Heat only Boiler - The sytem upgrade - boiler sizing and temporary hot water etc etc

[www.GymRats.uk]

Hi again all, If I can pick the collective wisdoms again..

To recap; our premium-grade (expensive) german combi completely failed beyond repair back in May after 17 years so I'm replacing with multi-fuel thermal store (250l) and a system boiler (+ woodburner boiler and possibly solar thermal in time).

As a temporary fix while waiting the "4 to 6 weeks" for the thermal store to come is it possible to have mains pressure cold water (from water softner)feeding directly into a "heat only" boiler with the outlet connected back into the DHW supply?

I appreciate water flow won't be great but my simplistic understanding is that a heat-only boiler sized to meet the demands of our central heating would be more than capable of supplying a single shower at mains pressure but presumably it would/might require a flow switch to fire up the boiler? Our mains water pressure is around 3 bar so not excessive but all heat-only boilers state "open vented" perhaps as that's the vast majority of situations they'd be fitted to rather than operating pressure?

System Boiler Sizing: I've just checked the original UFH documents and it had a 10KW boiler input requirement. Since then I've hooked in the downstaris (bedroom) radiators (x 6) we've also added an extension creating a larger hall/entrance, 1 small (3m x 3m) room and garage conversion fed by its own manifold and 4 circuits (1 for hall, 1 for small room, 2 for well insulated garage/gym)

Typically our old 32KW boiler would never run "flat out" even on the very coldest days the boiler fan would never run more than 3500 rpm with its maximum speed of 6000 rpm and the circulation pump would typically modulate down to between 20 and 60% for most of the season.

So... heat only boiler size, according to Valliant web-site they say: More than 4 bedrooms + Poor insulation = 16kW boiler More than 4 bedrooms + some insulation = 14kW boiler

Being over cautious, if I've now doubled the overall maximum heat output of C/H but significanty increased insulation including triple glazing in the upstairs lounge, dining room, kitchen would it be better to over-size the boiler if all sizes modulate down to the same minimum output? E.g. Viessmann Vitodens-100W

11kW 16kW 19kW 25kW System Boilers all modulate down to 2.9kW output at 80/60 F/R. Also just noticed they have an internal "pump flow switch" which I'm guessing fires up boiler on flow detection.

One final thing :) Presumably "weather compensation" has no place in a system with a thermal store as lowering the store temp would also reduce DHW supply or could there be a benefit of running weather compensation to lower cylinder temp by say 10 or 20 degrees through warmer periods?

Weather comp. seemed to work perfectly on my old combi suppling UFH and bedroom radiators but obviously it had no interation with DHW temps.

Thanks for all input (from those I can see!)

Cheers Pete

[Alan]

You want to run your domestic hot water through the heating heat exchanger, so you are running it like a combi boiler?

If so, then no, you cant do that. Firstly the water in the HE is not potable, and should not be mixed with mains water (unless it is well protected by check valves). The flow rate will be awful, the water is designed to be run through the central heating pump and radiators, so the flow rate will be low. Combis typically need a 24kW output to be able to keep up with a shower at mains pressure. Anything below that will struggle.Combis have an extra, smaller HE to concentrate the DHW heat into a small area to get a better output. The CH HE is larger,and designed to heat the CH slower. In hard water areas, doing that will scale up the HE soon, as they are designed to run with inhibitor in them, and could void the warranty if the inhibitor, and a magnetic filter is not used in the circuit. And CH HE's are just not designed to run the way you suggest, so it is a really bad idea. The easiest, cheapest option is to temporarily fit an electric shower.

[Roger Mills]

NO - DON'T DO IT!!

Your boiler is designed to be part of a sealed primary system, with inhibitor to prevent corrosion. It is not designed to handle a constant flow of new oxygenated water without inhibitor - you'd get scale *and* corrosion. [Even if part of a vented system, it's essentially the same water going round and round].

Also, a normal vented system will run at about 2 bar, with the over-pressure relief valve set about 3 bar. If your mains pressure is higher than that, the relief valve would open all the time.

Also, if the flow suddenly stops when you turn a tap off, chances are that the boiler will overheat and trip its cutout.

Other than that, it's a brilliant idea!

Regarding weather compensation when your thermal store is up and running, you will presumably have a mixing valve which controls the flow temperature to the radiators. It's *that* which needs to take account of the ambient temperature - NOT the boiler flow, which needs to keep the store good and hot. The store will, of course, be *very* well insulated?!

[Vir Campestris]

I agree it's a really bad idea & will kill the boiler - but he doesn't need the 24kW. An electric shower is likely only 7kW, and they are just about acceptable.

Andy

[rick]

My Thermal store only has a mixing valve on the DHW output (mains pressure) the underfloor heating cct is pumped around the heat exchanger, the Thermal store temp is set to limit max temp of water (65C) I have thought about adding a mixing valve, in theory I could then have Thermal store at higher temp.

Never bothered with weather compensation, unless you are going to have multiple external sensors and average the results, taking spot reading form sunny or shady side seems of little value.