, it would be nearly unlivable, but a dream system with modulating OAT reset of the distribution water temp. That's the missing element that you're trying to substitute for with the buffer tank.
I know you have posted but I don't remember all the specifics of the system, Windhager with conventional hydronic loads (baseboards radiant ?)
The oil boiler I would not tie to the Windhager. I would either have it manually off and turn it on or it would start in auto when the inside house temp fall below 50 deg, low limit emergency heat.
Heat through the boiler and heat out to the distribution zones can be two different independent variables. That is how I would implement the control scenario. Primary secondary loops with outdoor reset of the distribution loop water temp is the only thing I would consider for the application, unless you have forced hot air and need the hot boiler water temp at the blower.
By resetting the loop water temp lower based on outdoor air temp, which is a proxy for the structure heat demand, the distribution loop stays on longer at the lower water temp, and consequentially the boiler will achieve setpoint HWS faster and dial down the firing rate to meet demand, staying on longer at a lower firing rate as the loop stays on longer at a lower distribution temp. Both water temps respond independently to different control requirements.
If you have conventional hydronic baseboard and are running 180 deg boiler water though it, I would expect short cycling of the loop and then the boiler, which is to be avoided in the wood burning appliance. The Windhager is made to turn down the firing rate, which is the ideal control scenario. It is necessary to maintain constant load to do this, constant load but lower instantaneous heat output. If you had cast iron radiators or radiant at 180 deg water in the loop, it would be nearly unlivable, but a dream system with modulating OAT reset of the distribution water temp. That's the missing element that you're trying to substitute for with the buffer tank.
I know you have posted but I don't remember all the specifics of the system, Windhager with conventional hydronic loads (baseboards radiant ?)
The oil boiler I would not tie to the Windhager. I would either have it manually off and turn it on or it would start in auto when the inside house temp fall below 50 deg, low limit emergency heat.
Thanks again for your comments.
Yes it is a Biown with baseboard convectors and a conventional oil boiler.
For protection, I also have it arranged that the oil boiler starts up when the pellet boiler temperature is low and its induced fan is off, meaning the unit is off (ie, ran out of pellets).
What would probably also be ideal would be a modulating gas boiler to supplement the pellet boiler.
Yes it is a Biown with baseboard convectors and a conventional oil boiler.
For protection, I also have it arranged that the oil boiler starts up when the pellet boiler temperature is low and its induced fan is off, meaning the unit is off (ie, ran out of pellets).
What would probably also be ideal would be a modulating gas boiler to supplement the pellet boiler.
I might eventually come to these conclusions myself, but likely after experimentation during cold weather, customer complaints (!), etc.
NateB
Feeling the Heat
I would guess the PB would stop circulating water after the temp got too low in the boiler. Does the PB let you set that temp?
NateB
Feeling the Heat
Then you could turn off the pump till the PB gets up to temp, and kick the oil on around that temp.
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