Combi hot water temperature.

[Roger Hayter]

I have a 39kW Worcester Bosch Greenstar II oil boiler. When the heating circuit is set to maximum temperature, about 70 degrees and is running, and the hot water is set to minimum (50 degrees) the hot water gradually gets hotter to scalding temperature, which I don't want. The handbook shows a bizarrely complex flow chart regulating hot water temperature (which in itself suggests they have had problems) but it appears not to include a thermostatic mixing valve.

The only solution (assuming it is not a fault, or at least not a fault any local technician is likely to find) seems to be to fit an external thermostatic mixing valve. Can anyone recommend one which will not offer a significant flow resistance to the full 39kW hot water output? I assume that with the mixing valve I can set the hot water output to maximum temperature. (And see if it then runs too cool when the heating water temp is modulated down with lower outside temp!)

[TimW]

I have the same problem with a different oil fired combi boiler, eg Hot water is not hot enough - it starts hot but as you run the bath it gets tepid. The bloke who services it (and fixing boilers is his primary occupation) says likely the heat exchanger is full of sediment, so regardless of thermostatic mixing valves and burner efficiency the boiler simply cannot keep the water running out of the tap properly hot after a few mins.

TW

[Roger Hayter]

That is a problem I anticipate, if I set the HW higher. But at the moment my HW is too hot!

[John Rumm]

39kW in terms of hot water delivery is not actually that much - at a 35 degree temp rise, that is only 39,000 x 60 / 35 / 4200 = < 16 lpm

So pretty much any TMV will do that.

I used a Reliance TMV2/3 22mm "2 in 1" valve on the output of the unvented cylinder. At the ~3 bar the PRV on the cylinder limits the pressure to it is rated for 40 lpm.

Yup you set the max temp, it the progressively mixes more cold into the DHW as the temperature of the hot inlet rises. At inlet temps below the TMV setting, it should not mix in any cold.

[John Rumm]

TBH that is a problem with many combis - many don't have any form of flow regulation on the output, so if you draw water faster than the boiler can heat it to your desired temp, then the temp will fall. The solution being to turn down the hot tap a bit so the boiler can heat it for longer.

That might also be true - but a quick calculation will give an idea of what rate of delivery you can expect.

[Roger Hayter]

Thanks, I shall copy your setup (except 15mm), it is of great value to know a particular item works, and capacity x2 is reasonable headroom.

[John J]

It's a few years since I retired but I do recall servicing a couple of Firebird oil fired combis which had thermostatic mixing valves. There wasn't anything exceptional I recall about the valves save they had 15mm pipe connections all round and were similar to ones made by RWC (Reliance Water Controls). Once the customer had set a satisfactory temperature to suit their needs they were never altered again

[John J]

I had occasion to visit an oil fired combi which "ran out of hot water" as you describe. The boiler had a hot store tank alongside the boiler shell which was kept hot while on standby with its own thermostat. This store enabled instant feed to the tapwater plate heat exchanger to enable the burner to run through it's start up cycle, following which the burner provided heat to continue the supply of hot water to the taps. The opening of any hot tap in the house was detected by a water flow switch and this operated a relay to immediately call the burner to start rather than wait until the tank temperature dropped sufficient for its thermostat to call for heat. That particular house had a huge (really huge) bath together with high cold mains pressure and a high hot water flow rate into the bath. After measuring the conditions for some time and discovering everything was working as intended I determined the burner/boiler section were undersized to maintain the hot water flow temperatures for the length of time the bath required. With a conventional sized bath the boiler would have been perfectly adequate as the tap would have been closed before the heat store was depleted and the burner quickly raised the store temperature again. That particular combi model did occasionally suffer problems with the flow detector switches and burner instant start circuit but not in the case I encountered.

[Roger Hayter]

Interestingly, my previous oil boiler was a Firebird, and it served the final

10 years of its 30 year life with only the odd oil pump and capacitor on the spark primary circuit change. As you say, the thermostatic mixer valve was faultless.

Worcester Bosch seem to have gone to extraordinary (and ultimately failed) lengths to regulate water temperature *without* paying for and accommodating a thermostatic mixer valve.

[fred]

Roger Hayter [e-mail] wrote in news: [e-mail]:

Out of interest I did download and look at the installation and servicing manual for that range although I could not find one for a 39kw model, 32kw was the reported largest in the range.

Looking at the DHW flow diagram it appeared that there was no modulation control on the burner but instead a crude on-off control based on the temperature of water in an internal slave tank.

If your DHW is going overtemperature when the CH flow temp is high then I'd be looking at a fault in the diverter valve area resulting in leakage between the DHW heat exchanger and CH sections but from the diagrams that is very crowded fixed pipe area to investigate.

Also, does your model have a digital lcd display with an info menu? On the model I looked at info menu display i11 was a real time display of the DHW flow temperature. I'd check what temp the boiler thinks your temp is at when it is rising to scalding point. If it still thinks it is at a sensible temp I'd be looking at the temp sensors in that area. There's one on the cold inlet, one on the hot flow outlet (very accessible just where the pipe exits the case) and one on the slave tank. My guess is that the inlet & outlet ones will be the same electrically so measuring their resistance when the boiler is cold and power isolated could spot if one is out of spec. I'd pull off the connectors before checking.

Sensors would be an easy thing to check but a faulty diverter would be a pain so maybe the workaround of an external temp balancing valve may be expedient.

[Roger Hayter]

You sound like the voice of my conscience! That is exactly what I should be doing - and the fact it didn't work properly from new doesn't preclude a fault, though perhaps it makes it less likely.

I have my doubts about displayed figures on modern boilers, and other appliances like my oven for that matter - they often seem to be calculations of the average of what something should be rather than direct measurements. But, FWIW, the display does not show the rise in temperature.

Getting proper access to the boiler is tedious, one day I'll devote a whole day to checking the above - you've inspired me.

[AnthonyL]

At first reading this had me a bit worried as I am looking at getting a worcester-bosch-greenstar-2000-combi-gas-boiler and DHW is going to be relevant both from:

1) For shower 2) For low temp rinsing

I assume the issues only apply to the oil fired boiler and that the gas boiler will modulate?

However I also wonder that if the temp setting is right for the shower it'll be too warm for the low temp rinsing from the tap unless I have a mixer tap? Currently separate hot & cold.

PS Sorry for jumping in - just seemed a relevant place to ask.

[fred]

[e-mail] (AnthonyL) wrote in news: [e-mail]:

Absolutely, it's trivial to implement and so it will be.

Well that is the issue, I have room here for a large hot water storage cylinder and that is what I have, giving me a constant hot water temperature irrespective of flow. Personally I would avoid a combi at all costs.

Not at all, this is a truly tiny thread drift compared with many.

Oh and it's a WB heat only boiler I have here on similar grounds to the old IBM adage, no-one ever got fired for buying Worcester Bosch.

[Roger Hayter]

I think it will always be the case that comfortable and safe hot water temperature for sinks will be lower than the most efficient and effective temperature for transferring heat to a shower (which will of course have its own thermostatic mixing controls). The shower will be much less effective than it could be if a low temperature is used with long 15mm pipes. I wanted to use a low temperature because I don't currently use a mixer shower. But if you do use a shower then the best technical solution is either a mixer tap with manual control of temperature (which is regarded by H & S as unsafe for children and vulnerable adults) or a thermostatic mixer valve for each sink or bath. The latter is compulsory in hospitals and care homes, I don't know about industry and offices generally. Not enormously expensive, but fiddly to fit to an existing kitchen sink in most cases.

[Roger Hayter]

I'm not sure how that gets round the problem of hot water being cooler than CH temperature. Except that modern systems are usually run with a much lower CH temperature than used to be used. I really don't want to replace all my radiators and a lot of piping, so I use a more traditional 75 degrees in cold weather.

In an old cottage with little headroom and no spare space I really don't see where I could fit one. I would probably agree with you in principle.

[The Natural Philosopher]

You can fit a mains pressure water tank anywhere. Doesn't have to be high up. Could be even outside.

[Michael Chare]

The Worcester Oil Combi I look after has a thermostat which lets me set the hot water temperture. If this thermostat were to fail the boiler might behave like yours.

There is also a high limit thermostat which possibly stops the water boiling!

[Fredxx]

A thermal store is an alternative where the pressure is not at mains pressure but does supply hot water at mains pressure. It doesn't need servicing nor does it need a pressure relief valve. However they are not small, nor are they cheap.

However, I would not want to fit one outside, for rather obvious reasons.

I would have thought a combi with a good modulating range of output would be ideal for you?