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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorJustin R
    • CommentTimeNov 2nd 2020 edited
    Evening All,
    I'm thinking of installing an electric boiler to heat the radiators in our old cottage in Scotland, using the electricity from a mini-hydro scheme which should produce 3 or 4kW
    While the 4kW will apparently be plenty to cover the 33,000btu’s required, someone told me (and I think you will too) that I'm going to have a problem with efficiency trying to run the heating at such a low level.
    I notice that when I use the sizing calculators on the manufacturers websites what’s being recommended for a boiler is something between 12 and 16kW
    This suggests that for some reason, or reasons, heating a house with radiators must be done at high water temperatures for short stints, which fits with the typical arrangement of heating on a timer twice a day rather than continuously.
    Why is it always done like that? And why can’t you simply run your radiators at a lower temperature but for the whole day (using a low powered boiler) like with under floor heating?
    Alternatively are the boiler and radiator size recommendations based more on the ability to heat the house quickly from cold but then the actual kWs used to maintain that heat are much lower?

    A couple of other points I should mention:
    Assume we are off grid with a back-up generator for all intents and purposes.
    I realise we will have to design in some storage, to deal with other typical peak loads, but I’m trying to minimise this as much as possible (batteries expensive, accumulator tanks big). If I can heat the house using a couple of kWs continuously rather than storing and using the heat in high temperature bursts it would make things much simpler.
    Radiators and pipe-work (mostly 22mm) are already in. Some rads a bit small.
    Not much insulation (roof plenty, underfloor plenty, none in walls) and old thin double glazing.
    Probably can’t afford a heat pump, but the electric will be close to free and close to green so who cares?

    Any thoughts or solutions would be much appreciated. Even if it’s that using an electric boiler is a stupid idea in the first place!
    • CommentAuthortony
    • CommentTimeNov 2nd 2020
    Here is one I am on , trying for a grant

    Water powered water source heat pump

    Innovative heating for the Riverside Museum beside Blake’s Lock

    The exhibition space is sited above the river Kennet at Blake’s Lock and known as the Riverside Museum, it has been poorly utilised in recent years as it is difficult to heat.

    We propose an innovative solution to the heating problem, this is to use the river itself to heat the space via a water source water powered heat pump. Most people think that heat pumps need to be powered by electricity, in fact any motive force can be used to operate a compressor. One of these sits at the heart of a heat pump and our proposal is to use the potential energy of the water at Blake’s Lock weir to power such a compressor. Formerly the power of the river has been used for a variety of industrial processes including turning bells.

    By using the river to run a compressor the energy output is multiplied, typically a heat pump turns one kilowatt of energy into three or four kilowatts of heat. Our proposal increases the coefficient of performance even further by using the river water to extract the heat from, this increases the coefficient of performance of the heat pump to between 6 and 9 to one. i.e. 600 to 900%. The system would be controlled by a thermostat in the building that simply closes the sluice gate shutting off the flow of water to the heat pump’s compressor. The power from the pump shaft could also be use to power a fan or fans that would be use to circulate heat within the building. Interestingly this fan could also be used to introduce ventilation into the building too.

    There is also an opportunities to export any excess generated heat to the building next door either from this system or from a second hydro powered compressor.

    There are many good outcomes from this project all of which benefit the environment, the local community in several ways, educationally, increasing accessibility to exhibition spaces, providing a new source of heat energy, an exemplar innovative project. This is is ideal candidate for Silax funding.

    Tony October 2020
    • CommentAuthorJustin R
    • CommentTimeNov 2nd 2020
    Sounds nice Tony!
    • CommentAuthormikael
    • CommentTimeNov 2nd 2020
    Tony, your proposal sounds interesting, and you are perfectly right that heat pumps can be powered by any means of motive force. I have heard of people proposing wind powered heat pump compressors too.

    However reducing efficiency losses by driving a compressor directly rather than using the hydro power to generate electricity and then using this electricity to drive an electric compressor may not offer a huge advantage. The loses these days are not necessarily that large, and generating electricity can allow continuous generation of electricity which can be used for other devices and exported if not needed. Directly driving a compressor will mean that the energy from the flow of the water will not be harvested when heating is not required.
    • CommentAuthorEd Davies
    • CommentTimeNov 2nd 2020
    I like the general idea of heating continuously at low radiator temperature. Apart from the low power [¹] consumption there might well be a comfort advantage to having continuous radiation. I've been tuning the code I use to control the oil boiler in the old stone Scottish cottage I'm renting and definitely notice the difference when the radiator cools down even though the air temperature only fluctuates by about 0.25 °C.

    Posted By: Justin RWhile the 4kW will apparently be plenty to cover the 33,000btu’s required, …
    Could you clarify what you mean by 33,000btu here? At first glance I'd assume it means 33,000 btu/h which works out at 9.671 kW which would indeed be nicely covered by 4 kW into a heat pump but then your write:

    Probably can’t afford a heat pump
    leaving me a tad baffled.

    [¹] Note - I do mean low power consumption, not low energy consumption here.
    Why not use electric UFH.
    • CommentAuthortony
    • CommentTimeNov 3rd 2020
    100% efficient but downward losses are greater, heat pump 350% efficient
    • CommentAuthorJustin R
    • CommentTimeNov 3rd 2020
    Hi Ed,
    I'm also living in a small Scottish 'pile of stones' (I read your thread on radiator temperatures etc. that you posted recently and I really like that name).

    Yes I did meant 33,000btu/h per day.
    I wasn't envisaging using a heat pump at this time (maybe in the future). I was considering using a conventional electric boiler which seem cheap and relatively reliable and would utilise only part of our available electricity, still leaving enough for lights, appliances etc. with minimal battery storage for dealing with temporary peak loads.
    However it would require the low temperature water to the rads which you have been experimenting with.
    • CommentAuthorJustin R
    • CommentTimeNov 3rd 2020
    Oh sorry didn't mean to post that just yet! Must have clicked on the mouse with my elbow by mistake! Please wait before responding as more follows....
    • CommentAuthorJustin R
    • CommentTimeNov 3rd 2020
    So continuing on:
    Firstly sorry I meant 'post' rather than 'thread' in the first sentence of the other comment.
    Interesting that the difference to your comfort is so noticeable when the air temp. is only very slightly lower. This suggests that you are being warmed to a great extent by the radiation rather than the convection? I wonder do you sit close to the radiator?
    Anyway the small air temperature difference in the room is encouraging as it suggests that the efficiency of heat transfer from the radiator to the air is not much affected by running the rads at a lower temperature but for longer times.
    You mentioned in your post in the thread "How to choose an air source heat pump" that the radiators might provide the necessary heating at just 40 degrees in this continuous heating scenario. Have you checked what the actual temperature of the water in your radiators is?
    You also said that on average your boiler run time is 25%, which I assume is because it is cycling to produce the lower radiator temperatures. This would imply that the same effect to the air temp. of the house could be achieved from running an electric boiler at about 4.5Kw continuously.
    However there must be something missing in my assumptions above as it would mean that you were using 108kW/h in one day which can't be right. Particularly given that your house sounds quite similar to mine and I'm only expecting to need roughly 9.7kW/h. What is missing from the calculation?

    Renewablejohn: We want to minimise disruption in the house so we would prefer to avoid taking any floors up, not to speak of the fact that in the kitchen/dining room we have the original stone flags on the floor (one six foot long!) with the kitchen built on top....
    • CommentAuthorJustin R
    • CommentTimeNov 3rd 2020
    Sorry Ed I keep missing things... I re-read your post in the "How to choose an air source heat pump" thread and saw that you already gave a very detailed description of your actual radiator temperatures!
    Please disregard that question in my previous post!
    Hi Justin,

    I like the idea of using hydro, and as it's a continuous source, running continuous heating to match heat loss rather than high peaks and troughs makes sense.

    I'm slightly wary of your expectation of only needing 10kWh per day. That's one towel rail on for 24 hours. What external temps are you setting that energy consumption against? Could you paste your calcs on here for a quick look over?

    Finger in the air estimate would say that in winter (let's keep it simple and say delta T of 20oC, you'll have a house heat loss of 5kW, rising to 8kW for dT of 35oC (-15oC outside) - based on maybe similar cottages I've refurbished here in the Highlands.

    So you're looking at more like 120 to 200kWh per 24hours. That said, your continuous power feed of 3 or 4kW will make a big dent, that maybe a log stove could boost?

    Of course, a future improvement when cash permits might be adding the heat pump, a la Tony, rejecting coolth to the burn, assuming it's not a SEPA controlled Salmon feeder - if they were to be notified ;). Then your 4kW source, might go to 2 kW to the heat pump, and 2 kW for other uses, maybe heating DHW in a cylinder?

    Any chance of your river/burn freezing, giving reduced or no flow? Had one client up in the hills with that issue.
    • CommentAuthorEd Davies
    • CommentTimeNov 3rd 2020 edited
    Posted By: Justin RYes I did meant 33,000btu/h per day.
    Now I really haven't got a clue what you are talking about.

    For illustration of what units mean (which it seems you're confused about) let's take my 18 kW boiler running 25% of the time.

    The kilowatt (kW) is a unit of power: the rate at which energy is transferred (it's 1000 joules per second). Because kW is already a rate kW/h doesn't usually make sense. (There are a few weird cases like the output of a PV factory would could be measured in kW/h.)

    An 18 kW boiler running 25% of the time would have an average power output of 18 × 0.25 = 4.5 kW. That means that in 1 hour it would, on average, output 4.5 kWh of energy and in 24 hours it would output 108 kWh of energy (i.e, it'd have a power output of 108 kWh/d).

    A btu is a unit of energy: 1 kWh ~= 3412 btu so 33'000 btu = 9.671 kWh.

    If your expectation is that that's the amount of energy you'll need for heating in an hour (so your heat load is 9.671 kW) then that means your heat load is about twice mine which is entirely within the realms of possibility.

    If, on the other hand, that's how much energy you think you'll use in a day then it means that your average heat load is 9.671 kWh/d / 24 h/d ~= 0.4 kW. That's in the realms of very-well insulated small Passivhaus, at best.

    If, on the third hand, it's your average heat load per day over the whole year then maybe it makes sense but it's pretty much useless for sizing a heating system.
    • CommentAuthorJustin R
    • CommentTimeNov 3rd 2020
    Hi Paddy,
    you are right to be wary as I actually have no calculations at all! I'm just kind of ferreting around at the moment trying to see if it would be feasible to heat the house in this way.
    All I've done so far is to consult a couple of websites which have online calculators where you put in the volume of your rooms and whether the house is modern or old, and it spits out an estimate of the amount of energy you would need to heat it. Very basic, but I think I've misinterpreted even that!
    The average I got was 33,000 btu(?) and I assumed that was per day, but I now realise it should be per hour.
    That might explain the discrepancy when I was trying to interpret Ed Davies' post.
    I was misinterpreting the meaning of btu/h.
    It also explains why Ed was baffled by my saying that I probably couldn't afford a heat pump!

    So now the penny has finally dropped I can see that the heat pump is probably going to be necessary after all, and we will just have to find the money.

    Re the river freezing, it used to do so most years for a week or two though usually nor right across. Hasn't done so for a good while now though. We will be bringing the water in down an old mill lade and I'm sure it will freeze solid at times, in which case we will just have to rely on the mains or the generator depending on the final design of the system.

    Anyway thanks all for your comments and advice, you have resolved the main question for me though not in the way I expected!
    I'll probably be back to cause more confusion in the future....
    • CommentAuthorJustin R
    • CommentTimeNov 3rd 2020
    Oh damn I hadn't noticed that you had posted a comment Ed, before I posted my last.
    I'll read it now....
    • CommentAuthorJustin R
    • CommentTimeNov 3rd 2020
    Yep, as I imagined my misunderstanding clearly laid out.....
    V sorry to have caused such confusion! However as I said before you have not wasted your time as I now have a much better idea of the direction I need to head in, and that trying to heat my house using just 4kW with an electric boiler is going to result in a pretty chilly house!
    So back to the drawing board for me and many thanks to you for sorting this out.
    • CommentAuthortony
    • CommentTimeNov 3rd 2020
    4kW with a Cop of 3.5 (ASHP = warm house GSHP - warmer Water source = plenty left over for other things
    Your 6 foot slab is likely to be a cellar or priest hole. I have one here which has a void below it but as yet not had the courage to lift it.
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