How cool is too cool for condensate return temperatures? How do you choose?

ariccio

Once again, I was idly thinking about steam heat while I should've been doing something else. This time, I was thinking about options to extract more heat from a given baseboard convector style radiator with fan forced air… and I realized that I still have no clue how steam guys balance the factors I've learned about (like corrosion from heavily subcooling condensate absorbing CO2, vs the burner efficiency benefits of creating steam at subatmospheric levels) to choose an acceptable range. I have a lot of gaps and unknowns in my understanding. For all I know, it might be best to minimize subcooling beyond what's necessary to prevent flash steam (would that mean that the steam transfers heat throughout the building faster, by evacuating condensate from radiators faster?).

Overly verbose context first:

It's been on my mind while working through some theory of how it would be best to "dial in" a two pipe vacuum steam heat system after converting to orifice plates. Considering many of these convector style radiators in this building have been enclosed or otherwise had their output choked off over the years, I suspect that some may not heat rooms enough with the correct sized orifice plate installed; I suspect they essentially need to be "overdriven" to force heat into rooms.

It's all theory right now, but the problem I see is that there's always going to be a strong drive by maintenance staff to respond to occupant insufficient heat complaints by either trying to increase building wide heat output (by jacking up pressure or tweaking time controls) or to do something even more counterproductive like swapping in an oversized orifice plate. The easy and reasonable answer seems to me like it would involve either increasing the size of the radiator or reducing obstructions, but I know there are going to be all kinds of issues with occupants not wanting to change anything about their silly cabinets around radiators. At this point, the only option is to fan force air somehow. There seem to be options on the market that are close enough to this need that I'm not really asking about them here, but feel free to share any knowledge and experience if you have any!

This is where I come to my actual question. By fan forcing air, it would be almost easy to cool the condensate arbitrarily. I can picture a system that would extract every bit of extractable heat from the incoming steam, leaving the condensate at room temperature! So how would I know how much subcooling is too much? Is there a desirable target that is more than just a rule of thumb? Am I even thinking about this correctly?

I've read so many old timers share advice and rules of thumb about both condensate that's too hot (the usual problem, causing flash steam and loss of vacuum motive capability for other radiators) and condensate that's too cold (formation of carbonic acid as more CO2 dissolves in condensate in the return line, possible shock to the boiler as the far cooler liquid condensate returns, etc…) with all kinds of different different numbers and ranges stated for both. It reminds me of Dan Holohan's stories about being told to set pressuretrols to "whatever it needs", or "to the desired pressure".

So… how do you find an optimum target or otherwise choose and control this? The least bad outcome of getting it wrong here is just wasted efficiency. But the much worse outcome is pipe corrosion and boiler damage. Very unwanted.

Jamie Hall

Couple of exceedingly important points. First, the orifice plates and pressure and vacuum. The operation of the orifice plate — or any other throttling valve, such as are used in vapour systems — is critically dependent on… pressure differential, not pressure. We usually blissfully assume that it is pressure — but we also forget that we are really talking about gauge pressure, and that the outlet side of the plate or what have you is at, or very close to, atmospheric.

Now if you go and drop the pressure on the outlet side below atmospheric ("a vacuum") that has exactly the same effect as increasing the pressure on the inlet side by the same amount. so far as pounds of steam per hour. It does have a noticeable effect on heat output of the radiator in question, as the temperature of condensing steam under a vacuum is lower under a vacuum, and hence the heat output is less.

Bottom line: you will get less heat from a system under vacuum as you would from exactly the same system with the inlet pressure increased by the same amount.

It's all about pressure differential, not pressure. Our fearless leader has said that, but folks don't pay attention.

Now so far as extracting heat from the condensate. Keep in mind that the amount of condensate is astonishingly small. For each 10,000 BTUh of heating output, you will get a whopping 10 pounds of condensate per hour. About 6 quarts. Per hour. Or 0.1 quarts per minute. There is about 1,000 BTUh from that condensate cooling it to 100 F or so, but that heat is showing up in the heated space anyway, radiating from the returns. In terms of cold shocking the boiler — that is nowhere near enough water to cause a problem.

LRCCBJ

So how would I know how much subcooling is too much? Is there a desirable target that is more than just a rule of thumb? Am I even thinking about this correctly?

The answer to your question…………..is that you can cool it down to room temperature and not suffer any ill effects.

In reality, with the tiny fans that you will utilize, you'll be fortunate to drop the temperature from 205F down to 180F as it passes the fan. You don't exactly have a heat exchanger in there and the fluid passes that fan rather quickly!!

You get far more bang for the buck to utilize a tiny fan on the radiator itself………….now you have an effective heat exchanger with proper convective airflow. You can increase the energy delivered to the room significantly.

ariccio

@LRCCBJ Oh, I did mean fans on the radiator itself, not somewhere else. In this building there are mostly baseboard convectors - it would actually be fairly easy to bolt on some fans!

@Jamie Hall I've been letting this marinate in my head for a bit now. I know Mr. Holohan has spoken about it in detail many times, and I have a bit of an intuition from thinking about it as if it were refrigerant, but I can't *quite* picture the whole system well enough to answer all my own questions in my head.

I suspect I have been thinking too much in the line of keeping a good vacuum on the condensate side to speed condensate removal… but it's funny to realize now that we really are talking about less condensate than I pictured. 6 quarts is tiny!

I presume this is all something I could calculate? I am guessing I could get really close to answering all my questions if I could very roughly model it somehow by factoring in EDR of the radiator in addition to the orifice plate diameter and pressure differential? Most importantly, is it the kind of thing someone can do easily like calculating the output and cost of a water heater, or do I have to work out the entire system at once, more like trying to calculate the loads on a wood framed truss (way beyond me)?

The long term goal in my mind is still to figure out a way to (e.g.) dial in a system so that it's roughly capable of keeping an elderly lady on the top floor feeling warm enough without running the boiler longer/harder or accidentally leaking steam into the returns. If there's a way to get it kinda close without active controls (TRVs), I suspect the active controls will work a lot better when put in later. All of this is a somewhat neat learning exercise towards that goal 😅

Jamie Hall

Calculating the condensate volume is really simple. What you need is the EDR of the radiator to start. Then the radiator output is 240 BTUh per square foot EDR — so multiply to get the BTUh of the radiator. Then that heat (and it is a refrigerant problem — it's just that the refrigerant in this case is water!) is provided by condensing the steam — at the rate of about 1,000 BTU per pound of water or steam. Close enough anyway (it's actually 970). So a bit of math and you have pounds of water per hour, then get that down to gallons per hour then down to gallons per minute.

And it is astonishingly small. Actually that's one of the beauties of steam — you don't have to handle much water.

On the leaking of steam into the returns. There are really only two ways to do that — one is to limit the steam entering the radiator to what it can condense, and the other is with a trap. In the old days, with vapour systems with very tightly controlled pressure, the former was common — either with orifices or with special steam valves (which are still made) which have what amounts to a variable orifice which can be adjusted to limit the steam, but still allow the radiator to be turned down or off. Expensive. More important, either of those requires that the pressure be maintained in a rather limited band, as both are sensitive to pressure differential. A much more reliable way is to use a trap on the outlet…

Now since you are running two pipe steam, balancing demand is really simple, since on two pipe you can control the heat from the radiator simply by partly closing the inlet valve (unlike one pipe, where you can't do that). So, for your elderly lady and the rest of the house, get it all going so she's nice and comfortable — might even put the thermostat in her area if you can keep her from fiddling with it — and then throttle the rest of the radiators to get the results you want.

Later on if the budget permits (or you find that you want them) you can put TRVs on the radiators — just remember that they can reduce the heat, but not increase it.

ariccio

This is very good information and you did a very good job of explaining explaining it!

There's an idea for active controls that's crystalizing in my head now that I understand it all, and it's kinda the opposite of a radiator cozy… I'm thinking of almost a bolt on fan coil unit.

For buildings like mine where the radiators are baseboard convectors, it would be really easy to attach 120mm PC fans on the topside outlet grilles, and install a thermostatic switch on the inside of the connector, probably one that closes contacts at like 100°F or 120°F. This means that whenever heat is available, the fans would start extracting it into the room, but also would reduce the risk of steam leaving the radiator, and aggressively subcool the condensate. I'm picturing this as working exceptionally well with a TRV and an orifice plate: the orifice plate could be downsized to slow down the steam entering the radiator when there's a sudden pickup (person turns valve way up or steam starts), and the fans would keep the TRV on the lower side of it's range.

The aggressive subcooling might also help the burner in the boiler run in the higher efficiency regime, but I'm confident it would also help reduce unwanted heat from the condensate return pipes passing through other apartments.

As I see it, this would be even better than just the TRV + pressure control you've mentioned before (https://forum.heatinghelp.com/discussion/comment/1800033/#Comment_1800033) because the pseudo fan coil unit would let the cold old ladies get even more heat without running the boiler harder.