short cycling on combi boiler.

oreo123 · July 23

Have a combi that goes off and on when showering. This is on a low flow shower and rain head. This is due to not using enough hot water in summer showers. If the bath sink faucet is open on hot then there is enough flow to get constant hot shower water.

I am thinking the way around this is a small buffer storage tank. Not a 30 gallon one but somewhere between 2 and 5 gallons. I want to run it at say 140 degrees and then its output will go the existing mixing valve (output around 123 degrees).

Does anyone know of a small tank for this purpose? I would need a thermostat in the storage tank to bring on the boiler.

hot_rod · July 23

some of the “tankless”water heaters come with a small built in buffer

A 2.5 or 5 gallon point of use water heater might work

Does it have a recirc system on it?

Or a higher flow shower head😓

mattmia2 · July 23

Did this always happen or is it new? The flow sensor can get fouled and cause this sort of problem.

oreo123 · July 24

system is around 4 years old. This happened maybe 2 years ago. I removed flow limiter in shower rainhead and it worked till now. So much for the low flow things.

The residents will try running hot water in bathroom sink when showering tomorrow morning.

My plan is to remove the flow sensor turbine, inspect for debris, and clean with white vinegar. Shower head will be removed and checked for debris.

There is not a lot of real estate. I hate the idea of a 30 gal storage tank as that kinda defeats the combi purpose of fast hot water and no storage of hot water. This is one apartment in a three apt building. This is the only apt with the low flow stuff and the only one that is giving this problem.

I am no fan of short cycling a boiler. I will keep looking on line for a domestic approved small insulated tank. One with a thermostat well.

IMO the most reliable boilers are cast iron and have a stainless indirect storage tank. I have some indirects over 30 years old and still going.

GroundUp · July 24

Just switch out the low flow head? Unless these people are taking 75* showers….

hot_rod · July 24

A tankless is going to short cycle on low flow conditions, most lav faucets are down to .35 gpm now. Running a cleaner through it may help clean out the turbine and the HX. The short cycles and cold sandwich are the nature of the beast. Technology has gotten better to deal with those issues, but that low end is still a challenge with the larger btu units.

EdTheHeaterMan · July 24

I don't see how a tank of any kind will increase the flow thru the tankless water heater. If you have a low flow shower that is put thru a tank, the flow out of the shower will not increase, that won't stop the short cycling. If you add a circulator pump to maintain a temperature in the tank then you may stop the short cycling but then you are storing hot water outside of the tankless water heater. Why would you want to do that?

The proper fix is to clean up all the parts in the system that have to do with the flow rate. The flow meter, the orifice plates in the shower head, and any other restriction there may be. It worked before when these items were new. Make them like new again and see if that does not solve your problem. Cleaning something is often less expensive than replacing or adding stuff to a system that has operated properly for years. What changes in a year or two from something new? Corrosion and build up of debris on parts. That is why we do maintenance. To keep stuff looking like new and operating like new.

Just my two cents. (that today, cost more than a nickel to produce)

EBEBRATT-Ed · July 24

This is the problem with Combis and tankless water heaters. No free lunch you need storage to prevent short cycling

short cycling on combi boiler.

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