Buderus Pump Logic

Dan_15

Recently installed a 36,000 btu wood stove, and Ive been eagerly tracking how the stove output affects heat loss throughout the house. I've got the living room up to a balmy 89* while the dining room where the BFU room sensor is located stays a comfortable 73*. The room sensor is set for 70*. Its 35* outside.

Now I expected to see the G215 cycling much less since the room with the BFU is maintaining 3* above the set temp. But the boiler still cycles a couple times every 20 min for a couple minutes per cycle. Watching the logamatic readout, I can see that there is no call for DHW or space heat. However, I realized that the boiler's "pump logic" is set at 104* which is probably the reason for the cycling. The boiler begins cycling at about 93* until it brings itself up to 118*, then it shuts off and dumps into the heating circuit (constant circ with the BFU room sensor), until its time to repeat again.

Do I need to keep the pump logic at 104? I have a converted gravity system which pretty much stays warm all season. I only use a 2* setback on the room sensor because it takes so darn long to get those big pipes heated otherwise. What is the motivation for maintaining 104 if I dont really run the risk of a lot of cold starts. I cant help thinking that all this starting and stopping at 1.25 GPH is not good for efficiency.
Thanks, Dan

Dan_15

I guess I should not have posted during Sunday night football.

RoosterBoy._5

pump logic setting keeps the pumps off wile the burner is running till the boiler gets above 104 to protect the boiler from condensing.

your boiler fires to 118 then shuts off because it's within your heating curve. 3F is not much to effect your curve what you could do when your running the stove is dial it all the way down. this is what i do. then your boiler will take along time before it needs to fire.

or put your hotwater on 24hr and set your bfu to night mode and dial it down wile your up and before bed set it back.

i have a wood stove also and the bfu room sensor. this year i have it on summer mode :-) Ive just been burning wood i can burn wood just before bed and wake up to a nice warm house because it's well insulated and during the day i just need a small fire in the morning and the house will be warm all day.

thanks
Jason

Dan_15

Thanks, that makes sense in principle, but I dont understand why there would be a call for heat when the BFU is set at 70* and the room temp is maintaining 73*. I have the "room comp" dialed up to the max which gives it maximum control to continuously adjust my heating curve based on indoor feedback. So there should be no reason to anticipate a call for heat when the room sensor knows there is a 3* differential being maintained.

RoosterBoy._5

i agree with you mine dose the same thing. maybe joe can tell use why it works like that.

if my room temp is above the set-point on the bfu then why heat the boiler and how can we fix it?

thanks
Jason

S Ebels

Pump logic vs reset curve

I think you're getting the two issues entangled with each other, at least if I'm correctly picking up what you're saying/asking.

The pump logic feature is there for protecting the boiler from condensationdue to continuous firing at water temps below that level. Cold starts are not the concern. It will cycle the circ off during firing if your water temp is below 104*, no matter what else is going on in the house. It's strictly a boiler protection issue.

The reset curve on the boiler control however, is the gizmo that actually determines the water temp desired in the boiler at a corresponding outdoor air temp. In other words, the reset curve is what actually causes the burner to fire, based of course on the settings programmed into it.

These are determined by two things in your case. The first being the curve punched into the 2107 itself, then to a lesser extent, the indoor feedback the control is seeing from the BFU room sensor. The BFU will over ride, to a limited extent, the programming on the 2107 and adjust the water temp up or down.

If I recall correctly, the 2107/BFU combo will still provide constant circulation regardless until the system goes into Warm Weather Shutdown (WWSD), which is a function of outdoor air temp. It sounds to me like the two controls are still maintaining a very low water temp (probably as low as they can go) within the limits of their operating parameters.

I don't know if that answer helps you or not but it explains why the scenario you described is happening in the first place.

Dan_15

Yeah, you are right I was getting confused about pump logic, and mixing concepts. What you are saying makes sense, but regardless of the heating curve I should think that the boiler would not make a call for heat when the room temp is happily maintaining 3* above the set temp due to external factors. I'm sure that BFU makes adjustments for solar gain, so why not make similar adjustments for "wood stove" gain? I guess my question is that I'm surprised to see the boiler trying to maintain temp when there is an external heat source doing it. I was hoping to see no calls for heat as long as the room temp was being satisfied by the wood stove.

Ray Landry_2

Hello Dan what is your room comp set to? 18? How about your offset and building response?

jim_94

there may be no call for heat, but if you have it set up for constant circ it will continue to transfer hat. do you have temp gauges on the supply and return? it may not be dropping temp at all through the circuit. you might need a tstat for a high limit if you can't do it reset without overheating in shoulder seasons. also if you have an indirect, the boiler will dump it's extra temp into the zone, but that shouldn't be a problem with an old gravity system. what about trv's?

Joe@buderus_2

Educated guess

Pump logic is to prevent flue gases from condensing in the boiler when the burner is running. Pump logic does not effect the curve. The boiler is firing because based on the outdoor temperature the R2107 control is looking for a minimum temperature. You may not be pulling heat from the boiler into your home, but you are losing heat in the boiler (where the R2107 sensor is located). This could be caused by draft, pipe stand by loss or gravitational flow.You could "lower" the "OFFSET" by putting it into the negative number, but it could effect the heat when no woodstove is working. I have a similiar situation, Room Sensor in the main area with a fireplace. When I run the fireplace the area with BFU was hot. Area with the BFU was 72 to 75 degrees, rest of house was 62 degrees. Moved the BFU to the hall, put Wirsbo's telestats on the manifold loops feeding the main area of the house where fireplace is to stop flow into the radiant floor. WORKS GREAT!! Could be done with a thermostat and pump relay. If any questions please contact me.