Outdoor reset vs. thermostat setback?

jeds

I'm trying to wrap my head around the concept of allowing our new furnace to operate based on the outdoor temperature sensor curves vs. a programmable setback via the thermostat.



First, some background.  We had a Baxi HT380 boiler installed a couple of years ago replacing the 65 year old boiler in the house.  We are in upstate NY in a 1940's construction house with moderate insulation and finned baseboard and upright radiators. We have long used a programmable thermostat at 68 mornings and evenings, 65 during the day when people are in and out, and 60 overnight 10-7. In the first cold weather after the Baxi was installed the system was taking a very long time to recover to 68 in the morning, sometimes never reaching the set temp. On first call to the installer he came out and I believe changed the temperature curves, and when that did not entirely do it, he unplugged the outdoor temperature sensor entirely.



This seemed to fix the issue from our perspective in that the radiators get hot and the house temp recovers quickly in the morning.  I'm only now beginning to look in to how the system works again and wondering if we are wasting energy by using the furnace in this method where it comes on at full demand when necessary and then quickly shuts off once temperature is reached.  With this method it is very comfortable - you get a short blast of radiant heat when it comes on, nice in the mornings and evenings, and the temperature is generally quite stable.



However, if we could get similar comfort with better efficiency from using the temp sensor, I'd like to consider that option. What is the recommended way of setting up a system like this?  Would we be better off leaving the thermostat at 68 with perhaps a night-time setback to just 65 and letting the warm but not hot water circulate for longer, or are the short bursts of hot water to rapidly heat the house preferred?  (It rarely overshoots with this system as it would with the old - so it never gets uncomfortably warm).



Any feedback is much appreciated!

Gordy

To clarify

You do not have the 380 1.33 combi model correct?



If it is the 380 are you using an indirect domestic hot water tank?



Outdoor reset basis supply temps to your convectors on the outdoor temperature. So the warmer it is outside the lower the supply temp needed to off set the heat loss of the structure the thermostat is basically a high limit redundant control. Some boiler manufactures viessmann do not even have thermostats in their set up .





By your installer disconnecting the supply temp sensor either by not fully understanding the setup, or through frustration. You now have a similar operation to the old boiler, and not taking advantage of the efficiency offered through lower water temps. Now you are just running off the thermostat sounds like the boiler is running at full output then shutting down maybe short cycling not efficient.





Operating setback is argumentative around here. There are a lot of underlying factors as to whether or not it saves money at the sacrifice of comfort, and how much money saved as to its being worth while.



Generally if your house drops 5 degrees or more in 8 hours you may save a little. Obviously if you are gone for an extended period of days it will. My opinion is that deep set backs don't gain much in the way of savings, and sacrifice comfort with that being said some people like it cool during sleep hours. So setback is not a fits all to everyone. Bottom line are you setting back at night to save money, or because you like it cool for sleeping.ppp



I would ask the installer his reason for disconnecting the outdoor reset supply sensor.

jeds

Thanks Gordy.

This is the HT 380, not sure of any specific sub model below that. It is also used for domestic hot water.



The outdoor temp sensor was disconnected because otherwise, with the lower water temps, recovery times were way too long. When the heat cycled back on in the mornings, it would take many hours to warm the house back to 68. With it disconnected it heats the water to 80c and the house rapidly warms as expected.



This is the crux of my question: are we losing out on significant efficiency gains by continuing to use an 8 degree overnight setback and expect a quick recovery in the mornings? Would we be better served to keep the house at a more constant temperature where we can let the boiler operate more efficiently even though we won't be able to set it back as much because the recovery times are too long?



Setback at night is in part to save money and in part because I prefer cooler temps for sleeping.

Gordy

Reset curve

Maybe needs adjusted.



So you lose 8 degrees overnight through set back? That's a pretty aggressive setback, and heat loss. Was a heat loss done by the installer? Is there enough base board to cover the heat loss calculated.



Usually part of the advantage of running a mod/con with base board is the base board is overly sized because insulation improvements had been made since original structure was built. This allows lower water temps for the typical 180 degrees required for the maximum output of the baseboard emitter. But then that would only be needed at the coldest deign temp of the year which usually happens 1% of the heating season.





I believe you should use the outdoor reset with out setback or as aggressive of a setback, and dial in the reset curve. This would give the best efficiency to the boiler if it was properly sized.



Are you setup,for domestic hot water priority? Which means when there is a call for hot water the boiler lets the heating go, and makes domestic hot water until satisfied then goes back to the heating demand.

CMadatMe

Stop Setting Back

Leave the thermostat alone, hook up the outdoor sensor and let it do its job. Setback fuel savings is a myth. It is promoted because 99% of boilers are oversized and so are the heat emitters.



Heat Loss, heat loss, heat loss..

jeds

Yes, maybe

the curves need adjusting.  I can do so via the knob on the front of the unit that either controls the water temp (with no outdoor sensor) or adjusts the curve when it is.  I've currently set it up so that it is slightly above base-line to try it out again now that I have a bit better understanding of how it's supposed to work.



Yes, DHW has priority over heat and that seems to work fine.



I have the 8 degree setback still because that seemed to work fine with the old boiler (or this one with no outdoor sensor).  The temp drops steadily overnight with no heating needed at all overnight.  Depending on outdoor temps overnight it drops to between 63-60.  I am willing to consider dropping it less with this new system if that's how it's designed to work, but anything above 63-64 would probably be a bit warm for sleeping.



I'm quite sure that no heat loss was calculated by the installer - it was installed on short notice during the summer when I was 12,000 miles from home and the old hot water tank started leaking on my wife.  The installer was someone we'd had a relationship with for many years though.



Like I said, the house is 1940's construction of moderate insulation.  Several areas of the house have been re-insulated during improvements, but it is by no means tight.  With a fast responding system the house feels warm and stable though, and at 68 feels much warmer than many other houses I've had or been in at 70-72. 



The problem seems to be that even though there is enough radiant capacity when the water is 180F, it has a hard time keeping up when it's 140F or thereabouts.  I can try reducing the setbacks and see how that works for us, but it still feels like the boiler is running an awfully long time, even if it is at a reduced output, compared to the quick bursts we are used to.

jeds

I'm willing to give that a try Chris.

My concern is that with this lower temperature water circulating, it appears that the boiler is running a LONG time to make up even a degree or two. Is running the boiler longer at lower output really mor efficient than running it a short time at full output?

CMadatMe

Curve

May need to be adjusted but setback when doing outdoor reset is a no no unless the boiler has the capability to boost coming out of setback. The boiler has no idea that you are setting back so it has no way to react to it.



You need to let the boiler do its job but need to make sure it is set up with the proper curve. Need to do a room by room heat loss. Measure emitters for each room and find the capable output of the emitters for XX degree water to over come heat loss. Without it your just guessing on a curve.

Paul48

Memories

We all grew up standing by a hot radiator when we were cold. You have to lose that mentality. All homes have specific characteristics that allow the cold to enter the home, and the heat to leave the home. A mod/con boiler with the correct reset curve will produce just enough heat to overcome the losses. Once the structure is warm, it may only need to produce 20% of its rated output to maintain a very comfortable temperature in the home. By setting the thermostat back and letting the temperature get down to 60*, you haven't just done that to the air temperature. You've done it to the whole structure.Under normal conditions the system will run much longer than a conventional boiler, at a much reduced firing rate.It can take some trial and error to come up with the best reset curve for you. I would suggest that if you enjoy sleeping in a cold bedroom, you look into TRVs or zoning for the bedrooms, and not reduce the temperature of the whole house.If there are drastic differences in temperature from room to room, you need to do a Heat Loss of the cold rooms, you may have to add emitters to them. But,"Don't throw the baby out with the bath water", and go back to the old function boiler.

jeds

I can see that Paul

I am a couple of days in to letting the system work via outdoor reset at a consistent indoor temperature, and I am starting to see the reasoning behind this system.  While I would not have said that our house was uncomfortable before, I can now see that in reality there were always cold spots, especially in our half finished walk-out basement that is 2/3 below grade.  If my understanding is correct, the basement is acting as a large thermal mass (it is generally 7-10 F cooler there in the summer than upstairs), but by letting the house cool to 60 overnight, that mass was never really getting up to the full 68 degree temperature, and dragging down the temperature of the rest of the structure when we were trying to heat it up.  Immediately now that we are leaving the temperature stable there is less of a noticeable temperature shift between upstairs and down.  68 is a little bit warm for sleeping at night, so I may need to use a couple of degree shift, or, better yet, maybe we can stabilize at 66 or 67 at all times. 



So my question is, why doesn't anybody tell you that your old way of doing things no longer applies with these systems?? Not my contractor, who made a couple of free service calls trying to 'fix' things for me when in reality I was just trying to use the new system the same way we'd always used the old one, and while you fairly regularly read about thermostat set-back in the mainstream media, I've never seen any mention of needing to treat these systems differently.  Even in the Baxi user's manual for owners, all it says is something to the effect of 'we recommend turning the water temperature to maximum for winter heating.'  Why is this??

jeds

One other question

Looking at the circulation plumbing, I'm wondering if the output and return lines are switched?(!)



The DHW lines make sense to me, but if I'm understanding what I'm looking at correctly, it sure appears that the hot water output coming from the boiler is being dumped directly into the cooler return line before being pumped through the radiators.  I need to borrow an infrared thermometer from work to confirm the temperatures I'm feeling with my hands, but it seems pretty clear that this is causing the water temperature to drop drastically over just a few inches of piping as it leaves the boiler unit.  It sure would seem to make more sense to me if the water flowed in the other direction, but I am hopefully missing something.



If I were to post a picture of the plumbing here would someone have enough familiarity with this system to tell me if it's right or wrong?



Thanks again!

Paul48

Absolutely

We like pictures. Stand far enough back to get a good picture of the piping. You'll get an answer, as to whether there's something wrong with the piping.

jeds

Ok, here we go

I've labelled what I believe to be the flow path, which I'm not even sure how it works like this, but it is what appears to be happening.  It sure seems backwards to me, where the hot water exiting the boiler should be taking a left out what is now the return line.  The pump is definitely pulling from the other line though.



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