Hot water heater as a boiler?

paappraiser · Sep 17, 2007

Hi

Im new here and not a plumber by any imagination I am a real estate appraiser. I was doing a walk through on a multifamily house that was owned by a plumber. What he did was took your standard 30gallon hot water heaters and turned them into boilers for the radiators in the units (pumps on them and all). I asked him about it and he said that it was a lot cheaper than buying 6 new boilers. Basically he changed some valve, added the pump and some how got a thermostat on them.

Since then Ive seen this in a few other random buildings.

Questions
Is this safe, stupid or ? and very importantly 'How is this done?'

Thanks

Matt

Furd · Sep 17, 2007

How it's done is quite easy. You remove the drain valve and use that tapping for the return water. The outlet tapping is used for the supply and the inlet tapping is plugged.

Is it safe? Not in my opinion and not in the opinion of many building officials. Many jurisdictions prohibit using domestic water heaters for space heating systems. Most manufacturers void any warranty for domestic water heaters used for space heating purposes.

Sponsor · 2022-01-29T12:20:58-0800

Leejosepho · Sep 17, 2007

Furd said:
Is it safe? Not in my opinion and not in the opinion of many building officials.

Why not? What are the issues?

I ask because I have been planning to add a recirculation line and pump to feed a small heater in the kickboard area of the kitchen cabinets. The heater and pump would be thermostatically controlled, and the heater's blower has a low-limit control so it will not run unless the water is hot.

Jimbo · Sep 17, 2007

The issues are that domestic water heaters are not designed for the kind of duty cycle which they would likely see if used for heating. They will yield whatever the spec'd gallons per hour is, but if tasked to deliver that basically 24/7, there is no way that they will last. The burner components and the tank itself will "cook".

We do not see hot water heat out here, so I am quite uneducated in this area. But a question I would like to hear answered by some pros from back in "cold country" is this: when you put a WH in a closed system like a hot water heating loop, does that tank now require ASME cert, even though it is being operated at only 40,000 BTU, and fairly low pressure???

GrumpyPlumber · Sep 17, 2007

30 gal water heater is about 30K btu's hour.
Low end boiler for small residential application 80-100K.
The BIG difference: recovery rate.
32,000 BTU's will heat 30 gallons of water from 40 to 130 degree's in about 45minutes.
That might possibly work in southern Texas, but not in the north.

Jadnashua · Sep 18, 2007

Most radiators are spec'ed to produce their stated output with 180-degree water; radiant floor uses a lower temp (around 120 or so depending on load and design). Depending on what you have, you'd never get the comfort level required if you were using it for radiators. The other big thing is the duty factor...a WH is not designed to run continously, which could be required in the cold of winter.

MACPLUMB · Sep 18, 2007

I disagree to one point, if the water is used as closed system the water heater will last longer,
this is according to the mfg's,
MANY WATER HEATER MFG'S NOW MAKE UNITS THAT HAVE SIDE OUTLETS FOR THIS PURPOSE THIS ARE IN THE 50 GALLON ON UP RANGE!!
these are combo units for potable water and space heating

MACPLUMB. MASTERPLUMBER AND WATER HEATER SPECILIST

Leejosepho · Sep 18, 2007

MACPLUMB said:
... if the water is used as closed system the water heater will last longer ...

Does that mean a recirculation line can extend/enhance the life of a typical water heater?

The thing that comes to mind here is that a water heater in a house system with a recirculation line connected at its drain port might not always cycle as deeply as otherwise.

Jadnashua · Sep 18, 2007

In a potable water system, you are constantly inserting new water, with it's disolved contaminants, minerals, oxygen, etc. In a closed system, you've only got a finite amount, so things reach equilibrium quicker. You end up with water that isn't as reactive, thus it should last longer.

Master Plumber Mark · Sep 19, 2007

MACPLUMB said:
I disagree to one point, if the water is used as closed system the water heater will last longer,
this is according to the mfg's,
MANY WATER HEATER MFG'S NOW MAKE UNITS THAT HAVE SIDE OUTLETS FOR THIS PURPOSE THIS ARE IN THE 50 GALLON ON UP RANGE!!
these are combo units for potable water and space heating

I have an outlet in my own home on my 75 gallon gas

but it is meant for going through a heat exchanger in my
forced air system on the furnace...

that 30 gallon heater will burn out and leak or will not keep up with the demand in a bitter frigid temp

paappraiser · Sep 20, 2007

Hi thanks for the reply

The heaters were used in a 2500 Sf building. So there were 3 "boilers" total 1 for each unit. Does it get cold here yea im in Philadelphia.

Were they 30 gallon units, I don't know. I'm sure they no more than 40 mabey 50 gallon.

So what would you do to set for the thermometer? I'm curious really on the mechanics on the whole thing.

He did say that they would last quite a long time since they were in a closed loop. (My gas hot water boiler for my house is pushing near 50 years old)

GrumpyPlumber · Sep 20, 2007

Water heaters just aren't designed to return enough BTU's that a heating system requires, they're also awfully inefficient compared to a boiler.
A relatively cheap boiler runs at 80% efficient, a water heater runs at 60%, you'll see it on the gas bill.

Think of it this way, it's February, 24 degree's outside, the zone kicks on with a full 50 gallon tank maxed @ 160 degree's (boilers run at 180).
As the zone runs it draws the heat from the tank, the tank will lose it's heat very quickly, depending on size of loop.
Once that tank has gone to room temp it's just heating with the circ running overtime and you now have a zone thats trying to compete with 24 degree's but takes 45 minutes to recover @ 130, nevermind 160.
Your tennants might not notice the higher gas bill, but they will notice it's cold.

shashalou · Dec 8, 2007

I came across this discussion by chance and cant help commenting.

I live on the shore of Lake Ontario and have used a 30 gal 30000 btu water heater for all my heating and domestic needs in an 1100 sq ft home with minimal insulation, for well over 10 years.

The notions of inefficiency are false if the heater is located in heated space and the vent passes thru the building to the roof.

The notions of faster "wear out" or excessive condensation are false since the number of on off cycles is much less over the life of the heater and wear from expansion and contraction is thus minimized.

Scalding is no more of a problem than owning a dishwasher but can easily be solved with an old kitchen faucet as a mixing valve.

Recovery problems, although present in very fast outside temperature drops, are not what might be expected, since the pump runs continually. The thermostat is simply the delta t formula with a little help by manually adjusting the temperature at the heater.

Most recovery difficulty would be eliminated by correcting the very wide temperature differential (30 degrees) in the gas valve. If anyone knows of a valve with maybe a 10 degree differential that would fit a standard heater, I would sure appreciate knowing about it

Bill Arden · Dec 10, 2007

1. The normal element in a hot water heater will get to hot for the antifreeze solution and this will cause bubbles. This means that you have to replace the elements with a "folded" element.

2. These bubbles can sit at the top of the tank depending on how the tank is plumbed. (The top should be the exit)

3. The antifreeze tends to turn to goo if there is a leak. This can cause the T&P valve to stick.

4. Most people don't put a big enough expansion tank on the system and this causes the T&P valve to leak a tiny amount. (See above for the result)

They are not allowed here, but I've found that they make small electric wall mount heaters that are designed to be used as boilers.

So Physics says that they can be used safely, but idiots have used them incorrectly and the results were bad. (See myth busters TV episode)

I've even thought about starting with a piece of steel pipe and making my own heater since I only need about 5Kw to heat up the concrete floor.

shashalou · Dec 10, 2007

My system is not closed. Anti freeze is not necessary and air leaves via the domestic hot water taps.

Pete C · Dec 14, 2007

Air you also using this system for your potable hot water?

Yummy!

Not sure I'd wanna drink or shower in that stuff.

I have heard of using a single system for heating and potable hot water, but, I have also heard that some nasty critters brew in such setups.

Jadnashua · Dec 14, 2007

The efficiency of a heat exchanger is dependent on the surface area, the conductivity, the flow rates and the differences in temperature between the source and the destination. A potable water heater is usually not a good match for a heat exchanger since it is not normally run very hot. A typical boiler is often designed for 180-degree water. A WH is normally capped at 130, and that is potentially dangerous. Regardless of the heat source in the WH, you can't transfer all that heat in a typical heat exchanger unless it is huge to the secondary loop. Typical radiant infloor heat runs are in the 80-120 temperature range, depending on design. The burner in the WH is probably in the 800-1000 degree range, so it can transfer heat from the flame to the water fairly quickly. You have nowhere near that delta-T in your heat exchanger.

shashalou · Dec 14, 2007

nasty critters brew in such setups.

I've heard that too, when the water is not moving. My heater operates as a basic air conditioner when it's not heating by piping new street water thru it.

A WH is normally capped at 130, and that is potentially dangerous.

All I have owned say 160 at the top end. Why is 130 dangerous?

you can't transfer all that heat in a typical heat exchanger unless it is huge to the secondary loop.

I dont understand that.

Typical radiant infloor heat runs are in the 80-120 temperature range, depending on design.

I guess that would depend on the volume of water per square
foot but in general I agree altho some people around here run them at 180. I'd like to get 120-130 with 1 ft of 1/2" pex per sq ft.

You have nowhere near that delta-T in your heat exchanger.

That's true but I dont understand why that's a problem as long as the burner stays lit. My problem is in the long downtime of the burner--the 30 deg differential of the thermostat in the tank.

Jadnashua · Dec 14, 2007

For the young or old where their skin is thin, 110 can give a second degree burn...130 is down right nasty. Raising the temperature does store more heat, but the higher it gets, the more likely someone could get hurt. In a commercial operation where sanitizing dishes is important, they'll use the higher temperatures, and if you have an older dishwasher that does not heat the water if it isn't hot enough, then you need to run the temp higher, but you need to be careful as if anyone ran hot full on, they could get burned. There's a way around that, use a high temp and then put a tempering valve to adjust it to a safe temperature. You get the higher temp for your primary loop, but keep it safe for the potable water chain.

You're using a heat exchanger to transfer heat to the secondary heating loop. The hotter the water is in the heat exchanger, the more can be transferred to the secondary. Think passing your hand through a candle or a blowtorch. If you have a string of 100 candles and you want to move across them at the same speed, you'll get a lot hotter than going through one, but if you have a few blowtorches, you can achieve the same result in a shorter array. In either case, the source is running full tilt. Running lower temperatures in the primary loop means you need a bigger heat exchanger to transfer the same amount of heat in the same time.

shashalou · Dec 14, 2007

put a tempering valve to adjust it to a safe temperature.

I have an old kitchen faucet in reserve to use as a mixing valve. As yet, with anti scald showers and one handle faucets, I havent seen a need.

In either case, the source is running full tilt. Running lower temperatures in the primary loop means you need a bigger heat exchanger to transfer the same amount of heat in the same time.

Yes, but I dont understand why you have to transfer the same amount of heat in the same time--that's what a tank is for. If I have a problem, it's that the burner shuts off way before full tilt is reached. I suspect most of the problem is in the 30 deg differential in the gas valve thermostat.

Hot water heater as a boiler?

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