instantaneous water heater 1

[skbutler]

I have a customer with two Teledyne Laars hot water heaters. They supply domestic hot water to a mental health facility and are 1,100,000 btu/hr input each. There is no storage tank in this system and domestic hot water is circulated through the building. The two heaters are piped in series and the original operating instructions from the installing contractor is that the first heater is set to 140 Deg F and the second heater to 138 Deg F. These units are at least 15 years old so the set temperature would be lower today. These heaters have an odd (to me) model number that is not shown in Laars literature. SQ-1100-K-N-06-B.

Questions. 1) What is the possible advantage in piping the units in series if you are not going to split the delta T across them?
2) Don't instantaneous water heaters have to be able to take the full delta T across them?

Thanks

 

[imok2]

It sounds to me that piping the heaters in series is because if the volume of water being used is more then 1 heater can maintain at the required temperature then the second heater comes online to maintain a constant temperature

 

[carlosgw]

I agree with imok2. No tank? Sounds pretty scary to me.

 

[pennpoint]

I'm leaning toward supporting (imok2's)post. Tankless heaters are RATED at incomming water temperature vs peak flow. A small houshold can (still) exceed the capacity of these "RATED" systems. What happens is the seller forces the consumer to buy two units and parallel them to make the heat/flow rate. The cost over a simple tank type system are very high. Time to pay back can a long time. For Selling features they use the space savings benifits or hot water on demand feature for advantages.
Realizing that, maybe, I might be preaching to the choir here, these types of water heaters are mostly adaptable to long reach, point of use applications, they can be of benifit (IMHO). Realize when you open the tap theres cold water in the line which has to flow out. Unless you have a point of use heater at every hot water tap, you are going to have to endur a wait period.
There are otherways to save water and have warm/hot water at your tap without the expense of these costly things. Some of these are the constant loop heating system by using a pumping system that ciirculates hot water to the furthest tap in the system. Time based it is on/off during peak/low use periods. Search Danfoss for particulars.
On the whole I am not apposed to tankless water heaters. I truely believe there are very good applications for which they are very well suited, but not in every case as they seem to be advertised.

Best Regards
pennpoint

 

[joepilot]

Since yours is a large facility......
Using water heaters in this manner for health care facilities is not uncommon. The reasoning is that hot water is required 24hrs per day and the facility is very large. However, the demand is variable. Therefore, the piping acts as the storage tank (calculate the volume, you'll see).
The two heaters provide staged capacity. When one heater alone cannot maintain discharge temperature the second kicks in.
This saves the cost (footprint, maintenance, installation) of the tank. Which is, for all purposes, unnecessary.

 

[friartuck]

Hi skbutler

Just picked up on this item

In the UK, particularly in health facilities, it is a requirement for run and standby facility. i.e. one water heater should do the job, but just in case one fails, the other can cope...however the units have not been piped up properly (i.e. in parallel) to achieve this as it does not sound if one units breaks down, the other can't be mauintained without afecting the other.

We HAVE to have a secondary recirculation...(Legionella and Water Regs--to prevent water waste by running off cold water)

Unvented systems are common in Europe and the UK.....why do you need a storage tank if the main supply is reliable? (We still put them in sometimes, but there has to be a good reason)

I have seen a system like yours,...it was installed by an inexperienced installer who undersized the unit and so added another unit to increase the load...

So in answer to your query, It could be any number of reasons why the system was installed as it is.


Friar Tuck of Sherwood