Thermal Store with PVs and underfloor heating

[D Walter]

Advice would be most welcome on use of PVs to provide hot water and underfloor heating.  We are installing a 6.5kW PV system in addition to a gas boiler and are beginning to look at the systems available.  We have 4 bedrooms, 3 showers (one of which is a bath) and wet underfloor heating in concrete flooring.  Our assumption is that the primary benefit we will get from the PVs is water heating, until such time as battery technology is affordable, but our hot water usage will be modest except when we have guests.  Ideally, we would want to have the option to run both hot running water and underfloor heating from the PVs.  We are looking at 300 litre Thermal Stores with 6/9kW immersion heaters which should be able to do what we want but would really like any thoughts on:

• whether we are on the right track;
• Thermal Stores (thus far I have looked at Advance Appliances Energymanager Sealed System and McDonald Water Storage Thermflow 300);
• control systems for managing the overall system.

Thanks in advance.

[scottishjohn]

that might work ,but you going  to need a control circuit that makes your pV take primary role for  DHW --or your boiler will heat it up and you won,t get full use of PV

If i was doing this then I would have boiler primarly doing UFH  and only when  dhw tank is above 60c? would i divert it to UFH -

maybe a separate loop in  bottom  half of tank for that and  divertor valve and pump  so you taking water at  or below40c for ufh ?

then maybe a coil for boiler to top up tank if top half way  temp drops below  a target temp - so  3  coils in tank ? 

DHW out 

uUFH out

boiler in 

then your 2 pv heaters

lot of control system needed

could be complicated to get best out of PV-

sure someone will have a solution 

Edited March 27, 2019 by scottishjohn

[ProDave]

This is where I think an UVC is a better solution.

 

The problem with a thermal store is the temperature in the tank needs to be a lot hotter than your desired DHW temperature. If you heat it hot enough with the boier, there is not much capacity to heat it further with PV

 

On the other hand, I can heat my UVC with my ASHP to 48 degrees, which is plenty hot enough for DHW and that still leaves a lot of scope for the excess solar PV to heat it hotter.  If the PV heats it hotter then that just means it is a lot longer before the ASHP comes on again to heat it.

 

No complicated control system other than a time clock to say when the ASHP is on and off, and a home made solar PV diverter that sends any excess to the immersion heater.

[D Walter]

Thanks ProDave.  Using an unvented cylinder would presumably mean not utilising the PV for UFH (i.e. running UFH directly off the gas boiler), which would probably happen anyway as UFH would only be required in winter when PV is at low output.

 

One of my local PV installers is trying to persuade us to go electric only with a supplier like Green Energy or Octupus who do c. 5p per unit off peak, with a Thermal Store and a Tesla PowerWall on the basis that we can easily install an 8kW PV system.  It all loooks very expensive, complicated and possibly "bleeding edge" technology so we will probably go for the tried and tested UVC as you suggest.  I would feel more comfortable exploring the more complicated options if I could find a consultant (not a salesman) who understands the technology and practicalities of installation and management who could also do a cost benefit anlysis.

[ProDave]

I have 4KW (soon to be increased to 4.5) of PV.  At this time of year all our generation is self used by either the house loads or DHW heating.  So far since installing the immersion heater diverter I have exported just 2.9KWh.  Obviously the larger PV array you install the more surplus you will have.

 

I am not convinced about battery storage yet.  Even if the electricity is free, the "cost" of the stored electricity when you factor in the capital cost and life of the battery works out at about the same as just buying it from the grid, in other words a lot of technical stuff for no gain.  That I am sure will change as battery storage becomes more of a main stream product and prices will fall further.  So I would be in no hurry to put that in just now but factor in the possibility later.

 

I am going to wait a full year to see just how much of our generation we can actually self use and how much gets exported.  That will tell me how much storage capacity is needed and whether it is worth it.  High summer I am sure will be the time we simply can't use it all and some gets exported.

 

Yes you will find in the winter, when you really need it, you won't get much from the PV 

 

You even have to price the install cost of the PV.  For me it was only worth it by searching high and low to get the equipment at rock bottom price and self installing it.  I installed 4KW for £1500 and it will be about another £150 when I add the extra 2 panels and a small inverter for them.  I expect this to pay for itself in saved electricity costs in about 6 years.  If I had paid typical installed costs the payback time would probably be double and I am not convinced that is good use of capital.

[Alex C]

How well insulated and airtight is your house? If it is really well insulated it wont need much heating anyway. You are also not going to be generating from the pv at the time of year you need to heat your house so no point linking to ufh. We have similar no bedrooms and bathrooms with passive type build and have 300l super insulated unvented cylinder and a small gas boiler that moderates from 9 to 3 kw. Heating is wet ufh on ground only and not had that come on since january. We have wet towel rads in bathrooms and these come on for half hour a day and that is enough to heat the house along with normal day to day activities and solar gain. With a 5.5 kw pv system we started heating all of our hot water last week as we had sunny days and will pretty much continue to do so until October. Dont underestimate how low pv generation is in the winter. 5.5kw system will generate well under 1kwh on a dark grey winter day but has been doing over 30kwh for the last few days.

[D Walter]

Totally agree!  We are at first fix and doing roofing as we speak so are installing the SWA internal cabling from utility to roof for PV installation and will research the best PV/cost.  The PV technology has moved on in the last 9 months since we got our first PV quotes and there has got to be a step change in battery technology in the near future so not looking at any form of battery for the time being.  The immediate decision we need to make is primary heat source and HW cylinder and as we are on mains gas it looks like the sensile option for now is go with gas boiler and probably a 300l UVC with 3kW immersion.

 

Thanks to all for your very practical input!

[D Walter]

We are at a U value of 0.15 on walls, between 0.09 and 0.11 on roof and windows between 0.7-0.8 with predominantly glazed SW aspect so we should have low UFH requirement.

[D Walter]

On airtightness we are not sure what we will actually achieve but are aiming at better than 2 m³/hr per m²

 

We have solid concrete walls and floors, well sealed triple glazed windows and doors, sprayfoam insulation at roof plate plus taped foil backed PIR and continuous vapour control layer so we are hoping we will achieve very good airtightness for the MVHR system.

 

 

[D Walter]

To further impose on your patience I have a couple of questions.  Following the advice given above we are going to go for an unvented indirect cylinder with two immersions powered by either 6.5 or 8.4 kW PV system and a gas boiler.  We will not need greater than a 300 litre cylinder but the 300 litre cylinders tend to be min 2000mm high.  The cylinder will stand in front of the UFH manifold which is at approximately 1950mm making access difficult.  The 400 litre cylinders tend to be shorter at c. 1600mm high which will work better with the UFH manifold, though we would need to restrict the width (and therefore the insulation) to c. 750mm due to the size of the cabinet.  The 400 litre cylinders which will fit the cabinet tend to have c. 2.5 kW per day standing loss as compared to 1.7 kW for a better insulated 300 litre cylinder.

 

My assumption is that, apart from the greater standing loss suffered which will be a pence a day, the bigger tank should not have a greater running cost as the same volume of hot water will be used so very little more water will be heated on an ongoing basis.  Does this, if you will excuse the pun, hold water?

 

My second question is whether I should go for a tank with the standard placement of immersions at 799mm and 1117mm or would I be better going for a cylinder with lower located immersions at e.g. 432mm and 910?  My guess is that the lower immersion would be beneficial in the summer months as there will be more kWs from the PVs to heat the greater height of water above and detrimental in the winter where the immersions may never get the head of water to a satisfactory temperature.

[ProDave]

I chose the Telford UVC which at 1650mm for the 300L was the shortest UVC I could find. https://www.cylinders2go.co.uk/shop/renewable-energy/300-litre-telford-tempest-heat-pump-cylinder/

[Alex C]

400l is a lot of water, are you running a b&b. I have the telford 300l tempest plus which has extra insulation although I can't see this on the website now. We have never run out of hot water. The standing loss is going to be higher than you think and whilst not costing much financially  in lost heat, it may end up heating a well insulated house up more than you would want it to. I have thick insulation around all the associated pipework but the cupboard the cylinder is in still gets pretty hot. Also be aware that most pv diverters only run 1, 3kw immersion at a time so it will take an age to heat 400l of water from cold just using pv.

[bassanclan]

You can order a tank to your custom size.

Two places I would recommend are Newark copper cylinders (Notts) or World Heat cylinders (Cheshire)

[D Walter]

Thanks again for the good advice!

[ProDave]

24 minutes ago, Alex C said:

400l is a lot of water, are you running a b&b. I have the telford 300l tempest plus which has extra insulation although I can't see this on the website now. We have never run out of hot water. The standing loss is going to be higher than you think and whilst not costing much financially  in lost heat, it may end up heating a well insulated house up more than you would want it to. I have thick insulation around all the associated pipework but the cupboard the cylinder is in still gets pretty hot. Also be aware that most pv diverters only run 1, 3kw immersion at a time so it will take an age to heat 400l of water from cold just using pv.

We have ran out of hot water from out 300L cylinder 3 times now.  At 15L per minute on full tilt that is only just over 20 minutes of showering and you should never under estimate how long a woman can spend in the shower.  We only heat the hot water to 48 degrees to get the best COP from the heat pump so it is not diluted with much cold so probably gives a real world 30 minutes of showering.

 

I have "solved" this by adding a modulating instant electric water heater on the output that will take over if the cylinder runs out.

 

Be careful putting more than 3KW excess PV into the tank at one time.  You will either need 2 solar PV diverters or one custom made one to deal with that load.