System temp for hydronic baseboard

metrosilo

Hi

Just converted a building from steam to hot water. Installed all new baseboard rads and there is one air make up with hydronic coil. 
Is there a rule of thumb for minimum system temp? Im of the understanding is 140⁰F
Is that true?

hot_rod

The baseboard manufacturers have charts to show output at various temperatures, run it just as warm as needed to do the job. Same for coils, fan coils, etc. If you know brands and models, that output info is on line.

Here are a couple graphs for common fin tube. You can either trial and error it, or do a room b y room load calculation to see what output you need.

If you have a typical cast or copper boiler you may not be able to run much below 160- 170 SWT to stay above condensation temperatures. Or add a return protection valve.

Jamie Hall

No, not true. If you are referring to the water feeding the radiation, ideally it will be such that the radiation, operating at that temperature, will balance the heating load. This results in the circulation running at all times, which is ideal.

Depending on the amount of radiation and the related system load, this may be considerably lower than 140 F. It may, however, be considerably higher. The air makeup unit with the hydronic coil will require a higher supply temperature to avoid cool draughts, which are uncomfortable.

The 140 F value to which you refer probably derives from considerations for condensing boilers (commonly called md/cons -- but modulating boilers do not have to condense, nor do condensing boilers have to modulate). That is the return water temperature to the boiler below which significant efficiency gains start to be realised from the ability to condense. Conversely, of course, if the boiler is not designed and built to condense, the return water temperature must be kept above 140 for at least most of a heating cycle, to avoid damage to the boiler.

There are a number of control strategies and piping configurations available to accomplish either goal.

metrosilo

Thanks for info guys. After start up, I just set it for 120⁰ until I returned. I found that the building was still cool and it's been relatively warmer outside. I've set my reset to range from 140 to 180. I've set up my air make up today and the minimum temp if 140 is enough to satisfy the area it's serving. 
I was more looking to see if there was a 'out in the field' rule to set temps for baseboard rads. 

Jamie Hall

"I was more looking to see if there was a 'out in the field' rule to set temps for baseboard rads. "

Not really -- but if you have a room by room heat loss calculation (you do, don't you?) and you know the length of the baseboard in each room, you can look up the BTUh output of the baseboard at various temperatures and that will give you the temperature you need.

pecmsg

metrosilo said:

Hi

Just converted a building from steam to hot water. Installed all new baseboard rads and there is one air make up with hydronic coil. 
Is there a rule of thumb for minimum system temp? Im of the understanding is 140⁰F
Is that true?

That question should have been asked and answered much earlier in the design process.

What's the heat load / loss of the structure?

@180° X BTU's per ft.

Jamie Hall

pecmsg said:

metrosilo said:

Hi

Just converted a building from steam to hot water. Installed all new baseboard rads and there is one air make up with hydronic coil. 
Is there a rule of thumb for minimum system temp? Im of the understanding is 140⁰F
Is that true?

That question should have been asked and answered much earlier in the design process.

What's the heat load / loss of the structure?

@180° X BTU's per ft.

Well, yes @pecmsg . I might add why the conversion was made at all, as another question -- but it was done, and evidently without that consideration (or the question on temperature wouldn't be asked) and so the problem now is how to help the OP out of his hole.

Coolmen

180°

iced98lx

if you don't want to do the load calculation after the fact you're on the right track, guess, check, adjust.

Our current system started at 180 when we moved in, and throughout the winter we realized how oversized things were and started turning it down. We run 135 minimum to avoid condensation and the burner will not heat above 150. Burn times are very low due to boiler oversize and circ times are as long as I can get them at this point which was the goal. I'm not running an Outdoor Reset, that would be better but truthfully we have too much baseboard, and entirely too much boiler so even at as low as I can run we're able to stay as warm as we want.

If it's residential I'd personally start up around 180 as @Coolmen suggested (a lot of the output ratings on baseboards include outputs at 180) and then work downwards. At 180 you're going to have short cycles, if you have it zoned up well you can start to lower the temperature and let it circulate longer and longer until you reach a happy medium where you're not falling out of the desired heat range but also running as cool as possible. With an ODR you're basically looking for an offset (which traditionally would be based on the heat loss calculation many have suggested) so the temp scales properly at any outdoor temperature.

Hope that helps, good luck!

EBEBRATT-Ed

Most baseboard systems are designed at a 20 deg TD between supply water and return water. So for 190 supply and 170 return.

At those temps you use 180 as the temp to select the baseboard at. That is the average water temp the baseboard sees.

ced48

My system almost never runs at more than 120 degrees. It took a little getting used to, not thinking I was wasting energy by running for multiple hours per cycle, but it really is the way to go.