View Full Version : Water Heater as source for Baseboard Heat

01-31-2007, 10:53 AM
I'm hoping you guys can help me get a handle on this...

We bought our house about two years ago. Main heat souce is gas furnace hot air, but house has an addition (family room 16'x16' and bathroom 7'x6') that used a hot water heater with a circulator pump to provide heat to baseboards (22' of baseboard in family room and 5' of baseboard in the bathroom).

The first winter the water heater/baseboard system did a good job of heating the room, but I don't think it was very efficient (high gas bills). We wanted to upgrade the system as the water heater was old (probably 20 yrs or so) because I was afraid of tank failure (it had a lot of rust), but money was tight, so we just turned it off and used an electric oil-filled radiator to heat the space in the begining of this season.

When we could afford it last month, I had a heating contractor come out and give me a recommendation. He told me even the smallest boiler would be too big for my application and told me the best bet would be to just swap the water heater for a new one. ( The old heater was a 30 gallon 40k btu unit) He recommended a 40 gallon 40kbtu unit. I asked if there would be a difference in the max water temp between the old and new units and was told no.

Three weeks ago they put the new unit in and replaced my old thermostat for the room with a new programmable unit.

Here's the rub...

Even with the water heater set at high, the room won't get as warm as it did with the old heater. ( I even insulated the lines (36 ft total) that run through the unheated crawl space under the room with no increase in heat output. They were un-insulated with the old heater with no problems.

They came out today and checked the whole system and basically said it was fine and as good as I can expect.

My question is, did the old heater have a higher temp output?

I asked the guys if they new what temp the new heater would produce and they said they were unsure but it was probably around 130f and that was plenty.

When I asked if a unit that put out higher temp water woul help my situation, I was told no, that my baseboard would still only put out the same amount of heat no matter how hot the water flowing through the pipes was..something about the fins being heat-saturated. Which I find hard to believe.

After they left I did some reasearch...

Bradford White says the max temp for a residential heater is 160f and my old heater was probaly rated at 180f. They said their combination unit with the heat exchanger and their commercial units both do 180f (My heating contractor told me that both the combi and the commercial units would be the same max as my residential unit)

Did I get hosed?

Is the heater insufficient for my heating needs?

Any thoughts?

Sorry about the length of this post, but I really need to figure out what to do.


01-31-2007, 12:24 PM
The difference in temperature between the room and the water determines how much heat you can distribute along with the velocity of the water (i.e., the changeover). If you check the specs on the baseboard heaters, they will have values like x BTU/foot at 180-degree input. It will probably also be spec'ed at different input temps, and will also indicate the gallons/minute of flow. Measure the water temp - if you don't have a gauge, it wouldn't be a bad idea to put in two, one on the output and one on the return.

There could be a restriction somewhere (a valve not fully open maybe or a check valve stuck) that is preventing proper flow. Did they change the circulator?

If you search around on the net, you can locate a heat load calculator (spreadsheet, typically) that will tell you how much heat you really need to keep that area warm. If you have enough heat capacity in the heater, but can't get the rooms warm, then you may not be flowing it fast enough. There is a limit on how fast you can flow the water before you risk cavitation and wear on things. It is also possible that there is an air lock, and things just aren't circulating. Are there any air extraction devices in teh system? Did they purge the system properly? Is the pump the right size?

01-31-2007, 12:55 PM

Thanks for your reply.

I'm out of luck on the specs for the baseboard (installed in 1972 and long gone).

No temp guages, but they measured the temp of the output and return today (output 126f return 124f, although he wrote 126.5 and 121.5 on my paperwork).

As far as restrictions go; new valves and fingers crossed.

same ciculator pump (3 speed) now set on 3 to see if it makes a difference.

It's a closed loop, but I have had to add water 3 times since the install when the pressure has dropped to "0". Could this be air being purged from the air extraction valve? I heard a lot of gurgling in the baseboard prior to the last pressure drop.

They said they bled the system after the install.

Pump info: Grundfos type UPS 20-42, 40w outp, Class F,
3: 2620 rpm winp 95
2: 2300 rpm winp 70
1: 1800 rpm winp 50

Thanks again for your help.


01-31-2007, 03:11 PM
Since it is a closed system, when you have to add water, there must be a leak. If it leaks and you let the heater get too low on water, you will ruin it. For a boiler, there is usually an automatic water makeup valve with a backflow preventer. Nominally, you should have the water pressure at about 15 pounds or so and have both an air extraction device and an expansion tank.

SInce the return water was essentially the same temp as the supply, the water isn't circulating. My guess is that you would see maybe as much as a 50-degree difference between input and output (or more) on a cold day. I feed my in-floor heat around 130-degree water, and it returns at around 60-70-degrees on a cold day.

The pump should be taking water out of the hot side of the WH, and returning it to the cold side.

The system sounds like it is air locked and the pump can't push the water through the radiators.

A WH isn't really designed for this, but can work - you will wear it out quicker.

If you do the heat loss calculation you would know how much heat you need. Some of the boilers today are modulating - they can adjust their output down from their maximum level. The smaller ones generally available can go down to maybe 15K BTU. This type of boiler costs more to buy and install, though, than an older design.

01-31-2007, 05:20 PM

Thanks for the reply.

I've checked the whole system for water leaks and have found nothing.

My old water heater set up had a feeder valve that the contractor said I didn't need since it is a closed loop (he hooked up the new heater without the feeder in the loop).

I've been running 20 psi and have an air extraction device and an expansion tank.

I had the same thought about the water not circulating because of an air lock and asked the contractor about it...he said the pump would "push" the air through in that case and it would go out the valve. Is theis untrue?

Here's a couple of pics of my set up:




any additional thoughts?

thanks again,


01-31-2007, 05:28 PM
I had the same thought about the water not circulating because of an air lock and asked the contractor about it...he said the pump would "push" the air through in that case and it would go out the valve. Is this untrue?

I know nothing about your kind of heating system, but I know the heaters in the back of my old school bus have air bleeders for a reason, and that reason is that the water pump cannot push the air through the heater cores at the ends of long runs of 1" hose. I would say you have an air lock somewhere.

01-31-2007, 06:35 PM
Depends on the pump and the piping. You could have enough pump to circulate things if there was no air lock, but it may not be big enough to push the air out depending on the distance and height difference. Is there any way to force water through the circuit by playing with valves? I had to do that on one of my hydronic loops. Since I did that, that zone works fine.

John in herndon
05-27-2007, 10:09 AM
A couple of thoughts here:

1. To check circulation, let the baseboards cool off and switch on the pump. Hold you hand on the copper return line at the heater. Time how long it takes for the return line to go from cool to hot, this is the circulation time for the loop. If it doesn't heat up no circulation is going on. If it takes a very long time, like a minute or two, there is probably an obstruction in the line or the pump is not working properly. Get that squared away first.

2. The water temperature is much more important than the circulation velocity. (think old fashioned hot water radiator systems with not circulating pumps). You need to be much hotter than 125F. You heater should go hotter than that. Even if it only goes to 145F you will be a lot better off. 180F would be the ideal and would probably work very well. You probably do not have to have the pump set at high speed to get results if the water is sufficiently hot.

05-28-2007, 03:22 AM
I think the output and return temps tell the story. At 126 degrees and say 121 degrees you're only giving 5 degrees to the space. Heating contractors usually design for a 20 degree drop in a loop. The idea of running hot water through a baseboard element is for the water to lose its heat to the room, making the room warmer and the water colder.

Of course that doesn't answer why. One reason could be the hot water isn't lingering in the room long enough; that is the water velocity through the pipes is too high (pump oversized).

05-28-2007, 03:49 AM
One reason could be the hot water isn't lingering in the room long enough; that is the water velocity through the pipes is too high (pump oversized).

Yes, and that principle is confirmed when trying to cool an auto engine rather than a water heater. The flow is restricted/limited for two reasons:

1) So the source can warm up;
2) So the radiator has time to release the heat.

01-31-2008, 09:08 AM

Most residential hydronic baseboard radiation is rated for an average water temp. of 170 degress. Since you have an average water temp. of only 125 degrees it is never going to heat properly.

Your new water heater will not deliver the 180 degree water that you need to heat with, like the old water did.

You might need to replace the existing water heater with one that can deliver 180 degree water, like a Noritz tankless unit where you can over ride the 120 degree setting with the controller.

01-31-2008, 04:13 PM
High temperature is required since the baseboard is a convector.

You cant get the temperature from the tank because the pump destroys the stratification of the water, the water around the sensor remains warm and the burner wont fire.

Turn the pump off or set it at the lowest speed and restrict it. This will increase the temp since the burner will fire longer.

When that doesnt work you need to find a thermostat that will work under the conditions youve given it. When you find it, let me know, I need one.

02-01-2008, 05:20 AM
You would find a lot better efficiency with something like a Polaris. A friend of mine has used one for years.


02-01-2008, 05:23 AM
Shashalou. That explanation is about as lucid as the one the original contractor gave. Baseboard needs a high temperature and 130 degrees is not enough. All domestic heaters will go to at least 150, and most to 160 degrees. The maximum is controlled by the thermostat and 160 is about the maximum "safe" temperature in a domestic situation, even though it is really "unsafe". Some commercial heaters will go to 180, but few of those are as small as your needs seem to be.

02-01-2008, 08:46 AM
The theory (which combination of fins, water temperature and heatloss) is really not important. The room was being heated with a 40M burner previously, in addition to heating 36 ft (!) of crawl space. It's obvious that a 40M btu burner will do it now.

It's simple to check the output temp with the pump high and low. Just tape a meat thermometer to the outlet from the tank as read it as the burner shuts off. A few times with the pump on high then a few on low.

This is the maximum output of the heater not what the thermostat is set at because the thermostat is guessing, from its low position, what the temperature at the top of the tank is, presuming that the water is stratified. In the meantime, a pump has been added that "unstratifies" the water.

I must give some credit to Larry at the Tank for my education in this matter.

02-11-2012, 01:29 AM
It appears that the heated water is not hot enough to make a typical hydronic convection radiator deliver enough heat. The 120 temps indicate the temp is too low to be effective the similar temps on both sides of the radiator also indicate low heat transfer into the room your options at this point is to reset the hw heater to a higher temp and risk scalding danger or replace the hydronic bbaseboard with a electric baseboard my$.02