Tekmar tn4 thermostat cycling


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Old 01-12-09, 05:05 AM
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Tekmar tn4 thermostat cycling

Cold-start boiler with Tekmar 336 zone controller and a 54X series thermostat - do not have a 42X reset controller installed.

When I use the 54X - which I think is set to 'auto cycles per hour' - the thermostat will turn on the zone for a minute or two at a time and very often. Because I have a simple aquastat this causes the burner to cycles on and off really often. What to do?

Do I need a 42X or heat manager-type control to make it run the burner correctly, or will it still mess it up since the burner may not (is this so with the 42X?) come on when the zone isn't calling for heat?

Or do I need to adjust the cycles/hr on the 54X, and if so, what is a good number of cycles (I don't care all that much about comfort - our old 1 deg diff thermostat is fine for us).

In general, (how) would one design a system where the burner can run independently - maybe with some feedback like in the tn4 - so that it can run optimally? In my example I would expect the burner to run the full differential on cold days, since you know it's gonna call for heat often and the standby losses are really low in the boiler. Do I need to build my own controller?

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Old 01-12-09, 07:57 AM
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You need a boiler reset control. For tN4, something in the 4xx series.

The thermostats talk to the zone manager, but the zone manager needs the 4xx to control the boiler water temperature, cycling, differential, etc.

You could just set the thermostats for a set number of cycles per hour, but that is essentially defeating the purpose of the 5xx series of thermostats and you could have just used hardware store cheapies instead.
 
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Old 01-12-09, 09:39 AM
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Thanks - I figured I'd need a 4XX to get optimal performance, but I just want to make sure I don't end up with a system that still cycles too often after I spend the couple hundred on the boiler control.

How do you feel about buying warranty replacement units?
 
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Old 01-12-09, 11:36 AM
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Ultimately, the performance of a tN4 system will depend in part on the characteristics of the boiler and how it is piped, what kind of reset range you are using, etc. etc. What setup do you have?

Warranty replacement? Purchased from? A retailer or supply house who will warranty it, or feebay or craigslist?
 
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Old 01-12-09, 11:55 AM
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I have an oversized Biasi B10 and undersized output capacity. Remedying the second by adding one or two kickspace units on the open-concept first floor (which I may or may not run as 2nd stage - opinion?) and baseboard on the 2nd floor (none there at the moment) in some TDB zoning config.

Boiler temp would of course be limited by condensation temp, so I'll probably run in the 160 - 190 range. I currently manually adjust the boiler temp and diff based on the season, but that is of course limited.

What I'm hoping to achieve is that I can safely oversize the output without running into additional cycling, and with not losing too much comfort by the 4XX and 54X adjusting the water temp to match required output. That way I can setback and recover in the winter (which I now can't because of the lack out output - could take a whole day to recover 5 deg.) I _think_ this should also allow running at lower overall boiler temps, which _should_ save some fuel, correct?

Also, some of the other 42X features - like DHW post-purge - seem beneficial.

On a related question: I now run with a 20+ deg diff to maximize burner run time. Because of the oversize boiler and undersized output this is something like 20 mins off, 4 mins on during heat call. Does this sound excessively short for the burner run? I don't want to necessarily go auto-diff and find that it kills efficiency?


EDIT: warranty replacement units sold on eBay. I would be looking to snipe one for not too much. They're like $300+ retail. Not sure where else (local distributor maybe) to get them for a reasonable price?
 
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Old 01-12-09, 02:29 PM
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Well, this approach is kinda backwards from how it's usually done -- you're adapting the house to the boiler, radiation, and controls instead of getting the house load down, then figuring out the boiler, radiation and controls -- but so it is.

Just step back and take a holistic look at what you're doing, what the endpoint is you're trying to achieve, and then figure out the best way to get there.

A general goal could be to have all heated rooms provided with a similar proportion of radiation to their heat loss, so that the whole building heats evenly and can use the greatest reset range you can usefully implement. That could mean adding radiation to existing rooms, or just ensuring newly heated rooms have a similar proportionality to existing rooms.

If heating the 2nd floor is being done just to add load to the boiler, that's probably only going to increase the fuel use.

In the absence of specific piping for boiler condensation protection, with an oil boiler you can use a reset range down to about 120 or so if the firing cycles are long enough to dry out the boiler. Four minutes is somewhat short for an oiler by most standards, but I don't know a whole lot about oilers.

You can use an auto-diff or keep a set dT. Some systems work better with auto, some work better with a set dT.

A hugely oversized boiler will have shorter cycles. You can address this by adding a buffer tank. Several strategies exist for that. Kind of a different discussion, and could require a lot different piping and control than what you've got.

Buy a warranty replacement on feebay? Yeah right. Not for me, anyway.
 
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Old 01-12-09, 03:04 PM
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I trusted the contractor on the boiler choice - my bad. Now I have an oversized boiler and replacing it is a bit pricey. I could of course go back to the contractor and talk to him, but that seems like it wouldn't go very far.

The lack of output capacity was a problem since I bought the place. The former owner had a wood stove, but since it was on the same flue as the boiler I _had_ to take it off. I guess they never noticed the lack of baseboard length because the wood stove made up for it.

2nd floor is not just for load. It's so that I can a) get it warm at night if I want to and b) I can set back the ground floor without freezing upstairs. The convection is pretty good, but it gets pretty chilly if the downstairs gets to 61 (which it does on the coldest days of the year because of the baseboard issue.)

I just don't have too many choices (although I am also still improving the building envelope) to fix lack of heat. I just wish the contractor hadn't talked me into this size boiler
 
 

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