BTU Effect of zone valve constriction?

Bob_19

the Cv rating of the specific valve.

The Cv rating will give the gpm flow of a specific fluid (1) @ 1psi dp (pressure drop).
The valve manufacture should be able to provide you with these formulas for thier valves.

I would start with the actual pressure drop across the valve if this is a worry.

Lori

hydronic baseboard - zone valve

Hi,

I have baseboard convectors running off a domestic hot water heater. I noticed that the 3/4" pipe coming from the heater is constricted to 1/2" to go through the zone valve, then restored to 3/4" for the rad line. That seemed to me to be similar to having a TRV, set low, right at the start of the rad run, without a bypass line.

I asked about this at the local plumbing supply store, and was told that that is how the manufacturer makes them. I find zone valves on line sized for 1/2", 3/4" and 1". The store personnel also said that the short constriction would have no appreciable effect on the rads output. Is this correct?

My real question is, could someone tell me approximately what percentage (if any) this is likely reducing the rated BTU output per foot of the baseboards?

Thanks for any info!

Regards,
Lori

Lori

hydronic baseboard - zone valve

Hi,

I have baseboard convectors running off a domestic hot water heater. I noticed that the 3/4" pipe coming from the heater is constricted to 1/2" to go through the zone valve, then restored to 3/4" for the rad line. That seemed to me to be similar to having a TRV, set low, right at the start of the rad run, without a bypass line.

I asked about this at the local plumbing supply store, and was told that that is how the manufacturer makes them. I find zone valves online sized for 1/2", 3/4" and 1". The store personnel also said that the short constriction would have no appreciable effect on the rads output. Is this correct?

My real question is, could someone tell me approximately what percentage (if any) this is likely reducing the rated BTU output per foot of the baseboards?

Thanks for any info!

Regards,
Lori

Mark Eatherton1

The one correct answer to any hydronic heating question is...

It depends!! (Ta DAAA). It depends on whose valve, what size of pump, what length of run, an dhow much baseboard. My gut instincts tells me that the use of a 1/2" valve on a 3/4" system probably does produce a negligible pressure drop in relation to the rest of the system. Most baseboard is rated at a flow rate of 1 GPM, and a 1/2" valve is capable of passing that much flow.

I have some other serious concerns though.. You mention a domestic hot water heater. For your and your familys health sake, I hoope the water heater is not also doing your actual DHW. If it is, there should be a heat exchanger to isolate the heating fluids from the potable water. Otherwise, you are being unneccesairly exposed to water borne bacterium including Legionella.

The other thing that concerns me is that convective baseboard doesn't put out a whole lot of heat per foot at 140 degrees F, which is typically the maximum temperature of availability from a residential DHW system. If your system is a "combo" system, and you add a heat exchanger to the system, it will lower the temperature of availability even more, making the output of the base board even lower.

If you can give me an idea of how many feet of pipe and baseboard you have, and the number and type of fittings, along with the make and model of pump, I can tell you almost exactly how much GPM you would be moving through that circuit. Doubling the flow rate increase the output of base board insignificantly. An increase in temperature will REALLY increase the output of baseboard.

I used to heat my home with a dedicated 40 gallon water heater, until I got my hands on a Munchkin T-50 prototype boiler. It can be done, but everything has to be sized wioth the low temperature of availability in mind.

ME

Lori

Dedicated to space heating

Thank you for your responses, gentlemen! I greatly appreciate it.

To answer your questions on specifics, Mark:
The system is now a dedicated 40 gal. space heater. The loop is 150', 79' of which is baseboard. (I know this exceeds the recommended 67', but the last 16' are in the room where I spend most of my time, and I've been comfortable without overheating elsewhere.) All pipe and fittings are 3/4" copper. There are 41 elbows, 5 couplings and 2 unions. (Add 10% for any I can't see.) The circulator is Grundfos, and the label says Type UP 15-42 F. Is this the model info you requested? It also says 1PM and 10uF. I have assumed a 1 gpm flow in my calculations to date. (This system used to heat only 2 rooms at one end of the house, with a wood stove heating the rest. However, I'm now a widow, and the wood is tough to handle, so over the last 3 years I've extended the rad system to its present length.) The zone valve is Honeywell.

I am in a position where I must reduce my consumption of fuel oil, significantly. I figured the most direct route is to have the burner off as much as possible. To that end, this spring I removed my DHW supply entirely from the system. I've not burned a drop of oil for 6 months, which is a good start. I expect eliminating the need for the unit to raise half its volume 98 degrees every day, from my DHW draw, during the heating season should help considerably, too.

The system has worked well with the 140 degree water. My resources have gone further by choosing up front to use the rad line to meet 80% of my needs, with a second system - the woodstove - meeting the rest. I've done other things to reduce my need for heat, and in fact, the rad line was sufficient for over 90% of the time last winter, which was longer and colder than normal. Now, however, I need to reduce the water temperature to further reduce my oil consumption, and have set the aquastat to 120F. (I've not had the system on yet this year.) I will burn more wood, so most of the BTU drop will be happening overnight and when I'm not at home. Still, I would like to do whatever I can to get the BTU output closer to what I've managed in the past, to reduce the labour from burning wood.

I've spent the last 3 years learning as much as I can about heating - much from this site and much from reading your articles online, Mark. I know that to compensate for lower temp water, I need more output devices. Hence my question about the zone valve constriction. To answer your question, Bob - No, it is not a problem. I wondered if eliminating the constriction would be the equivalent to a few feet more of rads. I have had difficulty comprehending the concept of pressure drop, so decided that until it became a need-to-know issue I would concentrate on delta T and plumbing issues. Since both of your responses refer to pressure drops, it appears that the time has come to figure it out.

If the 1/2" constriction is not that significant, then perhaps it might be a gain to split the line? I've not done that, since the circulator is on the return, limiting that line to 3/4". I'm doing all this myself, and I'm not touching any of the original set up. A plumber who installs hydronic systems has looked at what I've done to date, and pronounced it excellent, so I'm doing all right so far. If the zone valve constriction makes no appreciable difference , then if I had the start of the line restored to 3/4", split the line, and have it come together into the existing 3/4" line just before the circulator, would I be simply relocating an insignificant constriction, while gaining the benefit of two shorter lines? I am also insulating the pipe in the basement, and putting up radiant barrier between those pipes and the outside wall, as well as behind the convectors. I will probably reverse the flow, so that this room gets first dibs on the hot water instead of last, but how I proceed will depend on the zone valve and line-splitting issues.

Again, thank you so much!

Lori

hot_rod

Just be sure

you have adequate combustion air for that woodburner! More than once my belt clip CO detector has gone off in homes heated with wood stoves or inserts.

Slow, smoldering wood stove burns, indeed produce CO. A top quality CO detector in the wood burners room may be a wise investment!

hot rod

Mark Eatherton1

I'm extremely humbled...

Lori, You've obviously been doing LOTS of homework. The calculations I ran actually show the Grundfos as moving just over 5 GPM. That is SCREAMING. 3/4" pipe should be restricted to flows of around 1-1/2 to 3-1/2 GPM. If your pump runs continuously, I'd recomend choking the flow back to avoid burning out the copper pipe with hydrolysis.

Again, I think a lot of your problem pertains to the temperature of availability more than it does flow, as is obviated by my calculations.

You have a few opetions. Yes, reversing the flow would give the primary room first dibs on the available BTU's, but then the subsequent rooms will suffer.

As a possible alternative to base board, you could install some small fan coil units in the rooms. Fan coil units scrub the btus out of low temp water much faster than natural convective tendencies do.

Also, splitting the loop into two parallel loops may help some.

Another alternative, although more expensive, would be to consider intalling a high eficiency boiler, er I mean converter:-) like the Munchkin. This unit has the capability to generate the required higher tmeeperatures that make baseboard work. It would also be the most fuel efficient unti you could install. The water heater, although least expensive up front cost is the most expensive to operate because fo the large hole (flue gas passage) running right through the middle of the stored water.

The actual temperature drop of the system can also be calculated if you could give the dimension of the series string, and what rooms they're in. By the way, for those interested, I'm using John Siegenthalers software for these calculations. I would also need to know the approximate room loss factors, i.e. lots of glazing, medium construction, versus little glazing and good tight construction, the actual square footage of the room and the linear footage of baseboard in that room.

Hang in there gal. We'll get you warmed up some how. Sorry to hear of your loss.

Mark Eatherton

Lori

Getting there

Thanks for the alert, hotrod! This 150 year old farmhouse has lots of "natural ventilation" and I've been mindful of the need for lots of combustion air. However, I hadn't thought about a CO monitor. I agree with you that it is a sensible precaution, and will get one in place ASAP.

Mark ... Thank you for your kind words. I think I might just be about to flounder, though. First of all, I finally figured out where my thinking went wrong re the zone valve constriction. I assumed (!) that TRVs worked by sending HALF or less of the water through the smaller bypass pipe, and this controlled the temp. Evidently, a closed TRV is exactly that ... and since a whole series of them could be closed, and heat would still be available to the last rad, obviously that 2" or so constriction at the zone valve has no impact on the BTU output. I surely am glad that I asked here!

"Choking the flow back" ... Would running two 3/4" rad lines out of the heater, and only one 3/4" back in cut the flow in half? If so, this puts me in the safe range, yes? (I've spent the morning cutting the line into bits, in preparation for putting it together as a split loop, so I really do hope the answer is a yes.) Would there be any advantage, or disadvantage, to 3 loops? The layout of the house is such that 3 loops would mean a lot less bare pipe running around down there, I think ... I'll look at that now, to see.

"Hydrolosis" ... This system has been in operation for about 12 years. What evidence should I look for? There is green gunk at a couple of places - does that signify anything? I had assumed (WHEN will I learn??) that it was from poor soldering.

I will do some research on the fan coils this evening. That sounds fairly doable. Once I realized that removing the constriction was not going to up my output, I decided to put in some radiant wall panels. I don't have enough baseboard space, nor the rads themselves, to compensate for the lower water temp. I would make the panels about 18" high with two 1/2" copper pipes, since the PEX needs 13F more degrees than copper to start conducting, sandwiched between aluminum plates (I have all these bits, so that determined the "design") and running parallel to and above the baseboard convectors. I'll use PEX for the connections. I've estimated that I might get 100-115 BTUs per linear foot from these. Does that sound in the ballpark? I will check out the fan coils, though, as an alternative or supplement.

I would dearly love to get a new boiler - and a contractor to handle this whole shebang - but I can't, so here I am.

I have my estimated BTU requirements per room, based on doing a paper heat loss calculation before I installed the extra rads last year, and modified by my subjective evaluation of how the house felt last winter with 140F water, and assuming the rads delivered 320 BTUs per foot (at 5gpm, I would guess 345 BTUs or so might have been more accurate?). I'm also operating on the understanding that 120F would drop my BTU output to about 200 per foot (using a fraction of the original 320, that is). Would this information, along with the baseboard footage, cover the data that you need?

I do know that the lower temps are not a "good thing" ... the 140F worked fine. But 120F is part of making my oil supply last so I'll do the things that will compensate. And throw another stick in the woodstove on the days when it doesn't cut it.

I am so grateful for the help! I was beginning to feel like I was going in circles, from trying to keep all the variables straight. You contractors have my utmost admiration, for doing this sort of thing all the time.

Lori

Mark Eatherton1

Responses...

Lori, I'm somewhat confused. It could just be semantics too. I didn't realize you had non electric thermostatic radiator (TRV'S) in the individual rooms. If you do, and the system is in series, when one shuts off none of the other convectors will get any heat. To that end, I'm only aware of one manufacturer of a TRV with a bypass for use with hot water baseboard, and that would be EnerJee Corp.

Hydrolysis is only an major issue of the pump runs ALL the time. If it is controlled by a thermostat, it is still a potential, but lacks the time necesary to do immediate damage.

Installing panel radiators parallel to the baseboard will only work with a balance valve on the base board circuit to force water through the smaller tubes.

If you give me a room by room loss calc, and the BTU/HR/Ft output of your baseboard at 200 deg F water temps I can tell you how large the convectors need to be at 140 degrees F. I would also need to know the rooms in the order that the water would flow through them.

Lower temps are a good thing provided the "system" as a whole was designed around it. I designed mine around a maximum water temperature of 140 degrees F.

Paralleling the lines into two or three would lower the velocity.

Where abouts are you located. We have friends ALL over the country that frequent this site.

ME

Lori

Hi, Mark

I'm gathering the info you've asked for, but I want to give you what I'm planning on doing, rather than the way it was, and I'm still waffling a bit on that.

I'm sorry ... I didn't express myself very clearly. No, I don't have any TRV's on the system. I had read about them, and misunderstood how they worked, then drew some wrong conclusions. That brought me to this site.

The radiant panels will be at the end of the baseboards, so the water will have nowhere else to go, although from what you say about pipe size, it may not move equally into each of the two half-inch pipes from the one 3/4" pipe.

I will split the loop. I'm now working on balancing it, and it will probably be 3 segments. As soon as I've worked that out, I'll send the room heat losses, lengths and order, etc.

I'm in the "other" country ... Canada. Prince Edward Island.

Again, thank you! I so appreciate you taking the time for this. I tracked down some articles by John Siegenthaler, so I uderstand what information you are asking for.

Lori