Temporary gas water heater for ufh

[Justin Adie]

First post. Limited experience of plumbjng design but adequate soldering skills and am not too shoddy at tightening threaded connections...

What do you think about using a cheap instant water heater (300 quid variety) to heat a buffer tank for underfloor heating. Gas bottles as no town gas. This is in France by the way. Propane and butane are easily sourced. I'm thinking the interior rated 10-13kg bottles only.

Need a stop gap for a couple of months! Until a new heat pump arrives.

I think instant water heaters work on pressostats or reed switches to sense flow or pressure drop from a tap. If a reed switch then that will work fine with a pump. If a pressostat then in the end it is just a switch and I can use a microcontroller to emulate the switch. So an external relay should be able to turn it on/off at the same time as the circulation pump. Put a temp sensor on the flow and return and control on off from the house thermostat plus the delta between flow and return.

Most cheap ones are 11l/min and about 14kW output. So that should be fine for the heat needs. The heat pump it is gap-stopping is an 11kW output variety. The instant heaters probably are not rated for frequent uses but the buffer tank should help short cycling and if it breaks in.a few months I'm not that bothered.

Need to find one that doesn't need venting ideally. The installation locale is in the basement and the easiest wall to get through goes under my terrace which would be a bad location to vent to.

I have an existing vent port for a tumble drier. This opens on to what is called a "vide sanitaire" which is a deep space (2m) around the foundations of the house. It is breeze block and open to the air (under the terrace which is slatted hardwood) on one side. Probably not much movement though.

Not sure whether that is allowed but providing it is not unsafe for the fabric of the building or health the rules can be broken for a few months.

Is this all a profoundly stupid idea or does it have at least temporary legs?

 

[Scott_D]

Sounds like a death trap to me.

 

[Justin Adie]

Sounds like a death trap to me.

Ok. Can you expand? Where is the danger?
Is it the venting? If so I could you use a hot water heater that does not require venting.

Either way a reasoned answer is more useful than an offhand comment!

 

[Scott_D]

The wrong type of appliance for the job.
Flue terminating dangerously.
Over riding controls.

 

[Justin Adie]

Thank you.
I understand it is the "wrong type of appliance" but broadly speaking it just heats water like a 'normal' gas heater. The difference is circulation pumps and how it turns on and off.

on controls: I trust controls that I have designed and built more than i'd trust any consumer grade product.

Flue termination: that's the question. Is it 'bad' to terminate a flue in a 'vide sanitaire'? If so why?

Alternatively there are variants of these boilers that require no flues but are slightly less powerful. So if that's the only beef then it's soluble.

 

[mattfrance]

Put a "resistance thermique" in the buffer tank.

 

[Justin Adie]

Put a "resistance thermique" in the buffer tank.

thanks MattFrance - not possible in this case as the buffer tank has no port for a resistance. In the past I have put an external resistance attached to a circulation pump looped into the buffer tank. It was a 7.5kW resistance and the electricity usage was about €20 a day. An expensive solution over a 2-3 months...

 

[mattfrance]

thanks MattFrance - not possible in this case as the buffer tank has no port for a resistance. In the past I have put an external resistance attached to a circulation pump looped into the buffer tank. It was a 7.5kW resistance and the electricity usage was about €20 a day. An expensive solution over a 2-3 months...

Bottles of butane will not be much cheaper than electric and I reckon you'll get through about 11kg a day, also you can only have one bottle in the house at a time and it can't be in the basement or a cupboard with a door on it.

If you do want to heat with gas I would buy a second hand gas boiler on, "leboncoin", you'll pick one up for less than 300 euros, it will be much better than one of those instant water heaters which arent designed to be run constantly for long periods of time.

 

[Justin Adie]

@mattfrance you may be right on the maths. My backup inline water heater is very much on the 'dead or dying' side of its existence, but I will see whether I can resurrect it for a couple of months. I'm loathe to invest in a new one as another is likely to be included in the heat pump unit that I buy. The attraction of buying a gas heater was the ease of plumbing it in and that it would be a backup if the leccy failed (which it does, all too frequently).

thanks