Number of items connected with heating my domestic hot water

[ericmark]

Praying on my mind are a number of items connected with heating my domestic hot water using my oil fired central heating boiler, thoughts welcome.

1) Plumber tells me I should not fit a motorised valve to the domestic hot water as there is no over run built into the boiler, so it could over heat if the thermo siphon is prevented, due to fitting a valve, the bathroom radiator and cistern hot coil can’t be turned off as it stands, except by turning lock shield valve on bathroom radiator, which only stops radiator working, hot coil is heated whenever boiler fires up, is he correct in what he says?

2) He says a by-pass valve is not required, the pump is on the return from radiators, and it has been found if one zone pump runs it causes the water to reverse flow in the other system, plus upstairs radiators heat up with thermo siphon even when no pump is running, so he is fitting motorised valves which will be electrical linked to pump so until valve is open pump will not run, unless the micro switch in the motorised valve sticks, is he correct that by-pass valve not required when every radiator is controlled by a TRV?

3) There is no thermostat on the cistern, so in the summer only control is time boiler runs for, at moment set to ½ hour a day, it would be hard to run wires between the cistern and the boiler heat link to control the temperature of the domestic hot water, and even if run, since no motorised valve it would only do anything in the summer, I note even at just ½ hour a day the boiler cycles off/on as the temperature hits 81°C when boiler turns off and 55°C when it turns back on again, so it would seem not needing full ½ hour, but that is the smallest time slot Nest will allow. Years ago I bought a wireless tank thermostat, but can’t seem to find any now, at least at a reasonable cost, I can buy a wireless room thermostat for £20 so to pay over £100 for a tank thermostat seems OTT. I am sure one can buy some wireless relay at under £30 but not found one that would not need an enclosure making for it which clearly lifts the price. However the first question is do I need it anyway?


So some back ground info, the boiler is a Greenstar Utility 18-25, the house is built into the side of a hill so three levels bottom side and two levels top side, boiler in the ground floor bottom side, which is a self contained flat which was a garage, when we moved in we found central heating had been altered so the time switch in the utility room centre floor simply turned it off/on and there was no longer an ability to select central heating or domestic hot water, there was a RF thermostat receiver down in the flat, but no transmitter, only the flat was temperature controlled with a wall thermostat switching on the pump for flat radiators, the pump for house radiators was simply on a 13A plug so you could run it if required. There were 3 fused connection units taking power from two independent fuse box/consumer units, and between main house and flat wire colours changed and there were only two out of three connected. Also floor failing under the cistern and electric shower, both showers leaking, both instant electric type.


So chippy and plumber employed to do emergency work to stop cistern dropping into floor below, and repair rotten beam under shower, replacing tiles and shower tray, he got a full set of TRV’s to replace all existing, but ran out of time, so is to return next week.


In the mean time I needed to sort out wiring, so pair of wires between flat and main house now used to connect Nest heat link to wall thermostat to give it 12 volt power, the heat link fitted in flat, and all power now taken from a single FCU and all ready to wire in the motorised valves when the plumber returns. The new arrangement gives house priority the flat thermostat will run motorised valve and pump, but not the boiler, it is likely to be rarely used, and if house has turned on the boiler it should be enough to heat flat when we have visitors, the main house central heating and hot water will be controlled by generation 3 Nest.


The hope is when winter arrives all will be in place, the Nest is paired with four energenie mihome TRV heads, so the three areas dinning room, hall and living room should all be at same temperature.


Considering adding EQ-3 Bluetooth Smart Radiator Thermostat (UK) 142461D0 to four rooms upstairs, at £15 they are a lot cheaper than the energenie mihome TRV heads, and should allow us to switch rooms off/on as required without needing to access the TRV manually. Plus maybe some manual programmable heads for kitchen and bathroom and maybe flat.


So thoughts please before the plumber returns, as he will be draining system and refilling with new inhibitor so if there is an error want it sorting before system refilled so don’t have to buy more inhibitor latter.

 

[D_Hailsham]

1) Plumber tells me I should not fit a motorised valve to the domestic hot water as there is no over run built into the boiler, so it could over heat if the thermo siphon is prevented, due to fitting a valve

Honeywell don't find this a problem! Here's their diagram for a C Plan showing the valve in the return from cylinder to boiler. If the valve is closed when the boiler turns off there is still a path through the heating circuit for any thermo-syphoning which occurs.
View media item 827

2) He says a by-pass valve is not required, the pump is on the return from radiators, and it has been found if one zone pump runs it causes the water to reverse flow in the other system, plus upstairs radiators heat up with thermo siphon even when no pump is running, so he is fitting motorised valves which will be electrical linked to pump so until valve is open pump will not run, unless the micro switch in the motorised valve sticks, is he correct that by-pass valve not required when every radiator is controlled by a TRV?

It's against the rules to have every rad controlled by a TRV. There has to be one rad without a TRV in the same room as the wall thermostat. Reverse circulation is normally controlled by a gravity valve in the return. Does the boiler manufacturer say that a bypass vlve is required?

There is no thermostat on the cistern

Fit one . It controls the motorized valve which you will be fitting to the return. Here's the wiring diagram. You will notice that the motorized valve has a 2-way microswitch, so a 6-wire cable is required to the valve from the wiring centre.

The Nest may have a 30 minute minimum slot, but if the water gets up to temperature in 10 minutes the cylinder thermostat will turn the boiler off/close the valve.

Did you have a full building survey done by a RICS qualified surveyor before purchasing the house?

 

[ericmark]

Honeywell don't find this a problem! Here's their diagram for a C Plan showing the valve in the return from cylinder to boiler. If the valve is closed when the boiler turns off there is still a path through the heating circuit for any thermo-syphoning which occurs.
View media item 827

There is a problem in that the central heating will thermo siphon so unlike your diagram there will be also a motorised valve in series with the pump to stop both thermo siphon and back flow when only one pump is used. So there is no flow if a motorised valve is fitted to the domestic hot water.

It's against the rules to have every rad controlled by a TRV. There has to be one rad without a TRV in the same room as the wall thermostat. Reverse circulation is normally controlled by a gravity valve in the return. Does the boiler manufacturer say that a bypass valve is required?

Nothing in the boiler (Wosrcester Bosch Greenstar Utility 18-25) instructions, the room with wall thermostat has a Energenie MiHome TRV head paired with it so both always set to same temperature, also the two joining rooms to hall also have the TRV heads paired. I have tried to work out what I have.

This is as far as I got, there are pipes going into loft space where there must be a top up tank, it would seem connected to the hot coil, but so much is behind plaster board a lot is guess work, you can see the unknown pipe, not a clue where it comes from?

Fit one . It controls the motorized valve which you will be fitting to the return. Here's the wiring diagram. You will notice that the motorized valve has a 2-way microswitch, so a 6-wire cable is required to the valve from the wiring centre.

I have included an extra triple and earth from heat link wiring centre to boiler wiring centre to allow for a motorised valve and thermostat to be added, however it is the route between the cistern and flat which is the problem, I remember years ago fitting a wireless tank thermostat for my dad, but can't find one now, at least at a reasonable price. Have looked at some sort of wireless relay but as yet not found one.

The Nest may have a 30 minute minimum slot, but if the water gets up to temperature in 10 minutes the cylinder thermostat will turn the boiler off/close the valve.

Correct it burns for 19 minutes then stops for 3 minutes, then burns for 4 minutes and off again but before it can fire up again the 30 minutes has lapsed.

Did you have a full building survey done by a RICS qualified surveyor before purchasing the house?

Yes full building survey done, they found leak on chimney, and noted there was a redundant consumer unit (actually a fuse box) between the original ceiling and new false ceiling, however not redundant it is still in use, missed the cistern floor was failing and if not corrected cistern would likely fall through the floor, and that the beams under the shower were rotten and needed replacing.

As far as central heating goes, it seems when the garage was turned into a granny flat an extra pump was installed so flat and main house independent controlled, there was a wireless thermostat receiver on the flat wall, but no transmitter in the house, when we arrived the pump for house had a 13A plug on it, is one pump is run, the water flows in reverse in the other pump, so as it stands until motorised valves fitted can't get independent control. We tested by turning off the isolation valves at the pump, and that does stop thermo siphon and reverse flow, however the plumber admitted he did not know how to wire, so I have altered wiring ready.

This is how I have wired ready for valves to be fitted, being careful to include colours so he knows how to wire, there is I know a fault that the flat heating will only work when the main house is calling for heat, however the flat will in the main not need heating, it is now a store, we will make it so visitors can stay, but it seems to heat faster than house, so don't see a problem.

The showers are electric, so the central heating boiler only provides water to hand basins and the odd bath, for the latter I would use the 30 minute boost to ensure enough hot water, there is an immersion heater, and an open fire, note some wood piled up on drive so assume they burnt wood, not keen on burning wood due to particular emissions, but OK for an emergency.

Trying to keep costs down at least until old house sold, but mothers house had problems with cheap wireless programmable thermostat, and had the heads we had used in her old house which I knew worked with Nest, so it seemed Nest was the only sensible route, it has resulted in being able to use ½ hour a day for DHW where as it was only option was 3 hours a day, and ½ hour a day does seem to give us enough hot water.

To be frank with an electric shower, cold fill washing machine and dish washer, we have very little need for DHW and washing hands really not enough time for hot water to get through the system, what I really need is extra insulation on the cistern, which will be done.

But I expect the plumber to arrive to finish the jobs he has started, so if there is some thing wrong with plan, I need to know before he arrives, I can see his point in that it will work, and the time to completely change things is when existing boiler fails, by that time there may be an Open Therm oil boiler, or LPG may be a better option, but I have future proofed as much as I can using a thermostat which can used Open Therm in the future.

I can work out the wiring, but plumbing is some thing else, so I am all ears.

 

[D_Hailsham]

Nothing in the boiler (Wosrcester Bosch Greenstar Utility 18-25) instructions

Oh yes there is! "An automatic bypass valve must be connected between the heating flow and return where TRVs are used on all radiators, fitted to give at least a 3 metre circuit when activated.

Correct it burns for 19 minutes then stops for 3 minutes, then burns for 4 minutes and off again but before it can fire up again the 30 minutes has lapsed

Preumably the 19 minutes occurs when heating the cylinder from cold, so all the heatfrom the boiler is being absorbed by the water in the cylinder. As the water temperature increases not all the heat can be absorbed, so the boiler return temperature, and consequently the flow, increases. The boiler stat ccuts the boiler off. If the cylinder water has not reached the required temperature, extend the on time to say one hour.

Even a full survey does not include lifting flooor boards etc

Fit a non-return valves after each MVs. This will prevent any back-flow.

Why is the flat valve's brown wire connected to terminal 5? Normally it would go to terminal 1.

 

[ericmark]

I have considered this slight variation which will reduce the load on the heat link relay contacts, running the pump direct from thermostat would release the micro switch in the motorised valve so the flat could run independent from house, but felt need to ensure the valve is open before pump starts?

An automatic bypass valve must be connected between the heating flow and return where TRVs are used on all radiators, fitted to give
at least a 3 metre circuit when activated.

Yes found it, I suspect that is why plumber says must not fit a motorised valve to the DHW as with no valve fitted there will always be a flow even if all TRV's are closed and all motorised valves are shut. However it does not protect the pump, only the boiler, so with TRV's on all radiators there must as you point out be a by-pass valve on both pumped systems, and with by-pass valves fitted then using the micro switch in the motorised valve to turn on the boiler and having valve and pump in parallel in the flat is not quite as bad, must ring plumber tomorrow to ensure he brings by-pass valves.

 

[D_Hailsham]

Why do you think the pump(s) need(s) protecting?

 

[ericmark]

Why do you think the pump(s) need(s) protecting?

Maybe it does not? I know in father-in-laws house the micro switch in the motorised valve failed, and the pump was pushing into a closed valve, and the boiler cycling, the boiler survived, the pump however would after that only run at high speed. But his pump was on the supply side, mine is on the return, there are loads of radiators in the house, 13 I think, and the bathroom towel rail has no TRV fitted, however there are two pumps, and it seems bathroom towel rail is thermo siphon.

The wall thermostat in house is linked to three of the TRV heads, so it should switch off wall thermostat which controls one pump, at same time as TRV electronic head closes the valve, but is should good enough?

In the flat the pump is on the wall thermostat, and the four radiators all have TRV's fitted.

I have not found any by-pass valves, and not sure they would work with pump on the return? I am having two motorised valves fitted, as tests showed when either of the pumps run, then we get reverse flow through radiators, and top floor radiators get warm, not hot, when the boiler only runs to heat domestic hot water.

Plumber is due Monday to fit motorised valves, if some thing else needs doing, now is the time, the system will need refilling with inhibitor on Monday, so don't want to drain it again.